By Pichu Rengasamy School of Agriculture, Food and Wine, University of Adelaide

Transcription

1 By Pichu Rengasamy School of Agriculture, Food and Wine, University of Adelaide

2 Title: GRDC Project Code: AADE00001 Author: Dr Pichu Rengasamy Senior Research Fellow School of Agriculture, Food and Wine University of Adelaide Waite Campus, PMB1 GLEN OSMOND SA Grains Research and Development Corporation. All rights reserved. ISBN Published January 2016 This book is copyright. Except as permitted under the Australian Copyright Act 1968 (Commonwealth) and subsequent amendments, no part of this publication may be reproduced, stored or transmitted in any form or by any means, electronic or otherwise, without the specific written permission of the copyright owner. A copy of this report can be found at Design and production: coretext.com.au Disclaimer: Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of the Grains Research and Development Corporation. No person should act on the basis of the contents of this publication without first obtaining specific, independent, professional advice. The Corporation and contributors to this Research Report may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. The GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication. CAUTION: RESEARCH ON UNREGISTERED AGRICULTURAL CHEMICAL USE Any research with unregistered agricultural chemicals or of unregistered products reported in this document does not constitute a recommendation for that particular use by the authors or the authors organisations. All agricultural chemical applications must accord with the currently registered label for that particular agricultural chemical, crop, pest and region. Copyright All material published in this Research Report is copyright protected and may not be reproduced in any form without written permission from the GRDC. 2

3 CONTENTS 1. INTRODUCTION PROCESSES OF SOIL SALINISATION Sources of salt Major types of salinity Extent of world salinisation DIFFERENT TYPES OF SALINITY IN THE DRYLAND REGIONS OF AUSTRALIA Groundwater-associated salinity (GAS) in Australia Transient salinity (non-groundwater-associated) in Australia Occurrence of salt bulges in deep subsoils in Australia Soil processes leading to salt accumulation (transient salinity) Potential transient salinity and subsoil constraints in cropping regions Climate change and salinity in dryland regions IRRIGATION SALINITY IN AUSTRALIA Irrigated soils Sodicity in irrigated soils Sodium adsorption in soils Natural processes of sodium input to soil profiles Addition of sodium through irrigation water Sodium from the dissolution of solids and minerals Sodium contributed by chemical applications Sodium input from saline groundwaters Sodium removal from the soil solution Sodium balance in soil profiles Precipitation and complex formation Threshold electrolyte concentration and saline irrigation Accelerated sodification under saline irrigation CATEGORIES OF SALT-AFFECTED SOILS Categories based on soil solution characteristics PHYSICS, CHEMISTRY AND BIOLOGY OF SALT-AFFECTED SOILS Soil structural stability in relation to sodium in salt-affected soils Sodicity in Australian soils Physical properties affected by the accumulation of sodium salts Mechanisms of structural change in sodic soils Initial wetting of dry aggregates Hydration reactions Comprehensive hydration mechanism for soil clays Slaking of soil aggregates Spontaneous dispersion Mechanical dispersion The Electrical Double Layer Net charge and clay dispersion Flocculation caused by cations in electrolytes

4 Contents Flocculation by alternative mechanisms Dispersive Potential (Pdis) Research reports on the effect of potassium and magnesium on soil structure Potassium Magnesium Recent research on the role of different cations on soil structure Ionicity and covalency indices for clay cation bonds Cation Ratio of Soil Structural Stability (CROSS) Development of CROSS Concept Biology of salt-affected soils Organic matter Microbial activity Soil ph in relation to salts Measurement of soil ph Managing high ph soils SOIL PROCESSES AFFECTING CROP PRODUCTION IN SALT-AFFECTED SOILS Effect of osmotic pressure of soil solution on plants Soil water and osmotic pressure dynamics in the field as related to climate Seasonal changes in transient salinity Other soil processes in relation to crop production in salt-affected soils DIAGNOSIS AND MANAGEMENT OF SALT-AFFECTED SOILS Dispersive soils Estimation of cations to calculate SAR or CROSS Dispersion (surrogate measure of cationic effects) 8.3 Managing dispersive soils containing Na and K Management of saline soils Measurement of soil salinity Calculation of Osmotic effect of salinity on water uptake by crops Gypsum and osmotic effects Assessing farm soils for salinity, dispersivity and alkalinity Soil sampling and identification of transient salinity Soil preparation Soil: Water (1:5) suspension for analysis Measuring dispersivity with a Dispersion Meter Measuring soil salinity (EC electrical conductivity) Measuring soil ph (Alkalinity or Acidity) Measurement of Soil Water Content Identifying soil texture classes by hand texture method...53 APPENDIX I THE EFFECT OF SOIL SALINITY ON PLANT GROWTH...55 APPENDIX II CONVERSION FACTORS FOR ELECTRICAL CONDUCTIVITY UNITS REFERENCES...56 NOTES

5 Contents INDEX OF TABLES AND FIGURES TABLES Table Area extent of salt-affected soils in different regions of the world... 9 Table Distribution of different types of salinity in Australia Table Irrigated area, irrigation water use and water resources in Australia Table Categories of salt-affected soils Table Area of different categories of salt-affected soils in Australia Table Slaking and spontaneous dispersion of aggregates (2-4mm) of an Alfisol at two levels of sodicity in various solvents Table Types of bonding and mechanisms involved in the linkages between clay particles in an aggregate Table Relative flocculating power of cations...38 Table Ionic potential, Misono s softness parameter, covalency index and iconicity index of the cations...35 Table Soil ph (1:5, in wter) measured after treating a non-calcerous soil with chlorides, sulfates, bicarbonates and carbonates of Na or K and washed free of salts Table Percentage of available water not taken up by plants in soils affected by transient salinity in southern Australia...43 Table Saline soil classes, on the basis of different soil textures and EC1: Table Approximate relationship between ECe and EC1:5 for different soil textures Table Percentage of available soil water not taken up by plants in different soil types due to osmotic pressure of soil water salinity in relation to laboratory measured soil salinity and field soil moisture Table Osmotic pressure caused by gypsum and sodium salts for different water contents Table Salinity classes (based on EC1:5, ds m-1) and soil gravimetric water content (%) below which water is not taken up by plants due to osmotic pressure > 1000 kpa...53 FIGURES Figure Major types of salinity in world soils based on salinisation processes... 8 Figure Groundwater-associated salinity (seepage or dryland salinity) in South Australian landscape Figure Schematic diagram of salt bulge in deep soils under native vegetation and cropping Figure Soil processes and accumulation of salt in the root zone layers (transient salinity) without the influence of groundwater Figure Different types of dryland salinity found in the Australian landscape Figure Crop production affected by transient salinity in subsoil in a paddock in South Australia Figure Relationship between subsoil exchangeable sodium percentage ESP and root zone salinity as influenced by annual rainfall at 30 sites in south-eastern Australia Figure Simulations of chloride accumulation using LEACHM models in a red-brown earth under restricted and free drainage at 1m depth Figure Map showing areas of dryland seepage salinity, regions with potential transient salinity and subsoil constraints and area covering grain production in Australia

6 Figure Quality of irrigation water affects the soil solution composition and subsequently affects soil physical properties and crop productivity Figure Threshold electrolyte concentrations distinguishing spontaneous dispersion (dispersive soils) or mechanical dispersion (potentially dispersive soils) from flocculation (structurally stable flocculated soils) on the basis of SAR (WESP) and EC (TCC) relationships in red-brown earths.. 20 Figure Schematic diagram relating sodicity (ESP) and salinity (EC) Figure Categories of salt-affected soils Figure Schematic diagram showing the change of a saline soil to a sodic soil by leaching by rain or irrigation Figure Schematic illustration of processes that take place and intensity of attractive and repulse forces involved when a dry aggregate of soil is wetted Figure Relationship between relative zeta potential and relative clay content of dispersed suspensions of soils with different mineralogy, exchangeable cations, organic matter and ph Figure Relationships between iconicity index of cations and relative turbidity and relative zeta potential...35 Figure Cation ratio of soil structural stability (CROSS)...36 Figure Active porosity in a soil dominated by a single cation...36 Figure Relationship between active porosity and relative hydraulic conductivity of cationic soils Figure The relationship between turbidity and CROSS or SAR in soils containing significant amounts of K and Mg in addition to Na and Ca...38 Figure An example of back alkali-spreading of dissolved organic carbon within a week of addition of bio-solids in a field soil in India with a ph of Figure Microbial respiration as influenced by the salinity levels induced by molar concentrations of NaCl or CaCl Figure Metabolic diversity of biological community as influenced by Cl- (low ph) and CO32- (high ph).40 Figure Classification of soils on the basis of ph (water, 1:5) and the possible deficiencies and toxicities Figure Factors causing abiotic stress in relation to ph in alkaline soils Figure Schematic diagram of the effect of electrical conductivity (EC) and osmotic pressure of the soil solution on the yield of wheat Figure Gravimetric soil water content (%), matric potential of soil water (-kpa) and total (matric plus osmotic) soil water potential (-kpa) of a clay loam layer (20-60 cm) in a Natrixeralf (with an EC1:5 of 1 ds m-1) during wheat growing season in Figure Symptoms of poor soil structure in the field caused by clay dispersion: (a) Soil crusting; (b) waterlogging...46 Figure Tunnels developed in a dispersive soil, northern Bruny Island, Tasmania Figure Soil structural deterioration in the field of a dispersive soil affected by sodicity near Horsham, Victoria, after a rainfall event...48 Figure Mechanism of reclamation of dispersive soils (containing Na and K) by using gypsum Figure Soil profile showing the upper 1 metre with differentiation of various horizons (soil layers) Figure Measuring dispersivity of a soil in water using a dispersion meter Figure Identifying soil texture by hand texture method

7 1. Introduction 1. INTRODUCTION Global food production will need to increase by 38% by 2025 and by 57% by 2050 (Wild, 2003) if food supply to the growing world population is to be maintained at current levels. Most of the suitable land has been cultivated and expansion into new areas to increase food production is rarely possible or desirable. The aim should be an increase in yield per unit of land rather than in the area cultivated. Increased efforts are needed to improve productivity as more land becomes degraded. It is estimated that about 15% of the total world land area has been degraded by soil erosion and physical and chemical degradation, including soil salinisation (Wild, 2003). Salinisation is the accumulation of water-soluble salts in the soil solum (the upper part of a soil profile, including the A and B horizons) or regolith (the layer or mantle of fragmented and unconsolidated rock material, whether residual or transported) to a level that impacts on agricultural production, environmental health, and economic welfare. A soil is considered saline if the electrical conductivity of its saturation extract (EC e ) is above 4 ds m -1 (US Salinity Laboratory Staff 1954). However, the threshold value above which deleterious effects can occur varies depending on several factors including plant type, soil water regime and climatic conditions (Maas 1986). For example, in rain fed agriculture, soil water can be far below field capacity and the salt concentration under field conditions is several-fold higher than that measured at soil saturation water content (Rengasamy 2002a). Saline soil water inhibits plant growth through an osmotic effect, which reduces the ability of the plant to take up water and by ion excess, which affects the plant cells (Munns, 2005). Soil salinity also induces nutritional imbalances in plants. When salinity is due to sodium salts, it can lead to the formation of sodic soils where salts are leached from the soil profile. Many saltaffected soils are also waterlogged and the interaction between hypoxia and salt has a powerful depressive effect on plant growth (Barrett-Lennard 2003). Salinisation of land has threatened civilizations in ancient and modern times. Problems associated with soil salinity are not new. Soil salinization in southern Mesopotamia and in several parts of the Tigris Euphrates valley destroyed the ancient societies that had successfully thrived for several centuries (Jacobsen and Adams 1958; Hillel 2005). The decline of civilisations in ancient Mesopotamia has been associated with soil salting under irrigation with rising saline water table. In modern times, specifically in the past two centuries, impact of irrigation salinity on agricultural productivity has been recognised in many parts of the world. Salt-affected soils are naturally present in more than 100 countries of the world and in some regions irrigation-induced salinisation is also present. Since the beginning of the last century, the scientific study of the processes and management of soil salinity induced by irrigation has gained importance in many countries, with pioneering work done by the United States Salinity Laboratory. Dryland salinity has become a major issue in natural resource management globally and in Australia has attracted increasing awareness from both farmers and politicians. Salinity reduces yields of agricultural crops in arid and semi-arid regions where rainfall is insufficient to leach the salts from the root zone. A range of engineering and farm management solutions is available to control soil salinity, but their costs and slow adoption mean that substantial soil salinisation is inevitable. In conjunction with normal reclamation and agronomic management, a genetic approach, involving breeding or genetic manipulation of cultivars with an aim to enhance the ability to grow on salt-affected soils, is also being pursued. Salinity, while directly affecting plant growth by osmotic and ion specific effects can indirectly impact on the soil by changing soil properties and soil processes. Literature is abundant with the scientific knowledge on interaction of plants in saline soils. Knowledge on both plant mechanisms for surviving in saline environments and the salinity affecting soil processes related to sustainable agriculture and environmental protection is essential to make productive use of salt-affected soils. However, the scope of this handbook is limited only to the discussion on the issues related to soils such as salinisation processes, types of soil salinity, categories of salt-affected soils, current knowledge on physics, chemistry and biology of these soils, and also their diagnosis and management. This book will focus mainly on Australian saline environments and research done in Australia, particularly by the author and his associates. 7

8 2. Processes of soil salinisation 2. PROCESSES OF SOIL SALINISATION 2.1 Sources of salt Salt accumulation in soil layers and groundwater takes place during the rock weathering process. Substantial amounts of salt accumulate in the soils of arid and semi-arid regions. The composition of the salt depends on the geologic materials in contact with the water and soil layers and the variations in the solubility of primary and secondary minerals. Rainwater contains low amounts of salt, but over time, salt deposited by rain can accumulate in the landscape. The composition of salt in rainwater varies with distance from the source viz. the ocean. The salt is mainly NaCl near the coast and is dominated by Ca 2+ and inland where atmospheric pollution dictates the nature of salts, it is SO Seawater intrusion onto land, as occurred in recent tsunamiaffected regions, can deposit huge amounts of salt in soils of coastal lands. Submergence of the low-lying lands by seawater causes salinisation of groundwater and soils, while soil surfaces could become saline through tidal fluctuations. In many geological periods, seawater has inundated large areas of the continents that have later been uplifted and salts have been deposited when the water evaporated. When inland lakes evaporate salt bodies are found beneath the soil surface. Wind-transported (aeolian) materials from soil or lake surfaces are another source of salt. Poor quality irrigation water also contributes to salt accumulation in irrigated soils. The particular processes contributing salt, combined with the influence of other climatic and landscape features and the effects of human activities, determine where salt is likely to accumulate in the landscape. 2.2 Major types of salinity There are three major types of salinity (Figure 2.2.1) based on soil and groundwater processes found all over the world (Rengasamy 2006) and these are different from the normal classification of Primary or Secondary salinity commonly used. Primary salinity refers to the salt accumulation that occurs naturally in soils and waters. Secondary salinity refers to salting that results from human activities, usually irrigation agriculture and land development. FIGURE Major types of salinity in world soils based on salinisation processes. Groundwaterassociated salinity (GAS) Primary (GAS) Secondary (GAS) Dryland salinity SALINE LAND Non-groundwaterassociated salinity (NAS) Shallow (NAS) (solum layers) Topsoil (NAS) Subsoil (NAS) Deep (NAS) (below solum) Transient salinity, dryland salinity, magnesia patches Irrigation Associated Salinity (IAS) Topsoil (IAS) Subsoil (IAS) Irrigation salinity with shallow or deep watertable SOURCE: RENGASAMY, Groundwater associated salinity (GAS): In discharge areas of the landscape, water exits from groundwater to the soil surface bringing dissolved salts with it. The driving force for upward movement of water and salts is evaporation from the soil plus plant transpiration. Generally, the water table in the landscape is at or very close to the soil surface and soil properties at the site allow a maximum rate of water movement through the surface layers. Salt accumulation is high when the water table is less than 1.5 m below the soil surface. However, this threshold depth may vary depending on soil hydraulic properties and climatic conditions. 2. Non-groundwater-associated salinity (NAS): In landscapes where the water table is deep and drainage is poor, salts introduced by rain, weathering, and aeolian deposits are stored within the soil solum. In drier climatic zones, these salt stores are usually found in the deeper solum layers. However, poor hydraulic properties of shallow solum layers can lead to the accumulation of salts in the topsoil and subsoil layers affecting agricultural productivity. In regions where sodic soils are predominant, this type of salinity is common. 3. Irrigation associated salinity (IAS): Salts introduced by irrigation water are stored within the root zone because of insufficient leaching. Poor quality irrigation water, low hydraulic conductivity of soil 8

9 2. Processes of soil salinisation layers as found in heavy clay soils and sodic soils, and high evaporative conditions accelerate irrigationinduced salinity. Use of highly saline effluent water and improper drainage and soil management increase the risk of salinity in irrigated soils. In many irrigated regions, rising saline groundwater interacting with the soils in the root zone can compound the problem. 2.3 Extent of world salinisation Szabolcs (1989) described the global distribution of saline and sodic soils in different continents (Table 2.3.1) based on the FAO/UNESCO soil map of the world and many other maps, data, and material available at that time. According to a more recent report published by FAO in 2000, the total global area of salt-affected soils including saline and sodic soils was 831 million hectares (Martinez-Beltran and Manzur 2005), extending over all continents including Africa, Asia, Australasia, and the Americas. The exact location and distribution of salt-affected soils have been studied in varying degrees of detail. Different systems of classification and grouping are employed in individual countries. In addition, the maps have not been prepared on a uniform scale. It should be noted that the threshold electrical conductivity (EC) and exchangeable sodium percentage (ESP) values are not the same in different classification systems, particularly in the Australian classification. For example, while the US system defines sodic soils as those having natric horizons with an ESP greater than 15 (Soil Survey Staff 1990), Northcote and Skene (1972), in mapping the Australian soils with saline and sodic properties, defined sodic soils as those having an ESP between 6 and 14 and strongly sodic soils as those having an ESP of 15 or more. The recent Australian soil classification (Isbell, 2002) defines Sodosol (sodic soils) as soils with an ESP greater than 6 and at the same time, soils with ESP are excluded from Sodosol, because of their very different land-use properties. Sumner et al. (1998) have reviewed the effects of different soil components on the effects of ESP on soil behaviour and advocated the development of a classification system based on soil behaviour rather than on arbitrary threshold ESP criteria. Here the term salinity is used to describe salt-affected soils, which include sodic soils (as defined by Isbell 2002); a distinction is not drawn between saline and sodic soils as in Ghassemi et al. (1995). these soils are found in various climatic zones. For example, 4.5 million hectares of dryland cropping are affected by salinity in the Canadian prairies (Wiebe et al. 2005). Similarly, all soil types with diverse morphological, physical, chemical, and biological properties may be affected by salt accumulation. Although NaCl is the dominant salt in many saline soils, the presence of soluble compounds of calcium, magnesium, potassium, iron, boron, sulfate, carbonate, and bicarbonate have been reported (Szabolcs 1989). The distribution of saline and sodic soils on several continents (Table 2.3.1) illustrates the predominance of sodic soils in Australia. While in other global regions the ratio of sodic/saline soils is in the range of 0.02 to 1:55, the Australian ratio is 5:17. This is consistent, however, with the high Na/(Na+Ca) ratio of soil solutions and groundwaters in the Australian environment. A large proportion (86.2%) of sodic soils in Australia have dense subsoils with an alkaline ph (8-9-5) trend. Their subsoil clay is highly dispersible due to adsorbed sodium. Of the remaining sodic soils, 6.8% have a neutral ph trend while 7.0% have an acidic ph trend in their profiles (Northcote and Skene 1972). TABLE Area extent of salt-affected soils in different regions of the world. Region Land area (millions of hectares) Sodic/saline Saline Sodic Total ratio North America Central America South America Africa South Asia North and Central Asia South-East Asia Europe Australia SOURCE: RENGASAMY, 2006 All soils contain some soluble salts, but when soil and environmental conditions allow the concentration in soil layers to rise above a level that impacts on agricultural production, then soil salinity becomes an issue of land degradation. Even though the general assumption is that saline soils occur under arid and semi-arid climates, 9

10 3. Different types of salinity in the dryland regions of Australia 3. DIFFERENT TYPES OF SALINITY IN THE DRYLAND REGIONS OF AUSTRALIA Salinity in the Australian landscape has developed under different environmental conditions over many geological periods. Recent agricultural activities in Australia have led to many soil processes resulting in various types of salinity in dryland regions. Local communities and Government agencies in Australia are concerned about the impact of salinity on land values and water resources. The major salinity focus in Australia is on irrigation-induced salinity in the Murray Darling Basin and dryland salinity associated with shallow groundwater, particularly in Western Australia. The following brief account of dryland salinity in Australia is also reflected in many landscapes around the world (Rengasamy 2006). 3.1 Groundwater-associated salinity (GAS) in Australia This is also commonly known as seepage salinity and is often, incorrectly, thought of as being the only form of dryland salinity. Dryland salinity is the visual scalding of soil surfaces associated with a rising saline water table. At the foot of slopes and in valley floors, the water table is shallower and closer to the surface than in higher regions of the landscape. In some instances, groundwater is forced to the surface in upper catchments due to barriers to its flow or thin regolith, before deep valley sediments have filled with water. Under native vegetation, the leaching of salts from the permeable soil due to natural processes led to salt storage in deep regolith or the accumulation of salts in the shallow groundwater. Groundwater salinity was often very high, ranging from EC (electrical conductivity) ds m -1. Where the water table was 4 m below the surface, saline groundwater did not affect native vegetation and some species were able to cope with the shallower water tables. With the clearance of perennial native vegetation and the introduction of agriculture, the equilibrium levels of the water table have changed (Hatton et al., 2003). In low-lying regions with shallow water tables, water, with salt, leaked to the groundwater from the upper horizons. Groundwater levels have risen as a result. With the introduction of pastures and annual crops there has been a lower evapotranspiration of water captured from rainfall under the natural ecosystem, where deep percolation of still more water occurred down the profile. As saline groundwater approached the surface, soil layers (top 1 m) were salinized and waterlogged. Generally, water tables around 2 m depth in the valley floor can cause salinity in the surface soils. Salts reach the surface in the discharge zones (areas of the landscape where water exits from groundwater to the soil surface) by capillary rise of saline water. On valley sides of the landscape, saline groundwater can seep to the soil surface. The National Land and Water Resources Audit (2001) estimated that the potential is high for approximately 5.7x 104 km 2 of Australia s agricultural and pastoral zone to develop salinity through shallow water tables. The report also warns that unless effective solutions are implemented, the area could increase to 17x 104 km 2 by 2050 (for comparison, the area of the UK is about 24x 104 km 2 ). This form of salinity affects around 350x 104 km 2 of land area in the world (Szabolcs, 1989). Perhaps the large areas once thought to be at risk from rising water tables have to FIGURE Schematic diagram of salt bulge in deep soils under native vegetation and cropping. Depth (m) >40 Water table Under native vegetation SALT BULGE Under cropping (after clearing native vegetation) Sodium chloride (mg L 1 ) in soil solution SOURCE: RENGASAMY,

11 3. Different types of salinity in the dryland regions of Australia FIGURE Groundwater-associated salinity (seepage salinity or dryland salinity) in South Australian landscape. be reconsidered as a result of climate change-induced changes in groundwater levels. 3.2 Transient salinity (Nongroundwater-associated) in Australia Occurrence of salt bulges in deep subsoils in Australia Over many thousands of years, salt delivered by wind and rain and the weathering of rock minerals during soil formation accumulated in the soil solum. Salt accumulation may also be associated with parna, wind-blown dust emanating from the west and southwest of the Australian continent. Before agriculture was introduced in Australia, salts were leached down the profile by percolating rain, and accumulated below the root zones of vegetation. Under semi-arid conditions, the rainfall was not sufficient to leach the salts to the deep groundwater. The clay layers in deep subsoils hindered the movement of water and salt. As a result, a bulge of salt accumulated in the soil layers below 4 10 m depths from the surface (Figure 3.2.1). The water table was generally below 30 m depth from the surface, the quality of groundwater being low saline (EC <3 ds/m). One of the earliest observations by Holmes (1960) identified salt bulges at depths below 4 m from the surface of a site in a semi-arid part of South Australia containing a virgin, mallee heath community. Investigating a large area (104 km 2 ) of fresh groundwater that occurs in an unconfined aquifer in the south-west part of the Murray Basin, Leaney and Herczeg (1999) reported salt concentrations ranging from 4000 to mg/l in the unsaturated zones (5 30 m), while fresh groundwater was below m. Figure shows the salt bulge occurring in the unsaturated zones above the water table, comparing a system under native vegetation with a system under cultivated land. Shapes of salt profiles are highly variable (Batini et al. 1976), but bulges as shown FIGURE Soil processes and accumulation of salt in the root zone layers (transient salinity) without the influence of groundwater. 0m A horizon 0.2m Sodic B horizon 1m 4m >15m Salt stores Seasonally saturated zone Reduced salt leaching Crop establishment Salt stores Salt bulge Soil weathering and transport Groundwater Flowering to crop yield Atmospheric and agronomic input Soil mixing Plant uptake capillary rise Accumulation of Na +, Cl, HCO 3, CO 3 2, BO 3 3, Fe, SO 4 2 Seasonally saturated zone Low permeable layer Capillary rise Evapotranspiration 11

12 3. Different types of salinity in the dryland regions of Australia FIGURE Different types of dryland salinity found in the Australian landscape. Soil 0.5m 0m surface 1.0m 4m 15m >40m Sodicity/alkalinity/acidity Root zone transient salinity (EC e 4-16dS/m) (not associated with groundwater) Salt Bulge (EC e 20-60dS/m) Salinity (EC <3dS/m) Agricultural productivity Groundwater No agricultural productivity Watertable-induced secondary salinity (seepage salinity) EC e 20-80dS/m) Valley seep Salinity (EC dS/m) Discharge zone 0m in Figure 3.1. have been identified in many parts of the country including Western Australia (Peck et al. 1981). A recent pilot study by Lawrie et al. (2000) using airborne electromagnetics has found the presence of salt bulges in deeper soils in central-west New South Wales. The total salinity and the composition of many saline groundwater samples in Australia are similar to seawater. Studies on the stable isotopic composition of saline groundwater (Herczeg et al. 2001) indicate that the source of salinity in the Australian continent is mainly through rainfall. The groundwater chemistry is a combination of atmospheric input of marine- and continentally-derived salts and removal of water by 2m evapotranspiration over tens of thousands of years of relative aridity (Herczeg et al. 2001). During salt flow through soil layers, chemical reactions such as cation and anion exchange, complex formation, precipitation and dissolution involving different ionic species have resulted in the composition of groundwater being similar to sea water Soil processes leading to salt accumulation (transient salinity) Due to sodium salt movement through soil layers, over 60% of the soils in agricultural zones in Australia have become sodic. Water infiltration is very slow where subsoils are sodic and water does not move down below that layer. This causes temporary waterlogging in the subsoil and a saturated zone which is commonly called a perched water table. Salts, derived from rainfall and soil weathering reactions, accumulate in the saturated zones in the soil profile. After the wet season, when the water evaporates quickly, salt accumulation in the sodic subsoil layers is exacerbated. The amount of salt accumulating is not huge, but can be detrimental to crops. This transient salinity fluctuates with depth and also changes with season and rainfall. The term transient salinity is used to denote the temporal and spatial variation of salt accumulation in the root zone not influenced by groundwater processes and a rising saline water table. Transient root zone salinity is caused by 2 major factors: water and solute flux and hydraulic conductivity of the root zone layers. Schematic explanation of the soil processes leading to transient FIGURE Crop production affected by transient salinity in subsoil in a paddock in South Australia (Left). Soil profile showing salt accumulation in subsoils (Right). 12

13 3. Different types of salinity in the dryland regions of Australia FIGURE Relationship between subsoil exchangeable sodium percentage ESP and root zone salinity as influenced by annual rainfall at 30 sites in south-eastern Australia. Average root zone (0-60cm) salinity ( EC 1:5, ds/m) Deep subsoil (50-90cm) ESP Annual rainfall mm Annual rainfall mm Annual rainfall mm SOURCE: RENGASAMY, 2006 salinity in root zone layers of sodic soils is shown in Figure In Australia, a country whose agricultural area is about 7.6x 10 6 km 2, sodic soils have the potential for transient salinity and other root-zone constraints such as alkalinity, acidity, and toxicity due to boron, carbonate, and aluminium occupy 2.5x 10 6 km 2. Sixteen percent of Australia s total cropping area is likely to be affected by water table-induced salinity, and of that area 67% is subject to transient salinity and other root-zone constraints, costing the farming economy around one billion dollar annually in lost production(rengasamy 2002a). The different forms of dryland salinity found in the Australian landscape are illustrated in Figure The distribution of the different types of salinity in Australia is illustrated in Table The problem of transient salinity is not, however, confined to Australia. About 5.8x 10 6 km 2 of soils around the world are sodic and have the potential for transient salinity. TABLE Distribution of different types of salinity in Australia. Types of salinity Water table-induced seepage salinity Transient salinity (nonwater table-associated) Approximate area (millions of hectares) Percentage of total land area Irrigation associated Transient salinity in dryland regions is extensive in many landscapes where there is subsoil sodicity. Shaw et al. (1998) have established a good logarithmic relationship between rainfall, subsoil ESP and EC e for north-eastern Australian soils. By analysing 660 soils in north-eastern Australia within an annual rainfall range of mm, Shaw et al. (1998) found about 78% of soils with clay content contained between 35 and 55% accumulated salt above EC e of 7.7 ds m -1 in layers between 0 and 0.9 m from the surface. Analysis of 151 profiles from reference soils of south-western Australia identified 91 profiles having salinity above an EC e of 4 ds m -1 (McArthur 1991). FIGURE Simulations of chloride accumulation using LEACHM model in a red-brown earth under restricted (a) and free drainage (b) at 1m depth. Depth (mm) 0 Depth (mm) Restricted drainage Chloride (mmol L 1 ) Series 1: 0 days Series 3: 180 days Series 5: 360 days Free drainage Chloride (mmol L 1 ) SOURCE: RENGASAMY, 2002a 13

14 3. Different types of salinity in the dryland regions of Australia This study also showed the high incidence of both salinity and sodicity in Western Australian topsoils and subsoils. Sixty-eight soils had an ESP >15 and an EC e >30.0 ds m -1. While a good correlation exists between ESP of subsoils and the root zone salinity, this correlation is highly influenced by the average rainfall (Figure 3.2.5). Soil survey reports indicate that about 4 million ha of dryland soils in South Australia, Victoria and Western Australia have subsoil salinity ( m) above an EC e of 4.0 ds m -1 (Rengasamy 2002a). As early as the 1930s, soil surveys in the Salmon Gums district of Western Australia identified salt accumulation in surface and subsoils (0 60 cm) in more than 50% of the 0.25 million ha surveyed (Burvill 1988). These surveys also found that virgin areas had more salts in the upper metre of the major soil types than in cleared areas. Hutson and Wagenet (1992) have described some of the major processes typically included in transient salinity models. The LEACHM model simulates drainage fluxes and salt transportation by incorporating soil chemical reactions, soil hydrological properties and water transport functions with coarse approximations of evapotranspiration and plant root development. Figure shows the simulation results of chloride accumulation, using LEACHM, in a red-brown earth soil profile in South Australia where there was restricted or free drainage below 1 m from the surface in a cropping year. The area has an annual rainfall of 400 mm and the annual evapotranspiration is 1400 mm. Salt input (0.5 mmol/l) is assumed from rainfall only. Simulations have been carried out for 1 year only (April 2000 to FIGURE Map showing areas of dryland seepage salinity, regions with potential transient salinity and subsoil constraints and area covering grain production in Australia km Approximate area covering grain production during (source: ABARE) Dryland areas that have, or are likely to have, between about 1% and 10% of land affected by seepage salting Areas where there is potential for transient salinity and subsoil constraints such as sodicity, alkalinity and toxicity due to aluminium, boron, carbonate and bicarbonate SOURCE: RENGASAMY, 2002a March 2001). The modelling results suggest that salt accumulation in the top layers is significant in 1 year under restricted drainage. In dryland regions with annual rainfall between 250 and 600 mm, sodic subsoils have an EC e between 2 and 16 ds/m which can dramatically affect crop production through osmotic effects during dry periods. Laboratory measured EC e increased several times under field conditions as the soil layers dried in between rainy days. The combination of poor water storage and osmotic stress increased the water stress of crops under dryland cropping Potential transient salinity and subsoil constraints in cropping regions Farming land, in regions where annual rainfall is less than the evapotranspiration and where there is no supplementary water input by irrigation, is identified as dryland. Australia is commonly referred to as the driest inhabited continent with an average annual rainfall of 420 mm, of which 87% is lost to evaporation and transpiration. However, successful crop production has been found to be sustainable in several regions across Australia with annual rainfall of between 250 and 600 mm and where soil management and agronomic practices have been developed to suit the rainfall pattern. Efficient capturing of rainfall, storage in soil layers, and subsequent water use by crops is critical for dryland farming. Sodic soils in Australia are defined as those having an exchangeable sodium percentage (ESP) 6 (Isbell 2002). These soils are widespread in arid and semi-arid regions of the world, representing up to 580 million ha of land. In Australia, more than 250 million ha are affected by sodicity and, on a world scale, it is by far the most extensive distribution of sodic soils (Rengasamy and Olsson 1991). Farming practices on these soils are mainly performed under dryland conditions. Sodicity is a latent problem in many salt-affected soils where deleterious effects on soil properties are evident only when salts are leached below a threshold level (Rengasamy and Olsson 1993). While soil salinity reduces plant growth and directly affects the physiological functions of the plant (through osmotic and toxicity effects), sodicity also causes deterioration of soil s physical properties which indirectly impacts on plant growth and survival. IIn Australia, about 23.7 million hectares was planted for the production of 42.3 million tonnes of both winter and summer grain crops in (ABARES Agricultural Production Statistics Report 2015). However, the yield per hectare in Australia still remains below other western countries. For example, average yield of wheat in Australia is about 1.71 t/ ha while the world average is 2.68 t/ha. Limitations to agricultural productivity imposed by the constraints in Australian sodic soils are severe and require remediation for improved dryland farming. The map showing areas of dryland seepage salinity, regions with potential transient salinity and area covering grain production in Australia is presented in Figure

15 3. Different types of salinity in the dryland regions of Australia Climate change and salinity in dryland regions In many dryland regions in the world, the extent and nature of salinity is influenced by a range of soil processes and climatic conditions. The major factors are: the amount and frequency of rainfall, evapotranspiration (caused by climatic factors), water use by vegetation, and soil hydraulic conductivity. Global warming and climate change impact on aridity (low rainfall and high temperature) and are conducive to salt accumulation leading to transient salinity. Lack of organic matter due to a dry climate can lead to the deterioration of soil structure, which reduces the ability of salts to be leached from the soil profile. If the change of climate resulted in high rainfall and low temperatures then salt accumulation will be prevented and transient salinity reduced. However, in locations where salinity is caused by altered groundwater levels, and where climate change has resulted in higher rainfall and lower temperatures, the groundwater perturbations will be greater leading to increased soil salinity. Where there has been a change to a drier climate, salinity levels in soils in regions previously influenced by groundwater fluctuations can be reduced. 15

16 4. Irrigation salinity in Australia 4. IRRIGATION SALINITY IN AUSTRALIA 4.1 Irrigated soils Irrigation in Australia, over a wide range of climatic zones including the Mediterranean, tropical and subtropical climates has increased and uses 10.2 million ML of water annually on 1.84 million ha (Table 4.1.1). Most of the areas developed for irrigation are within the Murray-Darling Basin which comprises one-seventh of the nation s surface area and produces about onethird of Australia s rural output (Simmons et al. 1991). The importance of irrigated agriculture in the overall agricultural sector can be seen from the current annual value of about $5 billion from irrigation. The need for drainage schemes to avert shallow water tables and salinity under irrigation has long been recognized. Irrigation in arid and semi-arid regions mobilizes the soil minerals and increases the salt load in the drainage water and groundwater or in those relatively impermeable soil layers. Moreover, instances of toxic concentrations of several trace elements and potentially dangerous pesticide levels have been found in drainage waters in the U.S.A. (Schilfgaarde 1990). The management of irrigation drainage waters has become both complex and difficult. TABLE Irrigated area, irrigation water use and water resources in Australia. State/Territory New South Wales Area irrigated (10 3 ha) Irrigation water (10 3 ML) Water resources (10 3 ML) Surface Ground Fresh Saline , Victoria , Queensland , South Australia Tasmania , Western Australia Northern Territory , , SOURCE: RENGASAMY AND OLSSON, 1993 Of the average annual rainfall of 420 mm in Australia, 87% is lost through evaporation from soils, plants and water surfaces while 12.8% runs off into streams. Less than 0.2% is added as annual recharge of aquifers (Australian Water Resources Council 1987). This explains the severe salinity problem in this continent. For comparison, evapotranspiration accounts for 60% of the total rainfall in Europe and North America. This also emphasises the need for irrigation to meet the water deficit experienced in agricultural and horticultural soils. The productivity of irrigated agriculture in Australia is low for many crops with the exception of rice, when compared with the yields obtained in comparable climates such as California. In many cases, the district average crop yields are far below the potential yields determined by climate or estimated from experiments (Rengasamy and Olsson 1993). Decreased productivity of the soils under irrigation in Australia is mainly caused by the physical and chemical constraints found in most of the soils under irrigation. Salinity in Australia is dominated by sodium chloride salt. The sodicity of the B horizon of many of these soils is responsible for slow water and nutrient transportation rates, anoxic and suboxic conditions and high soil strength, restricting root growth (Rengasamy and Olsson 1991). Saline-sodic soils become sodic when leached by irrigation or rain. Paradoxically, irrigation of sodic soils also leads to the accumulation of sodium salts, forming saline-sodic soils.irrigation management in Australia is closely linked with the management of soil sodicity. Irrigation commenced in Australia in the 1880s and has expanded from an area of ha in to 1.84 million ha in 1991 (Table 4.1.1). The major irrigation developments have occurred on the Murray River and its tributaries, with three states, Victoria, New South Wales and South Australia, sharing the same fresh water resources. In Queensland, 46% of total irrigation uses groundwater (Table 4.1.1). When the subsoils are gleyed (soils developed under poor drainage), periodic surface waterlogging occurs. The cracking clays of Vertosols and soils affected by sodicity and high ph swell on wetting, often preventing deep penetration of irrigation water. Therefore, in spite of their high water storage capacity, water availability to crops is low. Soil surveys in the irrigation areas of Australia indicate that most of the soils have inherently natric (sodic) horizons within 1 m of the surface, even before the influence of irrigation. 16

17 4. Irrigation salinity in Australia 4.2 Sodicity in irrigated soils The quality of irrigation water is determined mainly by the composition of cations and anions, ph and EC and affects crops and soils (Figure 4.2.1). When this water percolates through the soil profile, the soil solution composition is significantly altered through cation and anion exchange, ion complexation, formation of different ionic species and precipitation of compounds dictated by solubility criteria. Total electrolyte concentration (measured as EC) and its nature in soil solutions affect crop production through osmotic and ionic effects. The nature and concentration of cations and anions in soil solution affects soil structural stability and associated soil physical phenomena. Although both monovalent Na and K have been found to adversely affect soils, the current focus by land managers is only on sodium. The following sections concentrate on the role of sodium in irrigated agriculture. The recent findings on both K and Na effects are discussed in a later chapter Sodium adsorption in soils As salinity in soil environments in Australia is dominated by NaCl, any process that accumulates NaCl salt in the soil profile inevitably leads to ssoil sodification unless soluble calcium or magnesium minerals are present in the profile. Irrigation waters dominated by sodium salts accumulate in the soil layers if they are not either leached to depth or exported in the drainage water. The degree of sodification of soil layers depends on the proportion of sodium to divalent ions in the soil solution; this is generally measured as the sodium adsorption ratio (SAR). The SAR model was originally developed FIGURE Quality of irrigation water affects the soil solution composition and subsequently affects soil physical properties and crop productivity. Surface 1m >2m Root zone Soil physical properties (affected by the concentrations of Ca, Mg, K and Na, & EC and ph of soil solution) Rising watertable Irrigation water Quality (ph, EC, cation ratio, toxic ions, nutrients) Soil solution (soluble cations and anions including nutrient and toxic ions, ph, EC) Leaching Quality of leachate (will impact on deep soil) Groundwater (level and quality) Crop productivity (affected by the osmotic pressure, toxic ion concentration & ion imbalance in soil solution) Drainage Disposal Quality of drainage water (will impact on environment) on the basis of ratio law of Schofield (1947) to predict the adsorption of Na on soil exchange sites in relation to cation concentrations in soil solutions. The SAR is defined as follows: SAR (mol 0.5 m -1.5 ) = Na/ [(Ca + Mg) / 2] 0.5 (4.1) where the concentrations of these ions are expressed in mmol c L -1. The adsorption of cations by clays or soils from a solution containing cations of multiple valencies is very complex, and it is difficult to model the exchange isotherms. However, several correlation studies have shown a relationship between SAR and ESP (exchangeable sodium percentage) and are consistent with ion exchange models such as Gapon equation (Sposito 1989). Many research reports indicate that SAR e measured in saturation paste extracts is approximately equal to ESP of soils. Similarly, Rengasamy et al. (1984) derived the following approximate relationship between ESP and SAR 1:5 measured in 1:5 soil-water extracts: ESP = 2 SAR 1:5 (4.2) However, the co-efficient 2 can vary between 2 and 4, depending on the electrolyte concentration of the solution. Our studies (e.g. Marchuk and Rengasamy 2012) have shown that the exchange phenomena in heterogeneous soil clays (where they have come into contact with solutions containing cations of different valences) are influenced by both electrolyte concentrations and the net charge on soil particles. Estimation of ESP is laborious and costly compared to SAR and therefore, it is preferable to use only SAR in relating to soil properties in analysing sodicity effects Natural processes of sodium input to soil profiles The amount of Na + input through rain in Australia may vary between 0.6 and 130 kg ha -1 year -1. The amount of NaCl deposited on any area depends on the proximity to the coast, the nature of the coastline, local sources of salt, the direction and intensity of prevailing winds, and the distribution of the rain throughout the year. More salt is deposited from persistent light showers than from prolonged rains of tropical origin. The ionic ratios of Ca and Na in rainwater vary at different sites in Australia showing the influence of local environment which may enrich Ca more than Na. Rainfall originating from the ocean or land containing salt can account for some 10 kg ha -1 year -1 even in inland areas, about 300 km from the coast (Gunn and Richardson 1979). The saline-sodic soils that developed during the arid climate phase would be leached with the onset of pluvial conditions and transformed to sodic soils if calcium salts were low. The predominance of exchangeable Mg over 17

18 4. Irrigation salinity in Australia Ca in the subsoils (B horizons) of the irrigated Alfisols and its close association with the exchangeable Na + are considered to be the result of leaching of accumulated cyclic salts with the ionic composition similar to oceanic waters (Isbell et al. 1983). However, aeolian movement of salt is also significant in inland southern Australia, where over the last 20,000 years, bare salty surfaces have been exposed during hot windy weather (Blackburn 1974). The accumulation of salt in soil layers overthe past geological periods explains the presence of sodic horizons in many soil types across Australia Addition of sodium through irrigation water Fresh water (<500 mg L -1 solids, Australian Water Resources Council 1987) from surface water resources is used for 87.4% of total irrigation while 12.6% of irrigation uses groundwater resources (Table 4.1). Even the channel water used for irrigation in most regions contributes to the salt concentration in the soil during evapotranspiration. For example, irrigation with fresh water containing 100 mg L -1 of NaCl salt (EC = 0.17 ds m -1 ) adds 393 kg of Na + per ha for each annual application of 1 m water. Over 66% of the groundwater resources in Australia are saline (>500 mg L -1 solids). Groundwater is often used for irrigation in the cottongrowing areas of the Namoi and Condamine valleys, the sugarcane areas of the Burdekin and Bundaberg regions and the vine and vegetable-growing areas in South Australia. Long-term studies in Queensland have shown that soils have become more saline and sodic following irrigation with saline groundwaters (Bevin and Shaw 1980). Groundwater pumping is also used in the irrigation areas of the Murray-Darling Basin to lower water tables. Due to the higher Na + than Ca 2+ in these waters (SAR values up to 30) and irrespective of ionic strength (or EC values), the use of these waters for irrigation has led to the rapid salinisation and sodification of soil profiles (Rengasamy and Mehanni 1988) with the level dependent upon the salinity and SAR of applied water. The effluents from several industries and recycled waters, now used for irrigation, contain significant amounts of Na in addition to other cations such as K Sodium from the dissolution of solids and minerals The mean Na level in the lithosphere is 2.8%, with the principal Na-bearing minerals being the plagioclases and sodium feldspars, Nepheline, jadeite, analcime, soda-glass, high and low albites, paragonite and beidellite contribute to Na in soils through weathering and dissolution. Practically all of the secondary sodium minerals are highly soluble in water. Gunn and Richardson (1979) found that in the samples from both shallow and deep drills in the rocks in eastern Australia, Na + and Cl - were generally the dominant ions, although Ca 2+, Mg 2+ and SO 4 2- were often prominent. Their data showed that Na + was appreciably higher in the unweathered freshwater rocks than in those of marine origin. Weathered sedimentary rocks of marine origin, however, contained more Na + than those of freshwater origin. These data suggest that mineral weathering can provide Na salts in long-term soil formation and can accumulate in less permeable soil layers. Increasing salt accumulation also tends to enhance the solubility of weatherable minerals Sodium contributed by chemical applications Applications of chemicals such as NaCl, NaNO3, Na 2 SO 4, and other Na containing organic chemicals, also contribute to the Na + inputs. However, concentrations of Na salts in agricultural soils are generally limited and in many cases are negligible. The ameliorants used to rectify soil physical problems, such as lime and gypsum, reduce the SAR of soil solutions. These materials, especially mined gypsum in Australia, may be contaminated with Na salts which reduce their efficiency in reclaiming sodicity. Gypsum application to surface layers can contribute to the Na + input to the deeper layers when the exchanged Na + is leached downwards. In our experiments (unpublished), when gypsum was applied to an irrigated Natrixeralf, Na 2 SO 4 accumulated at depths below 0.4 m from the surface. The amount of Na accumulated and its depth were determined by the frequency of irrigation and the quantity of gypsum applied Sodium input from saline groundwaters Interactions between groundwater and soil layers near the surface in the Murray Groundwater Basin have been studied in detail by Macumber (1991). In northern Victoria, the regional groundwater flow passes northward, from recharge areas in the highlands, towards regional discharge zones situated on the lower Loddon Plain and in the Mallee. Whatever the origin of the salt, the groundwaters in this region are dominated by Na + while the proportion of cations and anions are similar to sea water. On the Riverine Plains, regional groundwater discharge occurs in the Loddon Valley, at the junction of the river basin with the marine basin. The rising saline water tables in this junction contribute to the high input of Na + by the capillary action in the soil profiles resulting in the formation of saline-sodic soils on the Tragowel plain and in the Kerang irrigation district. Over 90 years of continued fresh water irrigation in this region did not leach salt down the profile as the inputs of the sodic groundwaters were discharged from deeper aquifers. Even though it is argued that irrigation has accentuated the rise in regional water tables, these groundwater fluctuations could equally have been the result of longterm climatic changes as well as short-term seasonal variations in rainfall (Rengasamy and Olsson 1993). 18

19 4. Irrigation salinity in Australia Groundwater discharge in the Mallee region commonly occurs in the large discharge complexes (the Boinka) or in the many small salt lakes scattered throughout the Mallee. The locations of these discharge complexes are influenced by the geomorphology. In the lower topographic sections, groundwaters are highly saline ( mg L -1 ) and acidic (ph ) and have a significant influence in altering hydrochemical and biochemical processes in the discharge zones. Acidity in the unconfined Parilla Sand aquifer is generally caused by redox processes involving Fe and S. While the problem of discharge of regional groundwaters influencing soil sodicity in many regions of the Murray- Darling Basin has been studied in detail, the information on this problem in other irrigation areas was not available to the author Sodium removal from the soil solution Leaching and drainage removes some Na + from the soil solution in a given soil layer. In soils without any provision for artificial drainage, the leaching fraction influences the rate of Na + removal and the SAR of the soil solution. Water run-off from irrigated land increases the Na + levels in river systems. However, the mobilization of salt to water courses is greater from dryland salting than by drainage from irrigation. Uptake of Na + by plants contributes to minor export from the soil solution. Pasture and cereal crops can remove kg of Na + ha -1 year -1 from moderately saline soils while halophytes such as saltbush can remove kg of Na + ha - 1 year - 1 from highly saline soils. A saline-sodic soil with an EC 1:5 of 1 ds m -1 contains 1725 x 103 kg of Na + ha -1 in the rooting depth of 1 m. Compared with this level of Na + in the soil, removal of Na + by crops is negligible Sodium balance in soil profiles A sodium balance equation to determine the soil sodicity at a given soil depth can be calculated by summing the various inputs and outputs of ionic species to the soil solution and the proportion of sodium to divalent ions, as follows: SCM i+r + SCM s+m + SCM a+f + SCM gw SCM dw SCM p+c SCM cu = SCM ss (4.3) The S, C and M denote the concentrations of sodium, calcium and magnesium ions respectively and the subscripts i, r, s, m, a, f, gw, dw, p, c, cu and ss denote the sources of these ions, namely irrigation water, rainwater, solids, minerals, amendments, fertilisers, groundwater, drainage water, precipitation, complexation, crop uptake and soil solution respectively Precipitation and complex formation The formation of inner-sphere and outer-sphere complexes (see Sposito 2008) of various molecular and ionic species in a soil solution will reduce the activities of free ionic species. In sodic soils, Na +, NaHC0 3, Na 2 SO 4, Mg 2+, MgS0 4, MgHCO 3+, Ca 2+, CaS0 4, CaHCO 3 + are the principal species, which will alter the SAR of the soil solution from that of the input water. In sodic soils with high ph, alkalinity of the soil solution, defined by Alkalinity = [HCO 3- ] + 2[CO 3 2- ] + [OH - ] [H - ], (4.4) controls complex formation and precipitation of the ionic species. If the alkalinity is mainly due to HC0 3-, the ph value is given by ph = log(hco 3- ) log P CO2 (4.5) where PCO 2 is the partial pressure of carbon dioxide in a given soil layer. In the root zone, microbial activity and oxygen diffusion alter P CO2 leading to the precipitation or dissolution of calcite, dolomite and magnesite. Hence, the equilibrium concentrations of Ca 2+ and Mg 2+ in the soil solution will be completely different from those of the irrigation water. Computer methods (see Jurinak 1990) are readily available to calculate the free ionic activities in soil solutions. At present, various procedures are used to determine SAR. The free ionic activities of Na +, Ca 2 + and Mg 2+ are conveniently used to calculate SAR which is related to the thermodynamic ion exchange resulting in sodium adsorption by soil colloids. Chemical analyses of soil solutions give total concentrations of Na +, Ca 2+ and Mg 2 + which are used to calculate a practical SAR (SARp). When only the equilibrium concentrations of Ca 2+ and HCO 3 - in the root zone are taken into account to evaluate the precipitation and dissolution of Ca minerals, the SAR derived is known as the adjusted SAR (SAR adj ). Jurinak (1990) gave the following equation to calculate SARadj in the root zone soil solution, or drainage water, from the irrigation water composition: SAR adj = Na iw x F c /(Mg iw F c + Ca eq ) 0.5 (4.6) where Na iw and Mg iw are the concentrations of Na and Mg (mmol L -1, respectively, in the irrigation water and F c is a concentration factor (= LF-1 where LF is the leaching fraction). The value of Ca eq (equilibrium concentration of Ca in the root zone) can be calculated from the molar HCO 3 /Ca ratio and the ionic strength of irrigation water by using the method given by Suarez (1982). Our analytical data on the water extracts of several Natrixeralfs in southern Australia show that the SAR calculated using computed free ion activities is larger than the SAR p by a factor of in soils with neutral ph ( ) and in subsoils with alkaline ph (>8.0). Thus, SAR p may underestimate sodicity in soils with alkaline ph. 19

20 4. Irrigation salinity in Australia 4.3 Threshold electrolyte concentration and saline irrigation Quirk and Schofield (1955) demonstrated that soil permeability can be maintained at a stable state, even for a sodic soil, provided the electrolyte concentration of the soil solution is more than a critical value, known as threshold electrolyte concentration (TEC). This concept is based on the electro-osmotic effect of saline solutions in combating the repulsive forces caused by the hydration of adsorbed sodium ions. For a large group of redbrown earths (Alfisols), Rengasamy et al. (1984) defined an empirical linear function relating SAR to the EC required for flocculation. These authors also defined two separate functions for (i) spontaneous dispersion caused by sodicity and (ii) mechanical dispersion in saline-sodic soils caused by externally applied forces (Figure 4.3.1). Many guidelines based on the TEC concept to use saline water for irrigation have been suggested so as to avoid the adverse effects on water transport in sodic soils (e.g. Quirk 1971; Jayawardane 1979; Rhoades 1982; Rengasamy et al. 1984). In the schematic diagram in Figure 4.3.2, which describes the relationship between sodicity (ESP) and salinity (EC), the diagonal line represents the TEC and distinguishes between flocculated (structurally stable) and dispersed (structurally unstable) soils, the former being saline-sodic and the latter sodic. It should be noted that although the salt concentration is useful in maintaining soil structural integrity, it is harmful to plants when it exceeds a certain critical level related to salt tolerance of plants. FIGURE Threshold electrolyte concentrations distinguishing spontaneous dispersion (dispersive soils) or mechanical dispersion (potentially dispersive soils) from flocculation (structurally stable flocculated soils) on the basis of SAR (ESP) and EC (TCC) relationships in red-brown earths. ESP (%) SAR Dispersive soils Increasing salinity Limiting productivity Potentially dispersive soils Flocculated soils Non-sodic soils TCC (1:5) EC (1:5) Both SAR and EC (ds m -1 ) were measured in 1:5 soil:water extracts. SOURCE: RENGASAMY et al, 1984 Australian and New Zealand guidelines (ANZECC & ARMCANZ 2000) also follow the threshold electrolyte concentration developed for a few Queensland soils and many soil consultants use these guidelines for judging the water quality used for irrigation. Threshold electrolyte concentration is not, however, a unique function of SAR and EC, but varies with soil type and other factors. Rengasamy and Olsson (1991) concluded that the net negative charge on soil particles alter these functions. Although the negative charge is determined by the nature and content of clay minerals in a soil, the recent report by Marchuk et al. (2013) shows that dispersionflocculation phenomena are highly related to the zeta potential of the dispersed clay confirming the hypothesis that clay dispersion due to adsorbed cations depends on the net charge available for clay-water interactions. The distinctive way in which clay minerals and organic matter are associated and the changes in soil chemistry, specifically ph, affecting the net charge cause the TEC to be unique for each soil. In many soils, particularly Alfisols, soil layers are heterogeneous in both sodicity and leaching fraction, leading to dynamic changes in equilibrium SAR and EC. The irrigation season is usually followed by a rainy season in southern Australia when the salts are partially leached from soil layers and the balance between SAR and EC is altered. When saline-sodic topsoils are leached by rainwater, clay dispersion in the surface layers leads to waterlogging and soil erosion. Myers et al. (1990) showed that when saline water with electrolytes greater than TEC was used in raised beds wetted by furrow irrigation, Na salts accumulated in the shoulders of the beds. With subsequent leaching by winter rainfall, SAR remained high while EC was lowered. This resulted in soil sealing and the inhibition of crop establishment in the following summer. Therefore, future research is needed to model the dynamic changes in SAR and EC in root zone soil layers as influenced by climate and soil management so that TEC-based guidelines can be practical Accelerated sodification under saline irrigation Even though saline irrigation is not widely practised world-wide, demand for sharing scarce water resources have necessitated the use of saline groundwater and drainage effluents for irrigation in many arid countries. In Australia, only 8.5% of the fresh water resource is currently used for irrigation (Table 4.1.1). In Queensland, saline groundwater is used only in limited areas where fresh water supplies are not available. In the Murray- Darling Basin, irrigation uses only about 25% of the available fresh water resource. Hence, there is no necessity to consider the saline water as a resource for irrigation. As discussed, saline irrigation in the Australian environment risks the accelerated sodification of soil layers unless soluble Ca and Mg minerals are present in the soil profiles to minimize SAR of soil solutions. 20

21 4. Irrigation salinity in Australia FIGURE Schematic diagram relating sodicity (ESP) and salinity (EC), with the diagonal line representing TEC, distinguishes between saline-sodic (flocculated) and sodic (dispersed) soils. When EC levels are above the dashed line, salinity affects plants. Osmotic stress reduces plant growth Salinity (EC) Flocculated soil Dispersed soil Sodicity (ESP) However, the use of groundwater pumping, to control rising water tables in the irrigation areas of the Murray- Darling Basin, has led to the use of saline, pumped groundwater for irrigation as the least-cost option. Moreover, recent trends are to apply untreated drainage effluents from factories and municipalities onto agricultural soils. For example, drainage effluents from horticultural industries often contain appreciable amounts of Na + and can be highly alkaline. Several experiments on the effects of saline-sodic irrigation water on soil properties and plant productivity (see e.g. Bevin and Shaw 1980; Rhoades and Loveday 1990; Gupta and Abrol 1990) have led to the following conclusions: 1. When the irrigation water salinity exceeds 0.2 ds m -1 and the leaching fraction (LF) is below 0.5, salt accumulation in the soil layers of duplex red-brown earths (Alfisols) is inevitable. Empirical models relating electrical conductivity of the irrigation water (EC iw ), the average root zone salinity (expressed as EC e, e denoting saturation extract), and LF of the soil layers have been presented by Bower et al. (1969). 2. If the SAR of the irrigation water is greater than 3 and LF is below 0.5, sodium accumulates in soil layers. The accumulation of Na + in Alfisols (duplex red-brown earths), as related to LF, is given in Fig. 3. When the LF is below 0.1, the increase in SAR e of the root zone can be exponential. Most of the cultivated soils in the Victorian irrigation regions have LF < 0.1 while soils under continuous pasture commonly have LF ranging between 0.1 and 0.2 (Rengasamy and Olsson 1993). The SAR of the soil extracts generally increases with depth for a given SAR iw in red-brown earths (Natrixeralfs) where subsoils have very low LF compared with topsoils. In the Australian environment, when the EC iw values are greater than 0.5 ds m -1, SAR iw values, irrespective of their source, are generally greater than Average EC and SAR at the soil root zone increase with increasing EC iw. In the major irrigated soils of Australia, the increase is generally greater in subsoils due to the low LF. The LF of surface layers can vary between 0.2 and 0.02 while in the sodic subsoil layers LF it is generally below Under saline irrigation, the LF may increase with increasing EC iw. This is generally attributed to the electrolyte effect on soil colloids (Rengasamy and Olsson 1991). 5. The increase in both EC and SAR of the root zone soil solution above plant tolerance threshold levels leads to decreased yields in economically important crops. 21

22 5. Categories of salt-affected Soils 5. CATEGORIES OF SALT-AFFECTED SOILS 5.1 Categories based on soil solution characteristics Irrespective of how salt is accumulated in the soil, the soil properties and how they affect plant growth depend on the chemical composition of soil water. The electrical conductivity (EC) of the soil solution is a measure of soil salinity and plant growth in general and crop yields are highly influenced by this parameter. Sodium adsorption ratio (SAR) of the soil solution indicates the effects on soil physical properties and soil structural stability. Soil solution ph indicates whether the soil is acidic or alkaline and hence, the associated stress affecting plant growth and productivity. A saline soil with high EC is usually saline-sodic when the sodium salts dominate in the soil. When the soils are leached these soils become sodic. Similarly a sodic soil becomes saline-sodic when salts start accumulating because of the adverse soil physical conditions. Generally saline, sodic and saline-sodic soils have a spectrum of disorders and the soil solutions have a range of SAR and EC values. Further, as the ph of the soil increases above 8, soil becomes alkaline and carbonates dominate the anions. Salts affect plants through adverse soil properties of alkalinity and sodicity, properties imposed on the soil by mobile salts. Figure gives the different categories of salt-affected soils found in different parts of the world, with criteria mainly based on SAR e and EC e of the saturation extracts of the soil and ph measured in 1:5 soil water suspensions. The value of SAR e of 6 (ESP= 6) and above to classify a soil as sodic soil is based on the Australian criteria of sodicity (Isbell 2002). The soil structural effects due to sodicity depend on both the levels of SAR and EC. Rengasamy (2002b) has given a detailed methodology below to determine the threshold electrolyte concentration for a sodic soil and the soil structure being adversely affected. Generally, sandy soils with very low clay content, as found in Western Australia, can only be saline and will not have soil structural problems caused by high SAR, whereas, clayey soils are likely to be sodic with soil structural FIGURE Categories of salt-affected soils based on EC e (dsm -1 ), SAR e, and ph 1:5 of soil solutions, and possible mechanisms of impact on plants. <4 ECe(dS/m -1 ) >4 Saline soils 1 Acidic Osmotic effect; microelements (Fe, Al, Mn, etc) 2- (ph<6) toxicity; possible SO 4 toxicity 2 Neutral Osmotic effect; possible toxicity of dominant (ph 6-8) anion (Cl - 2- or SO 4 ) and dominant cation (other than Na + ) Alkaline Osmotic effect; HCO 3 and CO 3 toxicity; (ph >8) microelements (Fe, Al, Mn, tc) toxicity at ph >9 Accumulation of salts other than Na salts Leaching by rain or irrigation Non-salt-affected soils Can have problems due to factors other than salts (Normal soils: ph 6-8) Decrease in Na + Increase in Na + Accumulation of Na salts (e.g. irrigation with high SAR and EC water) Addition of amendments (Ca 2+ ) and leaching Reclamation using amendments (e.g. gypsum) Accumulation of Na + (e.g. irrigation with high SAR and low EC water) Saline sodic soils 4 Acidic Osmotic effect; microelements (Fe, Al, Mn, etc) (ph<6) toxicity: Na + toxicity 5 Neutral Osmotic effect; toxicity of dominant anion (ph 6-8) (Cl - 2- or SO 4 ); Na + toxicity Alkaline Osmotic effect; HCO 3 and CO 3 toxicity; (ph >8) Na + toxicity; microelements (Fe, Al, Mn, etc) toxicity at ph >9 Accumulation of salts due to waterlogging and restricted drainage Leaching by rain or irrigation Sodic soils 7 Acidic Indirect effect due to soil structural (ph<6) problems; seasonal waterlogging; microelements (Fe, Al Mn, etc) 8 Neutral Indirect effect due to soil structural problems; (ph 6-8) seasonal waterlogging; possible Na + toxicity at high SAR e 9 Alkaline Soil structural problems; seasonal water (ph >8) logging; HCO 3 and CO 3 toxicity; microelements (Fe, Al, Mn, etc) toxicity at ph >9 <6 SAR e or ESP >6 22

23 5. Categories of salt-affected Soils problems with increased SAR. The categorisation of salt affected soils in Figure is based on soil analytical values, irrespective of how salt is accumulated in the soil. These categories apply to soils affected by rising water tables as well as those not associated with groundwater movements. More detailed categorisation to distinguish the effects of different ph values of alkaline soils is presented in Table 5.1. Possible mechanisms by which soils of each category will affect plant growth are also given in Table In alkaline sodic soils the ph of a 1:5 suspension is usually from one-half to one ph unit higher than that of a saturated soil paste or a soil suspension prepared by using CaCl 2 (Northcote and Skene 1972). Similarly, in our experience, the ph of soil water suspension is greater than the filtered extracts. Saline sodic and sodic categories of salt-affected soils are based on the dominance of sodium salts (SAR e > 6). The recent trend of irrigating crops using recycled water containing dominant ions other than Na+ (e.g. Smiles and Smith 2004), saline soil category with EC e > 4 and SAR e < 6 has been included in Figure 5.1.The constituent cations of total soluble salts in soils are usually sodium, calcium, and magnesium and the anions are chloride, sulfate, and carbonate (including bicarbonate). However, sodium dominates the cations and chloride anions in the majority of saline Australian soils to the extent that sodium chloride comprises 50 to 80% of the total soluble salts (Northcote and Skene 1972). As sodium is absorbed by soil particles above a certain level the soil becomes sodic, and soil structure and hydraulic properties deteriorate. These effects of sodicity (characterised by exchangeable sodium percentage, ESP, of soil or sodium adsorption ratio, SAR, of soil solution) are evident only when salts are leached below a threshold level (for details see Rengasamy and Olsson 1991). When the rainfall is not sufficient to leach the salts throughout the soil profile, they accumulate in deep subsoils, keeping the layers above as non-saline, but sodic. However, with time, even the sodic layers start accumulating salts because of restricted water movement (Rengasamy 2002). Saline, sodic, and saline-sodic soils have a spectrum of disorders and the soil solutions have a range of values of SAR and electrical conductivity (EC). Further, as the ph of the soil increases above 8, soil becomes alkaline and carbonates dominate the anions. Salts affect plants through adverse soil properties of alkalinity and sodicity, with properties imposed on the soil by mobile salts. The different categories of salt-affected soils generally found in different parts of the world, with criteria mainly based on SAR e and EC e of the saturation extracts of the soil and ph measured in 1:5 soil-water suspensions are given in Table 5.1. As described in Figure , these categories are dynamic and will change because of soil management and other soil processes. The schematic diagram showing the change of saline soil to sodic soil by rain or irrigation is presented in Figure TABLE Categories of salt-affected soils based on EC e (ds/m), SAR e and ph 1:5 of soil solutions and possible mechanisms of impact on plants. Toxicity, deficiency or ion-imbalance due to various ions will depend on the ionic composition of soil solution. No. Category of saline soil Criteria Possible mechanisms of impact on plants 1 Acidic-saline soil EC e >4; SAR e < 6; ph < 6 Osmotic effect; microelement ( Fe, Al, Mn etc.) toxicity; SO 4 2- toxicity in very low ph 2 Neutral saline soil EC e >4; SAR e < 6; ph 6-8 Osmotic effect; toxicity of dominant anion or cation other than Na + 3 Alkaline-saline soil EC e >4; SAR e < 6; ph 8-9 Osmotic effect; HCO 3 - and CO 3 2- toxicity; 4 Highly alkaline-saline soil EC e >4; SAR e < 6; ph > 9 Osmotic effect; HCO 3 - and CO 3 2- toxicity; microelement (Fe, Al, Mn etc.) toxicity 5 Acidic-saline-sodic soil EC e >4; SAR e > 6; ph < 6 Osmotic effect; Na + and microelement (Fe, Al, Mn etc.) toxicity 6 Neutral saline-sodic soil EC e >4; SAR e > 6; ph 6-8 Osmotic effect; Na + toxicity; toxicity of dominant anion (Cl - or SO 4 2- ) 7 Alkaline-saline-sodic soil EC e >4; SAR e > 6; ph 8-9 Osmotic effect; Na + toxicity; HCO 3 - and CO 3 2- toxicity 8 Highly alkaline-saline-sodic soil EC e >4; SAR e > 6; ph > 9 9 Acidic-sodic soil EC e < 4; SAR e > 6; ph < 6 10 Neutral sodic soil EC e < 4; SAR e > 6; ph Alkaline-sodic soil EC e < 4; SAR e > 6; ph Highly alkaline-sodic soil EC e < 4; SAR e > 6; ph > 9 Osmotic effect; Na + toxicity; HCO 3 - and CO 3 2- toxicity; microelement (Fe, Al, Mn etc.) toxicity Indirect effect due to soil structural problems; Seasonal waterlogging can induce microelement (Fe, Al, Mn etc.) toxicity Indirect effect due to soil structural problems; Seasonal waterlogging; Na + toxicity at high SAR e Indirect effect due to soil structural problems; Seasonal waterlogging; Na + toxicity at high SAR e; HCO 3 - and CO 3 2- toxicity Indirect effect due to soil structural problems; Seasonal waterlogging; Na + toxicity at high SAR ee; HCO 3 - and CO 3 2- toxicity; microelement (Fe, Al, Mn etc.) toxicity 23

24 5. Categories of salt-affected Soils The critical value of SAR e is based on the Australian criteria of sodicity, which is ESP > 6 (Isbell 2002). Possible mechanisms by which soils of each category will affect plant growth are also given. In alkaline sodic soils the ph of a 1:5 suspension is usually from onehalf to one ph unit higher than that of a saturated soil paste or a soil suspension prepared by using CaCl 2. Similarly, the ph of soil-water suspension is greater than the filtered extracts. These categories of salt-affected soils are based on the assumption of dominant sodium salts. With the recent trend of irrigating crops using recycled water containing dominant ions other than Na+, there may be different effects on soil structure, and also different ion toxicity or imbalance effects on plants. Ideally, the ionic composition of soil solution, in addition to the above criteria, should be taken into account for proper interpretation of soil-plant interactions. 5.2 Prevalence of each category in Australia FIGURE Schematic diagram showing the change of a saline soil to a sodic soil by leaching by rain or irrigation. NaCl -Ca NaCl -Ca NaCl -Ca NaCl Saline Soil NaCl Sodium chloride (salt) Na Sodium Cl Chloride Ca Calcium Clay particle Rain -Na -Na -Na Ca Ca Ca NaCl NaCl Ca Ca Ca Sodic Soil Sodium, chloride and calcium are washed through the soil leaving sodium in the surface layers bound to clay particles. Soils with saline and sodic properties are very common across a large part of Australia. Generally, classification based on soil surveys all over the world identified only salt concentration and exchangeable sodium percentage (ESP) to distinguish saline and sodic categories. The importance of soil ph in affecting both saline and sodic soils was recognised by Northcote and Skene (1972) and they categorised the salt-affected soils as saline soils, alkaline strongly sodic soils, non-alkaline sodic to strongly sodic soils and non-alkaline sodic acidic soils. Soil surveys to distinguish the different categories (given in Table ) have not been undertaken so far. Therefore soil analyses of individual samples from paddocks are necessary to identify the category and the constraints to crop production. The area of different categories identified by Northcote and Skene (1972) and the corresponding category, as in Figure are given in Table TABLE Area of different categories of salt-affected soils in Australia. Salinity category as in Figure Salinity category of Northcote and Skene (1972) Area (km 2 ) % of total land area in Australia Acidic saline soils (1) Not identified 95,000** 1.30 Neutral saline soils (2) Saline soils 386, Alkaline saline soils*** (3) Not identified Not known Not known Acidic saline sodic soils (4) Acidic sodic soils (7) Non-alkaline sodic acid soils 140, Neutral saline sodic soils (5) Neutral sodic soils (8) Alkaline saline sodic soils (6) Alkaline sodic soils (9) Non-alkaline sodic and strongly sodic neutral soils 134, Alkaline strongly sodic to sodic soils 1,721, All sodic soils classified by Northcote and Skene, 1972 are potentially transient saline soils (Rengasamy, 2002a) and therefore can be either sodic soils or saline sodic soils. ** Area for acid saline soils was from Atlas of Australian Acid Sulfate Soils published by CSIRO Land and Water (2009). *** The area of alkaline saline soils in Australia is unknown. This category of soils, with low sodium but high concentration of soluble carbonates of K +, Mg 2+ or Ca 2+, are rarely found in Australia. SOURCE: NORTHCOTE AND SKENE,

25 6. Physics, chemistry and biology of salt-affected Soils 6. PHYSICS, CHEMISTRY AND BIOLOGY OF SALT-AFFECTED SOILS Water and air movements in soil depend on the soil structure in general and are important in agricultural systems and soil engineering operations. They depend largely on the size and shape of the soil particles, and the arrangement of pores, as influenced by pore fluids. The chemical composition of pore fluids dictated by the salt composition affects swelling, dispersion, and flocculation of clay particles associated with soil particles and causes changes in soil physical properties such as soil structure, architecture of pore systems, hydraulic conductivity and soil aeration. Several studies have shown that sodium in soil solution (in relation to adsorbed sodium) adversely affects soil structure and subsequently, leads to soil crusting, runoff and different forms of soil erosion. Recent studies (Arienzo et al. 2009; Rengasamy and Marchuk 2011; Jayawardane et al. 2011; Laurenson et al. 2012) have shown that both monovalent Na and K affect the soil structural stability and associated soil physical phenomena. Traditionally sodicity and sodic soils are considered as the only issue of salt-affected soils and remain the subject of many investigations all over the world. Therefore, the following discussion will centre on sodicity and sodic soils while the sections following (sections 6.7 and 6.8) will focus on the recent developments on the functions of all four cations. 6.1 Soil structural stability in relation to sodium in salt-affected soils Sodic soils exhibit poor soil-water and soil-air relations; these properties adversely affect root growth restricting plant production and making the soils difficult to work when wet or dry. In Australian soils, when the adsorption of sodium on the surface of clays exceeds 6% of the total cation exchange capacity (exchangeable sodium percentage, ESP= 6), the soil is considered sodic and is subject to serious structural degradation. ESP 6 is low compared with the ESP 15 adopted in the US (US Salinity Laboratory Staff 1954) and many parts of the world, as a criterion for the deterioration in soil structure. The lower ESP for sodicity in Australian soils is attributed to the very low contents of soluble minerals, especially calcium, necessary to maintain the electrolyte concentration during leaching (Northcote and Skene 1972). While the sodicity in subsoils is caused by natural processes, the sodicity of both surface and subsoils is increasing due to the recent use of saline groundwater for irrigation, and is aggravated by management practices such as cultivation which accelerate structural degradation. While both salinisation and sodification occur together, the destructive effects of sodicity are only evident after leaching profiles free of salts (nonsaline conditions). Due to the dominance of sodium chloride in salt-affected soils in Australia, the Na/ (Na+Ca) ratio of soil solutions approaching that of oceanic waters sodium adsorption is now excessive and removal by leaching is ineffective unless appropriate amendments are added Sodicity in Australian soils Northcote and Skene (1972), in their detailed study of Australian soils with saline and sodic properties, reported that sodic soils are widespread throughout both the intensively developed southern and eastern parts of the continent and the sparsely grazed lands of Queensland and Western Australia. They reported that 27.6% of total land area in Australia was affected by sodicity. Secondary salinisation, together with sodification, is occurring in these soils as a consequence of irrigation, inappropriate agricultural practices and the clearing of native vegetation. Northcote and Skene (1972) estimated that about 840 km 2 of irrigated soils and 1970 km 2 of dryland soils were affected by secondary salinisation. The area currently affected is not accurately known as few systematic surveys have been made since. The overall soil degradation due to sodicity in agricultural land (both dryland and irrigated) in Australia is expanding rather than contracting Physical properties affected by the accumulation of sodium salts Two processes, swelling and dispersion are responsible for the physical behaviour of sodic soils during wetting. On the other hand, in saline soils swelling is minimal and clay dispersion is absent due to the high electrolyte concentrations present. Both swelling and dispersive behaviour are governed by the balance between attractive and repulsive forces, arising from intermolecular and electrostatic interactions between solution and solid phases in the soil. The distinction between saline and sodic soils arises because these 25

26 6. Physics, chemistry and biology of salt-affected Soils forces vary depending on whether the soil solution is concentrated (saline) or diluted with a high proportion of Na to divalent ions sufficient to cause swelling and dispersion (sodic). In the past, soil scientists have used a model involving Lifshitz-van der Waals, ion correlation, hydration, and electrical diffuse double-layer forces generated between colloidal clay minerals suspended in water to explain sodic soil behaviour. However, in natural soils, clay particles which are bound together with silt and sand particles and form aggregates of various sizes are confined and not readily suspended in water. Consequently, for clay in soil to become dispersed, forces other than those which operate in colloidal suspension must be overcome. In addition, most investigations of dispersion and flocculation have used pure clay minerals rather than soil clay systems as models for soil behaviour (Shainberg and Letey 1984). Despite the fact that soils are often categorized on the basis of the dominant clay mineral present (e.g., smectitic soil), soil clay systems (which are complex heterogeneous intergrowths of different clay structures intimately associated with organic and biopolymers) do not behave in the same way as their pure clay mineral counterparts. Soil colloids are unique in their behaviour and are not appropriately represented by pure clay mineral systems. Consequently, in order to understand sodic soil behaviour, all the processes that occur during the initial wetting of dry aggregates, which results in swelling to the final stage of aggregate disintegration, and leading, in turn, to dispersion of soil clays when completely wet, must be taken into account. Classical theories of colloidal behaviour in suspension such as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (van Olphen 1977) may not satisfactorily explain the entire phenomenon of sodic behaviour. In the discussion to follow, the mechanisms involved in the disintegration of an aggregate on wetting, into its constituents with the release of dispersed clay will be explored. 6.2 Mechanisms of structural change in sodic soils Rengasamy and Sumner (1998) have reported the details of the mechanisms involved in the structural changes occurring in soils when wetted with water. The following discussion is based on their report. When Na is present in clay particles, soils tend to form massive structures without any hierarchical arrangement of different particle sizes into micro- and microaggregates. The stability of different aggregate sizes and their associated pore systems depends on the relative magnitudes of the various forces arising from interactions between aqueous and solid phases in the soil. As the soil water content varies, there is a concomitant change in total potential (energy level), which is made up of component potentials such as osmotic (due to interaction with dissolved components) or, matric (due to interaction with solid phase) potentials. When a dry soil aggregate comes into contact with water, these interactive forces lower the potential energy of the water molecules, resulting in the release of energy which is partly consumed in the structural transformation of the aggregate, with the remainder being dissipated as heat (Rengasamy and Olsson 1991). Slaking, swelling, and clay dispersion are the main mechanisms by which the massive structure of sodic soils disintegrate during rain or irrigation, resulting in the observed poor physical condition. The potential energy of interacting water molecules in a soil aggregate is changed by: 1. Intermolecular force fields whose intensity depends on the surface area available for water adsorption, 2. Hydration of cations and anions adsorbed by outer sphere complexation on soil particles, 3. Electrical fields induced by positive and negative charges on soil colloids, and 4. Chemical interactions such as hydrogen bonding or inner sphere complexation between water molecules and soil particles. As slaking, swelling, and dispersive behaviour of soil aggregates are not influenced by non-polar solvents (Table 6.2.1), the polar (electrical) nature of water molecules is crucial in determining the patterns of behaviour outlined above. Despite the importance of the polarity of a liquid described by its dielectric constant in sodic behaviour, solvation reactions of the liquid appear to be equally important in clay swelling and dispersion. Soil aggregates are composed of several structural units linked together principally by colloidal (clay and organic matter) particles. The surface atoms and electrical charges on the colloidal particles and their associated cations and anions which are held by outer sphere complexation contribute to the stability or breakdown of the units when exposed to water molecules. Based on this assumption, the changes, which are likely to take place in the structural behaviour of sodic and non-sodic aggregates upon wetting, will be compared. The magnitude and direction of energy changes taking place during wetting of an aggregate are illustrated schematically in Figure 6.2.1, involving aggregate slaking and swelling, spontaneous and mechanical clay dispersion, and flocculation of dispersed clay by electrolytes. The individual stages involved in these processes will now be discussed. 26

27 6. Physics, chemistry and biology of salt-affected Soils TABLE Slaking and spontaneous dispersion of aggregates (2-4mm) of an Alfisol at two levels of sodicity in various solvents. Solvent Dielectric constant (25ºC) Slaking (% <2 mm) Dispersed clay (% of total clay) ESP 1 ESP 20 ESP 1 ESP 20 Water Ethanol Benzene n-hexane SOURCE: RENGASAMY AND SUMNER Initial wetting of dry aggregates Clay particles in dry aggregates (Stage 1, Figure 6.1) are bound together by inorganic and organic compounds involving several mechanisms and different types of bonding which produce strong attractive pressures of the magnitude of megapascals (Rengasamy and Olsson, 1991). The water stability of an aggregate depends on the strength and persistence of these linkages which, in turn, are functions of the type of bonding. Bond strength in the presence of water generally decreases in the order: covalent, hydrophobic, Lifshitz-van der Waals, coordination complexion, hydrogen, and finally ionic bonds. However, the inherent mechanical strength of each type of bond may change. For example, the mechanical strength due to ionic bonding is greater than that due to Lifshitz-van der Waals forces. In contrast to covalent bonds, ionic bonds are readily solvated by FIGURE Schematic illustration of processes that take place and intensity of attractive and repulsive forces involved when a dry aggregate of soil is wetted. Net pressure MPa water molecules, for example, in pure Na compounds and Na-clay linkages where ionicity dominates over covalency. In fact, any given heteronuclear bond found in natural systems has a mixture of covalent and ionic character (Huheey et al., 1993). The degree of covalency in a bond involving metal cations is characterized by the Misono softness parameter Y derived from ionization and ionic potentials (Misono et al., 1967). This parameter is calculated as follows: Y= 10((I Z / I Z +1) (r i / z 0.5 )) (6.1) where, r i = ionic radius of metal ion, Z = its formal charge, I Z = Z th ionization potential (M (Z-1)+ M Z+ ). The Misono factor Y, which expresses the tendency of a metal ion to form a dative π-bond, corresponds to its softness or the ability to enter into electron-acceptor and electron-donor (EAED) or hard-soft acid-base (HSAB) reactions. For example, the tendency to form covalent bonding and complexes increases in the order: Na +, K +, Mg 2+, Ca 2+, Fe 3+, Hg 2+. For more information on bonding models, see Huheey et al. (1993) Hydration reactions With further wetting, solvation or hydration (when the liquid is water) forces, which are distinct from the DLVO or Lifshitz-van der Waals forces, come into play. Hydration forces arise from the solvent effects in which the molecular structure of the solvent is altered. Both DLVO and Lifshitz-van der Waals forces are changed by the solvent interaction, even when the solvent molecules themselves are not transformed. Solvation forces in water differ from those in other liquids by exhibiting a long-range monotonic component, in addition to a superimposed oscillatory one. The monotonic component can be repulsive or attractive, long (>4 nm) or short range (<1.5 nm), and appears to depend on both the chemical nature and the dynamic state of the surface (Israelachvili 1992). Attractive Repulsive KPa MPa Stage 1 Continuous hydration (sodic clays) Hydration AGGREGATE (Dry) FLOCCULATION Osmotic dehydration Stage 4 + chemical bonding Stage 2 SWELLING SLAKING Mechanical separation DISPERSION Stage Particle separation (nm) SOURCE: RENGASAMY AND SUMNER,

28 6. Physics, chemistry and biology of salt-affected Soils TABLE Types of bonding and mechanisms involved in the linkages between clay particles in an aggregate. Type of bond Van der Waals forces Ionic bond Covalent bond Hydrogen bond Coordination complex Hydrophobic bond Mechanism Bonding between permanent or induced polar units Cation and anion exchange, protonation, cation bridging O-H, Al-O, Si-O bonding, inner- and outer-sphere complexation Water bridging, H-bond with oxygen atoms of silica sheet, proton-mediated H-bond Ion-dipole interaction, ligand exchange, inner- and outer-sphere complexes Interaction between the hydrophobic portion of the clay surface and non-polar groups of organic molecules SOURCE: RENGASAMY AND OLSSON 1991 In clay systems where there is a uniform charge (positive or negative), monotonic hydration forces are repulsed (the pressure being in the megapascal range); the initial attractive force between clay particles in the aggregate decreases markedly on wetting; and, as the hydration continues, the distance between these particles increases. The result is reduced aggregate strength with increasing water content. Although very weak, the net force is still attractive and the clay particles are held together by hydrated cations. If the clay particles are Ca or Mg saturated, further wetting does not increase the interparticle distance beyond 2-3 nm, whereas in monovalent cation saturated clays, particles continue to separate. This type of swelling is due to divalent cation hydration called crystalline swelling (Stage 2, Figure 6.2.1) and occurs in clay particles even at high electrolyte concentrations. However, two regions of limited swelling in divalent ion saturated smectites have been identified by Slade and Quirk (1991). The change in basal spacing from 1 to 1.55 nm is not affected by the concentration of the wetting solution, whereas the ionic strength (salt concentration) below which the transition from 1.5 to 1.9 nm (an osmotic process) takes place, depends on the charge density and origin (tetrahedral or octahedral) in smectites. Smectites with high charge and tetrahedral substitution require a lower electrolyte concentration for this transition to occur than low charge octahedrally substituted analogues. This phenomenon has even been observed in two Na smectites (Nibost and Drayton montmorillonites) with high charge arising mostly from tetrahedral substitution, which did not swell beyond 1.6 nm even in distilled water (Slade et al. 1991) Comprehensive hydration mechanism for soil clays The following working hypothesis of hydration reactions in soil clays was proposed by Rengasamy and Sumner (1998): The attractive forces between clay particles depend on the nature of bonding mediated through the cations (Na, K, Mg, Ca, Fe, and Al) which are commonly found in soils. Generally, a bond is considered to be covalent or ionic as long as the bond in question is predominantly one type or the other. However, in hydration reactions, it is convenient to be able to characterize the bond as intermediate. In EAED interactions, where the cation has no polarizing effect on the clay surface atoms, the bond is essentially ionic. When the cation and the surface atoms are mutually polarizing, the resultant bond is polar covalent. The degree of ionicity (or covalency) of such bonds depends on the nature of the cations as characterized by the Misono factor, with Na contributing more to ionicity than Ca. When the polarization by the cation is sufficient, a covalent bond is formed. The linkage between Al hydroxy cations and clay surfaces is dominated by covalency because of the high polarizing power of Al. The Al and Fe hydroxy species are specifically adsorbed or inner-spherically coordinated to the clay surfaces. Hydration of Fe and Al in such linkages is difficult. Calcium forms polar covalent bonds in which the hydration of Ca is limited and determined by the polarity of the bond. For Na, the linkage of clay surfaces is ionic and hence, hydration is extensive. In addition, clay charge characteristics modify the EAED interactions and the nature of the bonding. If the anion were large and soft enough, the cation should be capable of being polarized, with the extreme case being the penetration of the anionic electron cloud by the cation resulting in a covalent bond. As the polarizability of the anion is related to its softness, that is, to the deformability of its electron cloud (Huheey et al. 1993), the large size and high charge of the anion favour this interaction. Here the clay particle is considered as a super ligand with heterogeneous charged sites having different polarizing abilities per unit of charge depending on location. These factors also determine whether cations link clay particles by ionic, polar covalent or covalent bonding. The ionicity of the clay-cation-clay bonds is determined by both the type of cation and the nature of the clay ligand. In the case of soil clays with net positive charge, bonding depends on the nature of the anions present. More detailed accounts of surface complexation have been presented by Sposito (2008) and Stumm (1992). These attractive forces should not be confused with those of Lifshitz-van der Waals interactions, which are not involved in the hydration reactions of clay particles. Forces arising from water interactions with clay particles 28

29 6. Physics, chemistry and biology of salt-affected Soils can be attractive when the interaction is hydrophobic or repulsive due to polar interactions (not electrostatic). For soil clays, these polar interactions, which are based on EAED or HSAB interactions between the polar solvent (water) and the polar moieties (charged clay particles), are repulsive and are functions of ionicity of the bonds between particles. These interactions can exhibit energies that may be up to two orders of magnitude greater than those commonly encountered in the components of traditional DLVO energy balances (van Oss et al. 1988). Earlier in Russia, Derjaguin and Churaev (1974) considered hydration forces as the structural component of disjoining pressure, and more recently, hydration forces causing repulsive interactions between mica surfaces separated by less than 5 nm have been extensively implicated by Pashley and Quirk (1984) and Pashley (1985). They concluded that, in dilute suspensions, the forces observed were generally in good agreement with DLVO theory, but in more concentrated suspensions (clay particle separation less than 5-10nm), short range non- DLVO forces (hydration forces) arose. These short-range repulsive interactions were clearly related to the type of cation adsorbed at the mica surface. AlthoughLifshitzvan der Waals were certainly present in concentrated suspensions, they were completely overshadowed by the repulsive hydration forces. The origin of non-dlvo forces lies in solvation reactions where polar solvents react with polar surfaces. Clays do not swell or disperse in non-polar solvents (Table 6.1). Solvation reactions are controlled by EAED interactions and not merely by the polarity of the solvent as measured by the dielectric constant. Only charged clay particles exhibit swelling and dispersion, which depend on the ability of water molecules to solvate the cations or anions involved in clay-clay bonding and are related to the EAED interactions. It is important to note that the extent to which free (solution) versus adsorbed cations and anions are hydrated are different. For example, the hydration of cations as shown by the hydration numbers follows the order Ca 2+ > Mg 2+ > Na + > K +, whereas the extent of swelling of smectites caused by the hydration of cationic clays in pure water at zero matric potential is in the order Na + > K + > Mg 2+ > Ca 2+ (Rengasamy and Sumner 1998). As previously stated, hydration reactions during limited crystalline clay swelling lead to the separation of individual crystals in a series of discrete steps, corresponding to 0, 1, 2, 3, and 4 layers of water molecules. These reactions were dependent on EAED interactions during hydration. Initial separation up to a distance of about 1.6 nm is independent of ionic strength, but further separation to 2 nm (Figure 6.2.1) appears to be determined by both ionic strength and EAED interactions. During extensive hydration, as is observed in sodic clays, separation of clay particles beyond 7 nm depends on both ionic strength and EAED interactions. When the ionic strength of the equilibrium solution is decreased, water molecules enter the clay particles. The magnitude of pressure developed during macroscopic swelling, usually known as clay dispersion (Stage 3, Figure 6.2.1), depends on the difference in chemical potential of water in the equilibrium and inner (i.e. in the space between the particles) solutions. Once the clay particles are completely dispersed (i.e., separated into distinct individual entities), electrostatic repulsive forces, as predicted by DLVO theory come into play with Lifshitzvan der Waals attractive e forces being negligible. As the difference in the chemical potentials of water in inner and outer solutions approaches zero (the result of increasing ionic strength in the outer solution), the clay particles begin to approach each other. The electrostatic repulsive pressure is balanced by the increasing osmotic pressure, at which stage Lifshitz-van der Waals attractive pressures become dominant. It is at this stage, flocculation, or coagulation (used synonymously), occurs in clay or soil suspensions in water (Stage 4, Figure ). On drying, the flocculated clays are increasingly attracted to each other by EAED interactions; resulting in far greater attractive pressures than predicted by Lifshitz-van der Waals interactions. This association is called aggregation, or flocculation plus. When a soil aggregate is wetted causing the water content to increase, clay particles will not separate and become dispersed if the electrolyte concentration of the wetting solution is sufficient to balance the electrostatic repulsive pressure. The hydration reactions in soil clays are influenced by both enthalpy and entropy effects. Enthalpy of hydration will depend on the nature of bonding between cations and clay surfaces, ligand field effects, and the steric and electrostatic interactions. Factors related to clay and associated organic polymers such as concentration and particle size and the presence of hydrophobic components will introduce entropy effects. However, when many of these factors are relatively constant, the nature of bonding between cations and clay surfaces are the the major determinant Slaking of Soil Aggregates Despite the fact that the clay particles are separated by water molecules (Stage 2, Figure 6.2.1), the net pressure, which is in the kilopascal range, is still attractive. If the clay particles are Ca- or Mg-saturated, further wetting does not increase the interparticle distance beyond 2-3 nm. Hydrostatic and pneumatic pressures in the range of kilopascals associated with pore filling are sufficient to break this weak attractive force. As a result, the linked units of clay particles in a micro-aggregate setting become separated, which is known as aggregate slaking. Unless these clay linkages 29

30 6. Physics, chemistry and biology of salt-affected Soils are weakened by hydration reactions, the low pressures associated with entrapped air or raindrop impact (in the range below kilopascals) will fail to cause disruption. The rate of wetting is critical in the initial hydration involving EAED reactions. Stronger attractive bonding between clay particles through water/cation bridging is maintained during slow rather than rapid wetting, when entropy plays an important role. When saturated with divalent cations, aggregate integrity is maintained and slaking is minimized. However, in sodic aggregates, dispersive breakdown is not prevented by slow wetting under suction. On the other hand, under saline condi tions (high EC) where hydration is controlled due to the osmotic effect, aggregate breakdown is minimized, irrespective of whether the clay is Na- or Ca-saturated. However, when the aggregates are initially Na-saturated or the solution in which they are sieved has a high SAR, severe slaking takes place at low EC values, with the effect being less marked for Ca- and Mg-saturated systems Spontaneous Dispersion Spontaneous dispersion often takes place without energy inputs when sodic clay is placed in water of very low electrolyte concentration. When the bridging between clay particles involves Na and the EC is low, the interparticle distance continuously increases, with continued wetting to beyond 7 nm (Figure 6.2.1). The proportion of clay particles separated in this way depends on the number of Na ions involved in the clay linkages and hence on the SAR of the soil-water system (Rengasamy and Olsson, 1991). At water contents lower than saturation, limited but more than crystalline swelling and incomplete separation of clay particles take place, with the interparticle distance depending on the water content. Similar mechanisms are involved in interlayer and interparticle separations in smectites and other clays (kaolinite and illite) (Quirk 1986) Mechanical Dispersion Hydrated clay particles which have undergone limited separation can be pushed further apart by applying external mechanical pressure in the range of Pascals to Kilopascals such as from raindrop impact. At distances of separation greater than 2/K, where K is the thickness of the electrical double layer in nanometers, electrostatic repulsive forces predominate over attractive forces. Hence, on continued wetting, which reduces electrolyte concentration and increases double-layer thickness, the particles become progressively more separated, finally reaching the stage of dispersion. Thus, a Ca or Mg clay aggregate can be dispersed when uniformly remoulded (energy input) at or above critical water content, defined by Emerson (1983) as the water content for dispersion. At lower water contents, attractive forces dominate in spite of the mechanical repulsive pressure introduced by remoulding. In subplastic soils (Norrish and Tiller 1976), the attractive pressures between clay particles have to be overcome by higher levels of mechanical energy involving sonification or grinding. These attractive forces increase with the extent of covalent bonding between particles caused by cementing agents, thereby decreasing the potential for solvation by water molecules. This is analogous to the difference in solvation and subsequent dissolution of CaCl 2 and CaCO 3. In the case of CaCl 2, solvation is complete and hence it dissolves, whereas for CaCO 3 the much lower solvation renders it insoluble. Similarly, Na-saturated Drayton montmorillonite did not swell on wetting (Norrish and Tiller 1976) because the high tetrahedral charge (Slade et al. 1991) promoted strong inner-sphere coordination between Na and the clay, making hydration more difficult. On the other hand, when lithium (Li)-saturated, the clay hydrated and dispersed readily. 6.3 The Electrical Double Layer In an aqueous suspension, the charge on clay particles is neutralized by hydrated ions of opposite charge. In sodic soils, clay surfaces usually always carry a net negative charge, which is neutralized by a diffuse cloud of ions in which the concentration of cations increases and that of anions decreases as the surface is approached. The electrical double layer consists of the surface charge and the surrounding ion swarm. Various theories (Gouy-Chapman, Stern) and modifications thereof (multiple layer models) have been proposed in an attempt to predict the behaviour of the electrical double layer present in dispersed colloidal systems. The classical DLVO theory (details in Rengasamy and Sumner 1998) explains colloidal stability (dispersion and flocculation) on the basis of the opposing forces of attraction (Lifshitz-van der Waals forces) and repulsion (electrostatic or columbic forces). The DLVO theory has been found to be unsatisfactory in clays with divalent cations, where diffuse double-layer formation is restricted due to stacking or aggregation of particles. This failure has been assumed to be caused by factors such as partial blocking of negative surfaces by adjacent particles, incomplete double-layer formation on some or all surfaces, and specific adsorption (inner sphere complexation) of cations with the negative sites on the clay. However, once clays become dispersed, either spontaneously or mechanically, the different forces operating in colloidal suspen sions can be described adequately by these models, dis cussed in detail by Rengasamy and Sumner (1998). 30

31 6. Physics, chemistry and biology of salt-affected Soils 6.4 Net charge and clay dispersion Because repulsion between particles increases with an increase in 1/K in a system of uniform polarity, the nature and concentration of the cations control the behavior in negatively charged systems and anions in their positively charged analogues. In mixed charge systems, charge reversal can be effected by changing ph and/ or adsorption of organic or inorganic ligands (e.g., phosphate on positively charged sites) by inner sphere complexation to the surface (specific adsorption). Thus adding phosphate to a soil promotes repulsion between particles because any positive charges present become negative, reducing the attractive forces between sites of opposite charge. In terms of sodic soils, in which there are elevated levels of Na, expansion of the double layer and hence repulsion between particles are promoted. The electrostatic interaction in dispersed clays depends on the net charge of the system. For example, Chorom et al. (1994) demonstrated a significant relationship between the net negative charge of soils (CEC at ph 8.0) and their tendency to disperse. However, the slope of the relationship was a function of soil type and ph, indicating that the actual charge present in the field was not reflected well by the CEC measurement at ph 8.0. FIGURE Relationship between relative zeta potential and relative clay content of dispersed suspensions of soils with different mineralogy, exchangeable cations, organic matter and ph. Clay content rel y = x R 2 = 0.72 p< Zeta potential rel SOURCE: MARCHUK et al, 2013 Net negative charge also influences the electrokinetic properties of the clays. For example, the electrophoretic mobility of clay particles or zeta potential depends on the net charge available for outersphere complexation of cations, the mode of attachment of Na on clay surfaces in most cases. On the other hand, divalention clays always exist as domains or quasicrystals, where a number of individual particles are linked by innersphere coordination. As a result, these clays have lower electrophoretic mobility than sodic clays (Chorom and Rengasamy 1995). Investigating soils with different clay mineralogy, organic matter, ph and different suits of exchangeable cations dominated either by Na or K, Marchuk et al. (2013) found that clay dispersion from these soils was highly related to the zeta potential of the dispersed clay confirming that clay dispersion due to adsorbed cations depends on the net charge available for clay-water interactions (Figure 6.4.1). The distinctive way in which clay minerals and organic matter are associated and the changes in soil chemistry affecting the net charge cause the quantitative relationship between exchangeable cations and EC in a dispersive soil to be unique for each soil. 6.5 Flocculation caused by cations in electrolytes The process by which the dispersed soil particles (colloidal in nature) join together to form bulky porous mass is termed flocculation, also known as coagulation. Flocculated mass transform into soil aggregates involving many soil processes. This can be either transport-controlled or surface reaction- controlled (Sposito 2008). The clay separation from aggregates during interaction with water surface reaction is the major factor in controlling flocculation. In dispersed phase, as the net negative charge increases, the electrostatic repulsive force in the double layer and its thickness 1/k increase. By increasing the electrolyte concentration or introducing multivalent cations, 1/k is reduced to a distance where the attractive energy becomes equal to repulsive energy and the clay particles flocculate. The electrolyte concentration at this stage, where the total potential energy per unit area of particle surface becomes zero, is defined as the critical flocculation concentration (CFC), which is a function of 1/ Z 6 (Z is the valence of the cation) and is popularly known as Schulze-Hardy rule (Details in Rengasamy and Sumner 1998). Although this rule explains the effect of valence on flocculation, quantitative predictions deviate from experimental results because flocculation by multivalent ions involves a charge reduction on clay surfaces by the screening effect of adsorbed cations and EAED interactions in addition to the reduction in the thickness of the double layer. Rengasamy and Sumner (1998) evaluated the critical flocculation concentrations for a number of homoionic clays by chloride solutions of the same cation and illustrated the major role that valence plays.. However, even with a given valence, flocculating power differs (K > Na; Ca > Mg). They concluded that compared to Na = 1, the flocculation power of the other cations would be K = 1.8, Mg = 27, and Ca =

32 6. Physics, chemistry and biology of salt-affected Soils Flocculation by electrolytes appears to be a combination of an osmotic phenomenon and EAED interactions. The osmotic pressure of the electrolyte in solution causes the removal of water molecules between the clay particles (dehydration), thereby reducing their separation distance. The cationic effects cause EAED interactions, resulting in particle bonding and reduction in net charge available for outersphere complex formation. On the one hand, the Misono factor and the ionic potential can be related to the EAED interactions, while on the other, the reduction in double layer thickness is related to Z 2. Combining these functions, the following relationship was used by Rengasamy and Sumner (1998) to calculate the flocculation power of the cations: Flocculating power = 100(I z / I z+1 ) 2 Z 3 (6.2) The flocculating powers (dimensionless) of Na +, K +, Mg 2+, and Ca 2+ calculated from this equation are very close to those determined experimentally (Table ). TABLE Cation Calculated using equation 6.2 Derived experimentally Na K Mg Ca SOURCE: RENGASAMY, 2002b Flocculation by alternative mechanisms The point of zero net charge at which clays are flocculated, can be attained by altering the total particle charge density. The total charge on a clay particle is the combination of permanent negative charge, variable charge due to ph, inner-sphere complex charge and outer-sphere complex charge (Sposito 2008). Thus, at the ph value of the point of zero charge (PZC), clay particles are flocculated. Outer-sphere complexation of hydrated Ca 2+ reduces the total charge effects by a charge-screening mechanism (Sposito 2008) and particle rearrangement (such as quasi-crystal formation). Inner-sphere complexation generally reduces the total charge. Complex formation involving iron and aluminium polycations may result in charge reversal (Rengasamy and Oades 1979). Adsorption of organic or biological polymers mediated by cation bridging may also reduce the negative charge (Theng and Tate 1989). Steric factors due to particle association can cause the adsorbed polymers with polar functional groups to repel each other thereby increasing the dispersive potential. Thus, depending on the extent of charge reduction and particle association, flocculation can take place at extremely low electrolyte concentrations. Also, clay dispersion can be prevented by heating a sodic soil or clay at a higher temperature (above 200ºC) when sodium forms covalent bond with clay surfaces (e.g. Chorom and Rengasamy 1996). A significant proportion of soil surfaces may have hydrophobic character caused by the adsorption of nonpolar organic molecules, often a feature of nonwetting sands. The ordering of water molecules on such surfaces tends to be limited by the inability to react with the soil particles. In water suspensions of materials exhibiting this character, the particles associate among themselves in such a way that contact with water molecules is minimized. Hydrophobic forces of attraction (in the range of kpa) can be stronger than van der Waals forces and can operate at distances of separation of about 80 nm (Gregory 1989). The adsorption of hydrophobic substances on soil clays leads to a reduction in net charge, making this mechanism even more important. 6.6 Dispersive Potential (P dis ) The improvement of the structure in sodic soils requires the application of appropriate amendments in order to promote clay flocculation. However, the critical flocculation concentration (or TEC, threshold electrolyte concentration) values obtained in dispersed systems are not directly applicable to soil at field moisture contents where clay particles occur as aggregates. The repulsive forces present in dispersed clay systems are far greater than those in the aggregated clay particles. Hence, the electrolyte concentration required for the prevention of clay dispersion from soil aggregates is always lower than the TEC obtained for suspensions. However, TEC is necessary for each level of sodicity to maintain favourable soil structure in sodic soils. But, TEC is not only a function of SAR and EC; it also varies with the net negative charge, as discussed earlier. In order to derive a single parameter that will combine the effects of SAR and EC, Rengasamy (2002 b) developed the concept of Dispersive Potential, which is derived from the electrolyte concentration and composition at which the tendency of soil aggregates to disperse spontaneously is prevented. This potential Pdis is defined as the difference in osmotic pressure between the concentration required to flocculate (or prevent dispersion) from aggregates Ptec, and the ambient solution concentration Psol. P dis = P tec P sol, for P sol <P tec (6.3) Using the established (US Salinity Laboratory Staff 1954) relation between the osmotic pressure and the electrical conductivity (EC) of soil solutions and also the relation between EC and ionic concentration (1 ds m -1 = 10 mol c m -3 ), osmotic pressure (P osm ) is derived as: 32

33 6. Physics, chemistry and biology of salt-affected Soils P osm = 3.6 kpa per mol c m -3 (remember, mol c m -3 is meq L -1 ) (6.4) The flocculation-dispersion phenomena are influenced by osmotic pressure of the soil solution and the flocculating power of individual cations in solution, as discussed in detail earlier. Using the flocculating power also in calculating the osmotic pressure involved in dispersionflocculation processes, Rengasamy (2002 b) derived the following equation: P tec or P sol (kpa) = 3.6x [45x C Ca + 27 C Mg C K + C Na ] (6.5) Where C is the concentration of Ca, Mg, K and Na (mol c m -3 ) in equilibrium solution which contains threshold electrolyte concentration (TEC) or the original soil solution in which clay dispersion is observed. The methodology to estimate the dispersive potential of an individual soil (spontaneously or mechanically dispersed) is detailed in Rengasamy (2002b). Dispersive potential indicates the energy associated with the dispersive reactions in a soil-water system. Because it is determined using a given soil, it eliminates the differences due to soil factors other than cations such as mineralogy, organic matter, cementing agents and ph. Further, the different effects of the cations are also taken into account in the calculation. An application of Pdis is in calculating amendments, such as gypsum, required to flocculate the dispersed clays and favour the formation of stable aggregates. Thus for a P dis =1000 kpa, the above equation is used to calculate that 6.2 mol c m -3 of Ca or 0.53 g L -1 of gypsum is required to prevent dispersion. Comparing this with other cations, 10.3, 154.3, mol c m -3 of Mg, K and Na respectively is required to prevent clay dispersion. 6.7 Research reports on the effect of potassium and magnesium on soil structure It is well known that sodium adversely affects soil structure and associated physical properties. The role of monovalent potassium and the controversial role of magnesium on structural deterioration, although debated in the literature, have not received attention in the management of salt-affected soils. The following sections give the various results reported on these issues Potassium Potassium exists in soils in structural, non-exchangeable, exchangeable and water soluble forms. The major mechanisms involved in the decrease in permeability due to monovalent exchangeable cations are swelling, dispersion and clay migration which affect hydraulic conductivity of the soil (Quirk and Schofield 1955). Irrigation with wastewaters from agri-industry processes is commonplace nowadays and these wastewaters all have high concentrations of potassium (K). Long term application of such wastewaters may lead to build up of potassium in soil resulting in a decrease in the hydraulic conductivity of the receiving soils (Arienzo et al. 2009). Reported results on the effect of exchangeable potassium on soil permeability differ or are conflicting, which may be attributed to differences in clay mineralogy and sample preparation procedures (Levy and Torrento 1995). Some studies have found that exchangeable sodium and exchangeable potassium had similar deleterious effects on hydraulic conductivity of the soil (Quirk and Schofield 1955). Other researchers have reported that the effect of exchangeable potassium on soil permeability was not as negative as that of Na, but not as favourable as that of divalent cations (Ca and Mg) (Reeve et al. 1954). The literature shows a broad spectrum of possibilities for potassium effect on infiltration, ranging from being similar to sodium (negative effect) to being similar to calcium (positive effect). However, it seems that the overall effect of increasing exchangeable potassium can negatively impact on soil hydraulic conductivity. For example, it was reported that soil permeability relates to exchangeable cations in the following order: Ca Mg>K>Na (Chen et al. 1983), although difference in the relative values of permeability have been reported: Mg>K>Na (Reeve et al., 1954), Ca>Mg>Na=K (Quirk and Schofield 1955); Ca>K>Na (Gardner et al. 1959); NH4=K>Na; Ca=Mg>K>Na (clay loam); Ca>Mg>K=Na (clay) (Swaify et al. 1970). Levy and van der Watt (1990) observed that an increased amount of exchangeable K + in soil clay resulted in a decrease in hydraulic conductivity. These authors commented that the extent of this phenomena depended on the clay mineralogy of the particular soil. The smallest effect was found in kaolinites and the greatest in the illitic soils. In contrast Cecconi et al. (1963) suggested greater stability in K+ saturated soils than in those saturated with divalent cations; K>Ca=Mg>Na. Furthermore, Chen et al. (1983) reported that an exchangeable potassium percentage (EPP) in the range of 10-20, improved the hydraulic conductivity in some Israeli soils. Shainberg et al. (1987) concluded that effect of K depends on charge density of the smectitic clay. They found that the higher the charge density of the clay the more favourable the effect of K + on hydraulic conductivity. In some soils potassium effects on hydraulic conductivity intermediate between Ca 2+ and Na +, whereas in others K + improves permeability. It seems that potassium fixation could be a possible mechanism that affects permeability. Potassium K + is a major nutritional element for plants and enrichment of K + in the exchange sites due to fertiliser practice can be expected. Therefore it is important to understand 33

34 6. Physics, chemistry and biology of salt-affected Soils the effect of potassium on soil structure and its role and position in the exchange complex. Potassium, a cation with low hydration energy, produces interlayer dehydration and layer collapses and therefore is fixed in interlayer positions. The degree of cation fixation depends on the layer charge of the mineral. In vermiculite, K + saturation effects interlayer collapse producing 10A structure, but in montmorillonite with a smaller layer charge than vermiculite, K+ saturation produces only a partial layer collapse. (Sawhney 1972). In strongly acidic soils the tightly held H + and hydroxy aluminium ions prevent potassium ions from being closely associated with the colloidal surface, which reduce their susceptibility to fixation. As the ph increases, the H + and hydroxy aluminium ions are removed or neutralised, making it easier for potassium ions to move closer to the colloidal surface, where they are susceptible to fixation (Brady and Weil 2008) Magnesium Traditionally, the negative effect on hydraulic conductivity and infiltration rate due to soil chemistry has been attributed to exchangeable sodium, or low irrigation water salinity, or both. However, magnesium may have been partially responsible (Emerson and Smith 1970; Oster 2001; Rengasamy et al. 1986). Some of the irrigation water in many regions of the world contains a high concentration of Mg, which results in increased concentration of exchangeable Mg in the soil. In some soils the exchangeable Ca/Mg ratio is less than 1 (Shainberg and Levy 1992). Ca and Mg, have, for practical purpose, generally been grouped together as similar ions in maintaining soil structure when quantifying sodicity of soil and irrigation water (US Salinity laboratory Staff 1954). While several reports on laboratory experiments have shown little or no differential effect of Mg and Ca on soil structure, a few laboratory studies have shown that exchangeable magnesium can cause structural deterioration in some soils under specific conditions (Rahman and Rowell 1979; Zhang and Norton 2002). Quirk and Schofield (1955) observed that saturated hydraulic conductivity (Ks) of Mg saturated illitic soils was much lower than Ca saturated soils. Emerson and Smith (1970) reported a difference in the ease of dispersion of surface soils when saturated with Ca or Mg ions. They found that Mg-soil dispersed when remoulded at a water content of 15 percent by weight, whereas Ca-soil started to disperse at 20 percent. Alperovitch (1981) found that in a calcareous soil exchangeable Mg had no specific adverse effect on the hydraulic conductivity whereas in non-calcareous soils Mg causes a decrease in hydraulic conductivity. Exchangeable magnesium in soil can directly influence soils structural properties: the effect is known as a specific effect, which has been reported in soils dominant in clay mica, but not in smectitic and kaolinitic soils (Emerson 1977; Rahman and Rowell 1979). Direct negative effect results from the hydrated radius of Mg ion being 50% greater than that of Ca ion. Therefore, soil surface, where exchangeable Mg is present, will tend to absorb more water than where exchangeable Ca is present, which will tend to weaken forces that keep soil particles together, resulting in increase in clay swelling and dispersion (Oster 2001). Magnesium can have an indirect effect on soil structural properties by influencing higher adsorption of sodium than in calcium dominant soils (Rahman and Rowell 1979). Keren (1991) studied the effect of adsorbed Mg and Ca on soil erosion and infiltration rate on two soils exposed to rainfall, in the presence and absence of adsorbed Na. He concluded that the erosion rate of the soils was higher for the Mg soils than for the Ca soils. Moreover, the infiltration rate and the cumulative water depth required to reach a steady state infiltration rate were lower for Mg soils than for Ca soils. Adsorbed Mg by the montmorillonitic soil increased erosion and lowered infiltration rate, regardless of CaCO 3 present. The Ca aggregates were more stable than Mg aggregates, even in the presence of Na. Shainberg et al. (1988) concluded that the low hydraulic conductivity values of the Na-Mg smectite systems compared to the Na-Ca system are related to the effect of Mg on hydrolysis of these clays. In order to prevent structural problems in sodic soils, the high level of exchangeable Mg (Ca/Mg <1) needs to be minimised to maintain an electrolyte level above the threshold value for a particular soil (Rengasamy. 1986). In fact, Rengasamy et al. (1986) observed that redbrown earth soils with an exchangeable Ca/Mg ratio less than 0.5 have an adverse permeability problem in the presence of sodium. 6.8 Recent research on the role of different cations on soil structure The discussions in the previous sections ( and ) reveal that there is no consensus on the role of potassium and magnesium on soil structural stability. The views on theoretical basis for the effects of these cations also differ among the researchers. Our group worked on the hypothesis, described in earlier sections, that the interactions between polar water molecules and the charged clay particles are functions of the ionicity of bonding involved. The following section deals with the degree of ionicity of a clay-cation bond and how it affects clay behaviour and soil structural stability in water. 34

35 6. Physics, chemistry and biology of salt-affected Soils FIGURE Relationships between ionicity index of cations and a) relative turbidity and b) relative zeta potential. a) Relative turbidity b) Relative zeta potential R 2 = 0.93 Y = 2.752X R 2 = 0.84 Y = 1.717X Li 1.0 Li K Na K Na Mg 0.5 Mg 0.5 Ba Sr Ca Ca Ba Sr Ionicity index Ionicity and covalency indices for clay cation bonds Every heteronuclear bond contains a mixture of covalent and ionic character. Covalent bonding between a cation and an anion is favoured on the basis of their polarizability. Small sized, highly charged cations will exert a greater effect in polarizing anions than largesized and/or monovalent cations (Huheey et al. 1993). This is defined by the ionic potential (IP): IP = Z = R (6.6) where Z is the charge of the cation and R is its radius. Hardness and softness of a cation, based on hard and soft Lewis acid theory, are indicative of low and high polarizability, respectively (Sposito 2008). The softness parameter for polarizability of a cation is quantified by Misono s softness parameter (Y). Metal ions with low Y values are termed as hard Lewis acids and with large Y values as soft Lewis acids (Sposito 2008). If the anion were large and soft enough, the cation should be capable of polarizing it and the cation penetrating the anionic electron cloud would give a covalent bond (Huheey et al. 1993). Thus, the resultant ionicity or covalency of a cation bond with an anion will be influenced by the nature of the anion. For example, the ionicity of Ca 2+ in CaCl 2 is different to that in CaCO 3. Generally clays are very complex with a variety of crystal structures and charge distributions on clay particles. However, in the development of the model in this paper, we consider clays as large sized anions with high charge both charge and size being several times larger than the cations. The polarizability of a clay anion will be related to its softness, or the deformability of its electron cloud. Both increasing charge and increasing size will cause Ionicity index this electron cloud to be less influenced by the nuclear charge of the anion and more strongly influenced by the polarizing cation (Huheey et al. 1993).Therefore, the covalency or ionicity index of a cation alone will indicate the degree of covalent or ionic character of the clay cation bonds. Both the Misono softness parameter, Y, and the ionic potential, IP, are important factors in the complex formation by a cation. Combining the ionic potential (IP) and Misono softness parameter (Y) by way of multiplication, we derive the covalency index (CI) of a cation, defined as: CI = (I z / I z+ 1) Z 0.5 (6.7) The ionicity index (II) is then defined as: II = 1 CI (6.8) The values of ionisation potentials given in the Handbook of Chemistry and Physics (Weast 1978) were used in the calculation of the covalency indices and the ionicity indices of cations Li +, Na +, K +, Mg 2+, Ca 2+, Sr 2+ and Ba 2+ are given in Table TABLE Ionic potential, Misono s softness parameter, covalency index and ionicity index of the cations. Cation Ionic potential (IP) (nm -1 ) SOURCE: MARCHUK AND RENGASAMY 2011 Misono parameter (Y) (nm) Covalency index (CI) Ionicity index (II) Li Na K Mg

36 6. Physics, chemistry and biology of salt-affected Soils Ca Sr Ba SOURCE: MARCHUK AND RENGASAMY, 2011 Marchuk and Rengasamy (2011) showed that the ionicity indices of cations in clay-cation bonds were strongly correlated with the behaviour of homo-ionic clays in aqueous suspensions such as dispersibility, zeta potential of the dispersed clay and mean particle size of the clays in suspension (Figure 6.8.1). As the ionicity index decreases in the following order Li + > Na + > K + > Mg 2+ > Ca 2+ > Sr 2+ > Ba 2+, the tendency to covalency increases and hence, the proclivity to break the clay-cation bonds in water increases. Transmission electron micrographs (Figure 6.8.2) of Urrbrae soil clays (illite dominant) treated with chlorides of Na, K, Mg and Ca, confirm the relationship between ionicity index and dispersibility and also between particle sizes. While Na-soil is highly dispersed, Ca-soil clays are highly aggregated. Marchuk et al. (2012) characterised the changes in pore architecture in soils as influenced by the cations (Na, K, Mg or Ca) by using non-destructive X-ray computed tomography (µct) scanning. Pore architectural parameters such as total porosity, active porosity, and pore connectivity depended on the ionicity of cations bonded to soils. As the ionicity increased in the order Ca < Mg < K < Na, active porosity decreased (Figure 6.8.3). The relative hydraulic conductivity of the cationic soils also followed the same order and was highly related to active porosity values (Figure 6.8.4). FIGURE Transmission electron micrographs of Urrbrae soil clays (illite dominant) treated with chlorides of Na, K, Mg and Ca, and dispersed in deionised water after washing free of electrolytes. Na Mg K Ca FIGURE Active porosity (P act ) in a soil dominated by a single cation (Ca, Mg, K or Na). P act % % 17.9% 8.7% 2.6% Ca Mg K Na SOURCE: MARCHUK et al, Cation Ratio of Soil Structural Stability (CROSS) Traditionally, exchangeable sodium percentage (ESP) is used as a measure of soil sodicity and is related to soil structural degradation. Critical values of ESP to define soil sodicity differ in different parts of the world because several factors including electrolyte concentration, ph, organic matter content and clay mineralogy affect the ESP value above which clay dispersion or reduction in soil hydraulic conductivity occurs (Rengasamy and Olsson 1991). Furthermore, measurement of ESP (Rengasamy and Churchman 1999) is time consuming and costly, therefore, sodium adsorption ratio (SAR) measured in soil solution which is highly correlated with soil ESP is conveniently used as a measure of soil sodicity and, in part, the effects of sodium on soil structure. Recent reports draw attention to elevated concentrations of potassium and/or magnesium in some soils which arise naturally and also as a result of increasing irrigation with waste or effluent or recycled water in Australia. There is also a tendency in industries to use potassium or magnesium salts instead of sodium during recycling processes to prevent the increase in sodium concentration in effluents. High levels of potassium in piggery effluents have been reported by Kruger et al. (1995) and Smiles and Smith (2004) and also in effluents from different sources including winery wastewaters by Arienzo et al. (2009). Long term application of such wastewaters may lead to build up of exchangeable potassium in soils (Arienzo et al. 2009). Smiles (2006) reported that there is, on average, more water-soluble and exchangeable potassium than sodium across a range of soils in the Murray-Darling Basin. 36

37 6. Physics, chemistry and biology of salt-affected Soils Exchangeable potassium can also cause effects similar to sodium, but has been neglected because of low amounts usually present in salt-affected soils. Potassium, being a monovalent cation, can cause clay swelling and dispersion. Conflicting reports on the effect of exchangeable potassium either being equal to or less than that of sodium are found in the literature (see section for details). Early basic colloid studies showed an almost exact correspondence between the effect of sodium and potassium in simple aqueous suspensions of lyophobic colloids (Hunter 1993). The neglect of potassium and simple appeal to SAR to infer soil structural stability will be misleading and, to meet this need, Smiles and Smith (2004) suggested a Monovalent Cations Adsorption Ratio (MCAR), which includes Na+K in the calculation of SAR. Nonetheless Rengasamy and Sumner (1998), in their study on flocculating and dispersive powers of cations found that potassium is not equivalent to sodium in causing clay dispersion in soils, and Mg is not equal to Ca in their flocculating powers Development of CROSS Concept The development of the cation ratio of soil structural stability (CROSS) is analogous to the formulation of sodium adsorption ratio (SAR) and monovalent cations adsorption ratio (MCAR) which is defined as follows: MCAR = (Na + K)/ [(Ca + Mg)/2] 0.5 (6.9) where concentrations of Na, Ca and Mg are expressed as milli moles of charge/l. FIGURE Relationship between active porosity and relative hydraulic conductivity of cationic soils. K srel Mg Ca Rengasamy and Sumner (1998) derived the flocculating power of the prevalent cations such as Na, K, Mg and Ca on the basis of the Misono softness parameter responsible for hydration reactions and the ionic valence (see section 6.5). The derivations of both flocculating powers and the ionicity of clay-cation bond are based on same parameters such as ionisation potentials and valence of the cations which determine the water interaction with soil. Based on these principles, a ratio analogous to the MCAR but which incorporates the differential effects of Na and K in dispersing soil clays, and also the differential effects of Ca and Mg in flocculating soil clays, may be written as: Cations Ratio of Soil structural Stability (CROSS) = (Na K)/ [(Ca + 0.6Mg)/2] 0.5 (6.10) where the concentrations of these ions (Na, K, Ca and Mg) are expressed in milli moles of charge/l. The total concentration of the cations, together with this formula should parameterize soil structural effects of the relative amounts of monovalent and divalent cation in the soil solution more generally and effectively than any previous approach. The SAR model is based on the ratio law of Schofield (1947) which explains the adsorption of monovalent cations by soils from solutions containing both mono- and di- valent cations. The ratio law stipulates that the amount of monovalent cations adsorbed will be dictated by the ratio of monovalent ions to the square root of the divalent cations. CROSS is also developed on the basis of same ratio law, but the differences among monovalent and divalent cations are taken into consideration. Rengasamy and Marchuk (2011) and Marchuk and Rengasamy (2012) obtained good relations between CROSS and exchangeable cation ratio, although these relationships depended on soil type, particularly clay mineralogy and organic matter. Theoretically, exchangeable cations control dispersion-flocculation phenomena. However, as CROSS is related to the exchangeable cation ratio, it can be directly used to relate to the clay dispersion and associated soil physical properties. Thus, CROSS was found to be superior to SAR in relation to soil dispersibility, particularly in soils containing K and Mg more than Na and Ca (Figure 6.9.1). In these figures turbidity is used to represent quantitatively the amount of dispersed clay Na K Active porosity P act % y = x R 2 = 0.76 p= SOURCE: MARCHUK et al, 2011 In soils where Na and Ca are predominant and K and Mg are very low, CROSS will be similar to SAR in predicting clay dispersion. In other situations when all these cations are present in significant amounts, particularly when K > Na and Mg > Ca, CROSS will be more effective than either SAR or MCAR. Marchuk et al. (2013) also reported that the clay dispersion influenced by CROSS 37

38 6. Physics, chemistry and biology of salt-affected Soils FIGURE The relationship between turbidity and CROSS or SAR in soils containing significant amounts of K and Mg in addition to Na and Ca. Turbidity (NTU) 16,000 Y = x R 2 = Turbidity (NTU) 16,000 Y = x R 2 = ,000 12, SAR CROSS was dependant on the net charge on soil particles which is determined by the unique way in which clay minerals and organic matter are associated and the changes in soil chemistry. Threshold electrolyte concentration (TEC) and dispersive potential (P dis ) determined in each individual soil were highly related to CROSS (Marchuk and Rengasamy 2012). Soil hydraulic conductivity and soil pore architecture were found to be highly correlated with CROSS, than with SAR (Rengasamy and Marchuk 2011; Marchuk et al. 2013). Future work is needed to establish the critical values of CROSS and the electrolyte concentration so as to differentiate between dispersive (structural instability) and flocculated (structurally stable) soils. Sodicity and sodic soil were regarded as important in salt-affected soils because of the predominance of sodium salts. However, the roles of other cations becoming clearer, it is necessary, in future, to classify the soils as dispersive soils instead of sodic soils in order to consider the effects of all cations together Biology of salt-affected soils Organic matter There are a number of reports on the effects of organic matter and sodicity on soil structure (e.g. Emerson 1983; Churchman et al.1993), but studies are limited on the interactions between salts and organic matter in soils. In general, soils all over the world affected by sodium salts have very low organic carbon. Spain et al. (1983) reported that all forms of sodic soils in Australia had low organic carbon content than all other soils except sand and arid zone soils. They also found, by correlation studies, ph and rainfall are the major determinants of soil carbon in Australian soils. High ph (alkaline) and low rainfall lead to low soil organic carbon. As noted earlier, 72% of the salt-affected soils in Australia have alkaline ph (Northcote and Skene 1972). Setia et al. (2012), using a modified Rothamsted Carbon model, estimated that world soils currently affected by salts have lost an average of 3.47 t of soil organic carbon (SOC) per hectare since they became saline. Their modelling also suggested that world soils may lose 6.8 P g SOC due to salinity by the year The input of organic matter to soil depends on plant productivity which is very low, both in saline soils (where osmotic stress and ion toxicity affect the plant growth) and sodic soils (where poor soil aeration and, water retention and movement limit the growth). Loss of organic matter in high ph soils occurs by dissolution and subsequent leaching or erosion. Organic matter in alkaline-sodic soils dissolves easily and spreads over the surface and such soils are termed as black alkali in the literature (Figure ). Increasing solubility of organic matter is affected by, in addition to ph, sodium which forms weak bonds with organic matter compared to calcium. In saline soils, while organic matter input is low due to low productivity, the decomposition of organic matter by micro-organisms is restricted because of the low microbial activity caused by the salinity induced osmotic stress (Setia et al.2012). Whereas, in sodic soils loss of organic matter is related to high rates of mineralisation (Nelson and Oades 1998) Microbial activity Soil biota regulate a number of biological functions that directly affect above and below ground plant growth and indirectly through the effects on physical and chemical properties of surface soils. Biological activities in the soil 38

39 6. Physics, chemistry and biology of salt-affected Soils rhizosphere can have negative effects of soil borne plant diseases and positive effects on plant growth through mineralisation reactions. In general, increasing levels of salts adversely affect microbiological processes in soils, by decreasing the input of organic substrates through low plant productivity and directly affecting microbial activity. These include effects on carbon and nitrogen mineralisation and soil enzyme activities which are important for the decomposition of organic matter and release of nutrients to plants (Pathak and Rao 1998). Increases in soil salinity have been shown to decrease soil respiration rates and the soil microbial biomass. While osmotic stress usually limits microbial growth in saline soils, ion toxicities associated with high ph inhibit microbial growth in alkaline-sodic soils. The mineralisation and immobilisation of nutrients such as N and P are always low in salt-affected soils. The ability to form and maintain nitrogen- fixing nodules is severely impaired both in saline soils and alkaline-sodic soils (Rao et al. 2002). High salt concentrations in soil have been shown to reduce the hyphal growth of arbuscular mycorrhial fungi whereas some type of fungal pathogens were found to increase with increasing salt concentrations in soil (Rao et al.2002). Alkaline ph and high carbonate concentrations can also adversely impact on metabolic potential and the catabolic diversity of bacterial communities. Gupta and Rengasamy (unpublished), found that microbial activity measured as respiration rate was affected by increasing concentration of salt irrespective of whether the salt is of sodium or calcium, confirming the osmotic effect of salinity (Figure ) However, when salinity was induced by molar concentrations of NaCl and Na 2 CO 3, while increasing salinity decreased community metabolic diversity, CO 3 2- toxicity (or high ph) affected community metabolic diversity more than Cl -1 ( or lower ph) (Figure ). Both salt type and concentration significantly influence the activity and catabolic diversity of soil microorganisms. The development of new techniques in microbiology such as stable isotope probing (SIP) will allow, in future, to link community structure to specific functional activity in saline and alkaline soils and to identify novel organisms uncultured until now (Gupta et al. 2008) Soil ph in relation to salts Soil ph is a measure of hydrogen ion concentration in the soil solution and is expressed as negative logarithm of [H + ]. Low ph values (<5.5) indicate acidic soils and high ph values (>8.0) indicate alkaline soils, although in chemistry it is thought that ph < 7 is acidic and > 7 is alkaline. Hydrogen ion concentration in soil solutions is influenced by several chemical reactions of soil components with soil FIGURE An example of back alkali-spreading of dissolved organic carbon within a week of addition of bio-solids in a field soil in India with a ph of 10. PHOTO: TIM SETTER 39

40 6. Physics, chemistry and biology of salt-affected Soils water. Different combinations of cations (such as sodium, potassium, magnesium, calcium, aluminium, manganese and iron) and anions (such as sulfate, chloride, bicarbonate and carbonate) contributed by salts in soil usually dictate the resultant ph in soil solution. Organic substances can also affect soil ph. Similarly, the soil ph also influences the nature of different species of cations and anions. As a result, the forms and concentrations of nutrient ions in soil solutions available for plant uptake differ with ph. Also, the concentrations and forms of microelements (such as iron, aluminium, manganese, boron and molybdenum) will vary with ph, becoming toxic or deficient at certain ph values. Acid soils are generally formed in high rainfall zones where non-acid cations ( or basic cations viz. sodium, potassium, magnesium and calcium) in soils are heavily leached and when aluminium and iron (acid cations) from soil clay minerals react with soil water to produce hydrogen ions in solutions. Nitrate leaching can also cause soil acidification. Sulphur dominant soils also become highly acidic due to oxidation-reduction reactions. In some cases growing legumes continuously can decrease ph in non-calcareous soils. Alkaline soils are usually found in arid and semi-arid landscapes where salts of non-acid cations (Na, K, Mg, and Ca) and anions accumulate in the soil layers due to high evaporation and reduced leaching. Alkaline ph is caused by the predominance of bicarbonates and carbonates, while phosphates, borates and some organic molecules can also contribute to high ph. In the literature and text books, generally sodic soils are defined as having ph >8.5. In soils with high exchangeable sodium, it was thought that hydrolysis of Na resulted in OH - ions, thus increasing the ph. However, in Australia, sodic soils with neutral and acidic ph have been reported (Northcote and Skene 1972; Rengasamy 2010). When accompanying anions in soil solution are either Cl - or SO 4 2-, the ph is less than 8.The hydrolysis of either Na or K is negligible and the hydrolysis of anions mostly contributes to OH - or H +. While Cl - and SO 4 2- increase H +, HCO 3 - and CO 3 2- increase OH - (Table ). The reactions of CO 3 2- and HCO 3 - can be represented as follows: CO H 2 O <--> HCO OH - (6.11) HCO H 2 O <--> H 2 CO 3 + OH - (6.12) H 2 CO 3 <--> H 2 O + CO 2 (gas) (6.13) Soil ph is also controlled by the concentration of CO 2 in the soil atmosphere (see equation 4.5), in addition to the presence of CO 3 2- and HCO 3-. While the concentration of CO 2 in the atmosphere is about %, it can increase in soils, where biological processes are active, up to 0.5%. The alkalinity of a saline soil solution, which leads to alkaline soil ph, is based on the concentrations of various ions in solution. Equation 4.4 defines alkalinity only on the basis of HCO 3-, CO 3 2-, H + and OH - ions. Considering all inorganic species, alkalinity is defined (Sposito 2008) as follows: Alkalinity = [HCO 3- ] + 2[CO 3 2- ] + [H 2 PO 4- ] + 2[HPO 4- ] + 3[PO 4 3- ] + [B (OH) 4- ] + [OH - ] [H + ] (6.14) When carbonate and bicarbonate ions are predominant in soil solutions, carbonate alkalinity can be written as follows: Carbonate alkalinity = [HCO 3- ] + 2[CO 3 2- ] (6.15) FIGURE Microbial respiration as influenced by the salinity levels induced by molar concentrations of NaCl or CaCl 2. Note that at 0.1 molar levels, CaCl 2 has more osmotic pressure than NaCl. µg CO 2 C/5h FIGURE Metabolic diversity of biological community as influenced by Cl - (low ph) and CO 3 2- (high ph). NaCl and Na 2 CO 3 solutions were added in molar concentrations LSD (P<0.05) Control NaCl CaCl Control NaCl Na 2 CO

41 6. Physics, chemistry and biology of salt-affected Soils FIGURE Classification of soils on the basis of ph (water, 1:5) and the possible deficiencies and toxicities. ph scale Interpretation: Acidic Neutral Alkaline Highly alkaline Nutrient deficiencies & toxicities (Fe, Mn, Al) Ideal ph for crop growth Nutrient deficiencies, toxicities (CO 3 2-, HCO 3-, AIO 4 ) and sodicity Recommendation: Apply lime Add acidifying agent TABLE Soil ph (1:5, in water) measured after treating a non-calcareous soil with chlorides, sulfates, bicarbonates and carbonates of Na or K and washed free of salts. Anion Na-treated K-treated Chloride Sulfate Bicarbonate Carbonate SOURCE: P.RENGASAMY, E.TAVAKKOLI AND G.MCDONALD, UNPUBLISHED Micronutrient deficiencies and toxicities in alkaline soils depend on a certain ph range within the alkaline regime. For example, boron can be toxic in soils with ph < 9, and can be deficient in soils with ph > 9. At high ph values boron is adsorbed strongly by clays and not released in soil solution. Similarly at high ph, copper is found in low levels and molybdenum is highly soluble. Phosphorus and zinc deficiency is common in alkaline soils, ph determining the ionic species and solubility. Brautigan et al. (2012) have shown that Al (OH) 4 - species are predominant in soil solution only when ph is above 9 and these soluble species are toxic to plants, mainly affecting root growth. Incidentally, at these high ph values CO 3 2- concentrations increase and are toxic to plants. They have shown that Al toxicity is additive to CO 3 2- toxicity. Several factors leading to abiotic stress in relation to ph in alkaline soils are given in Figure Measurement of soil ph Soil ph is usually measured in soil water suspension. In Australia the common practice is to measure ph in 1:5 soil-water suspensions. Scientists dealing with acid soils prefer to measure soil ph in soils suspended in calcium chloride solutions. In reality, calcium chloride will induce more hydrogen ions by hydrolysis of chloride ions. Further, in alkaline soils, bicarbonate and carbonate ions will be precipitated as calcium compounds, masking the ph caused by these ions. Soils with ph in water more than 9 will have a value around 8 in calcium chloride solution and will lead to misinformation regarding ionic composition of soil solution in the field. Therefore, measuring ph in water suspension or extract is necessary for alkaline soils. FIGURE Factors causing abiotic stress in relation to ph in alkaline soils. Productive soils Increasing abiotic stress and decreasing yield of crops Lime (calcium and magnesium carbonates) occurs as insoluble solids Carbonate toxicity Increasing levels of carbonates in soil solution due to sodium carbonate Soil structural deterioration Increasing amounts of sodium carbonate leads to soil sodicity Nutrient deficiency Decreasing levels of calcium, magnesium and zinc. Boron deficiency when ph is above 9 Low levels of iron Microelement toxicity Increasing levels of aluminium, manganese, molybdenum and iron in soil solutions Soil ph 1:5 (in water) Managing high ph soils While acid soils are reclaimed by using lime, alkaline soils need acid input to reduce the ph below 8.5. Calcareous soils with ph around 8.0 have no chemical limitations described above. Amendments such as sulphuric acid, elemental sulphur and pyrites have been used to reduce alkaline ph. These are highly hazardous to use and also uneconomical, particularly in dryland agriculture. Gypsum application can reduce alkaline ph and also reclaim sodicity associated with these soils. In addition to gypsum application, growing legumes in crop rotation may help sustaining ph reduction. While more research has been undertaken in ameliorating acid soils, the knowledge about treating alkaline soils is very limited. In high ph soils using alkalinity tolerant species/varieties of crops and pasture can reduce the impact of high ph. 41

42 7. Soil processes affecting crop production in salt-affected soils 7. SOIL PROCESSES AFFECTING CROP PRODUCTION IN SALT- AFFECTED SOILS Crop growth responds to salinity in two phases: a continuous osmotic phase that inhibits the water uptake by plants due to osmotic pressure of saline soil solution lowering its potential energy (water always moving from a higher to lower potential energy levels); and a slower ionic phase when the accumulation of specific ions in the plant over a period of time leads to ion toxicity or ion imbalance (Munns and Tester 2008). Elucidation of these mechanisms as well as the development of plants tolerant to salinity, in many instances, are based on the evaluation of the genetic materials in simplified conditions such as solution culture, hydroponics or sand culture. Sometimes genetic materials are evaluated in lysimeter or micro plots filled with saline or sodic soils or in actual field conditions. However, the interactions between root-zone environments and plant responses to increased osmotic pressure or specific ion concentrations in the field are complicated by many soil processes such as soil water dynamics, soil structural stability, solubility of compounds in relation to ph and pe (electron concentration related to redox potential) and nutrient and water movement in soil. For example, Tavakkoli et al. (2010) found fundamental differences in salinity responses of barley varieties between soil and solution culture. In their experiments, Tavakkoli et al. (2012) showed that the diverse genotypic variation found in hydroponic studies did not correlate with pot and field experiments. The following discussion, as reported by Rengasamy (2010) and based on the results of experiments using soils either in pots or in the field, emphasises the soil processes to be considered in the evaluation of salt tolerance of plants and the activities related to plant development to adapt to saline lands. 7.1 Effect of osmotic pressure of soil solution on plants Rengasamy (2010) conducted pot experiments on wheat growth using a sandy loam soil and various electrolyte solutions such as NaCl, CaCl 2, Na 2 SO 4 and Hoagland nutrient solution, inducing different EC levels of the soil solution. The water content of soils in the pots was maintained at field capacity for the first 25 days of growth. The resulting differences in dry matter production after 40 days of growth indicated the continuous operation of osmotic effect as the EC of the soil solution increased from 0.7 to 41.0 dsm -1. The osmotic effect became dominant and severely restricted plant growth when the soil solution EC increased above 25 dsm -1 in this set of experiments. Below this EC value, ionic effects due to Na +, Ca 2+, SO 4 2- and Cl - were also evident. Tavakkoli et al. (2010) also found in their experiments with barley varieties that specific ion effects tented to be more important at low to moderate levels of salt (EC at field capacity up to 10 ds m -1 ) but osmotic stress became more important at higher salinity levels. They (Tavakkoli et al. 2011) also showed that high Na +, Cl - and NaCl separately reduced the growth of barley, however, reduction in growth and photosynthesis were greatest under NaCl stress and were mainly additive of the effects of Na + and Cl - stress. Kelly and Rengasamy (2006) produced a schematic diagram relating EC of soil solution (and related osmotic pressure of soil solution) on the basis of pot experiments and field observations and concluded that osmotic effect of salinity is an important factor in reducing yield under dry land conditions. Their schematic diagram (Figure 7.1.1) shows that the increases in effect of the osmotic pressure of the soil solution on plant yield were greater when the pressure raised over 700 kpa. It is clear from Figure FIGURE Schematic diagram of the effect of electrical conductivity (EC) and osmotic pressure of the soil solution on the yield of wheat. Yield (%) EC 1:5 of soil water (ds/m) Osmotic pressure (kpa) SOURCE: KELLY AND RENGASAMY,

43 7. Soil processes affecting crop production in salt-affected soils that osmotic effect, reducing the plant water uptake, reduces the yield to uneconomic levels when the soil solution osmotic pressure is above 1000 kpa. The data in Table support that the reduction in yield due to salinity is related to the percentage of available water not taken up by plants in soils affected by transient salinity. The data presented in Table are averages of observations in seven different soils in South Australia. The reduced water uptake by wheat due to salinity has also been shown in pot experiments (Rengasamy 2010). It was found in these pot experiments that as EC of soil solution increased, the unused water in the pot soils increased and a high percentage of unused water (89 96%) was evident when the soil solution EC was >22.6 dsm -1. TABLE Percentage of available water not taken up by plants in soils affected by transient salinity (average of several observations from 7 locations) in southern Australia. Average EC (ds m -1 ) of soil water under field conditions Percentage of available water not taken up by plants due to osmotic effect SOURCE: RENGASAMY, 2010 Munns et al. (2006) suggested that the two responses to salinity osmotic stress and ion-specific stress can occur sequentially, giving rise to a two-phase growth response. Using the model suggested by Munns et al. (2006) and Rengasamy (2010), growing wheat in pot soil treated with NaCl or Hoagland nutrient solution at different salinity levels, concluded as follows: (i) at a low level of salinity (7 dsm -1 ), the osmotic effect was continuous during the entire period of growth. However, after ~25 days of growth, there was a difference between NaCl and Hoagland solution, indicating the ionic effect of Na in reducing growth. This agrees with the conclusions by Munns et al. (2006); (ii) however, at a higher level of soil solution salinity (30.0 dsm-1), the osmotic effect was predominant and the ionic effect of Na was minimal. Thus, it appears that above a threshold value of soil solution EC, osmotic effect is the dominant mechanism and ionic effect may be significant at lower levels of EC. In saline soils, particularly when soil fertility level is low and nutrient deficiency is an issue, application of fertilisers alleviates the salinity stress on plants. Elgharably et al. (2010) found that, at low salinity, addition of nitrogen to a sandy loam soil increased the dry matter production compared with control (non-saline) treatment. However, at high salinity levels, addition of nitrogen was not beneficial because of increased osmotic pressure of soil solution. Similarly, adding a small amount of calcium has been reported to enhance salt tolerance of plants at moderate levels of NaCl salinity (Cramer 1992). However, our experiments have shown that even calcium can reduce the plant growth at higher concentration (Rengasamy 2010). 7.2 Soil water and osmotic pressure dynamics in the field as related to climate The soil salinity measured in the laboratory either as EC 1:5 (soil: water suspension) or EC e (in soil saturation extract) may be low to moderate because of high soil water contents. In the field, generally, soil water is near field capacity after rain or irrigation events. As the soil dries due to evapotranspiration, the salt concentration increases, as does then osmotic pressure of soil water. Concomitant changes in matric and osmotic potentials determine plant water uptake in the field. The influence of soil texture and type of clay on plant-available water compounds the effect of matric plus osmotic potentials (Rengasamy 2006). He showed that in a loamy soil, in the absence of salt (EC 1:5 of 0.16 dsm -1 ), the plants can take up water from the soil having 25 5% water content (field capacity to wilting point), but when the soil is saline (EC 1:5 of 1 ds m-1 or higher), the plants cease to take up water even when the soil dries only to 18% water content because the total water potential (matric plus osmotic) in the field soil, at that point, reaches 1500 kpa. In these experiments, while soil water matric potential was measured, osmotic pressure of soil solution was calculated using the following relationship between EC and osmotic pressure: EC of 1 dsm -1 = 36 kpa of osmotic pressure. In dryland cropping, fluctuating soil moisture level during the growing season is an important factor while considering the effects of transient salinity on crops. Actual salinity (EC) of the soil in the field and the osmotic pressure of the soil solution in the field can be calculated from the laboratory measured soil salinity (EC 1:5 ) and the percentage of gravimetric soil water content in the field by using the following equations (Kelly and Rengasamy 2006): EC of field soil water (ds m -1 ) = [EC 1:5 x 500] / % field soil water content (7.1) Osmotic pressure (kpa) of field soil solution = [EC 1:5 x 18000] / % field soil water content (7.2) These equations are not appropriate when sparingly soluble salts such as gypsum are present. The solubility 43

44 7. Soil processes affecting crop production in salt-affected soils of gypsum in water is 2 g L -1 but that of NaCl is 360 g L -1. For example, if a soil contains 3% salt as NaCl, the entire salt will dissolve in the soil water content range between 100 and 10%. Whereas, if a soil contains 0.2% gypsum, the solubility will vary from 0.2 to 0.02 g in the same water content ranges Therefore, if sparingly soluble salts are present, psychrometric measurement of osmotic potential will be necessary. Kelly and Rengasamy (2006) have presented a Table giving the percentage of available soil water not taken up by plants in different soil types due to osmotic pressure of soil water salinity in relation to laboratory measured soil salinity and gravimetric field soil water. In dryland cropping, changing soil water content during growing season is an important factor influencing the effects of salinity on crop production. During wheat growing season in 2003, we measured the soil water content of a clay loam (20 60 cm) layer of a calcareous sodosol (Natrixeralf) in South Australia, affected by transient salinity, on the first week of every month from April to November, including 3 different days in September when there was a prolonged dry period. The laboratory measured salinity, EC 1:5 was 1 dsm -1. The gravimetric soil water content (%), the measured matric potential and total water potential (which includes measured matric potential and the calculated osmotic potential using the above equation) are plotted in Figure If the soil was non-saline, the plants would have the ability to use the soil water during the entire period of growth as the lowest value of matric potential of soil water was 400 kpa. However, the soil being saline, water availability to plants diminished significantly FIGURE Gravimetric soil water content (%), matric potential of soil water (-kpa) and total (matric plus osmotic) soil water potential (-kpa) of a clay loam layer (20-60cm) in a Natrixeralf (with an EC 1:5 of 1dSm -1 ) during wheat growing season in Gravimetric soil water content (%) Apr May Jun Jul Aug 1 Sep 15 Sep Total water potential (-kpa) Sep Plant available water diminished Oct Nov 50 Soil water content Soil water total potential (matric plus osmotic) Soil water matric potential SOURCE: RENGASAMY, 2010 between 15 September and first week of October, because the total (matric plus osmotic) soil water potential ranged between 1000 and 1500 kpa. This occurred during the critical physiological period of wheat growth flowering and grain filling. Even though the wilting plants recovered after the rainfall in the second week of October, the final grain yield was reduced by 50% compared with the yield in non-saline soils in the district (Cooper 2004). In saline soils influenced by saline groundwater, commonly found in Western Australia, salt concentrations are high in topsoils in the beginning of the sowing season. Similar situation occurs in magnesia patches found in South Australia. High concentrations of salt combined with low rainfall in the start of the season severely affect the germination of seeds. As the season progresses and with normal rainfall, salt concentration may decrease and germinated plants can survive. However, in Western Australia, there is a probability of increasing soil salinity at the end of the season, when rainfall may be scanty. Thus, changes in salt concentration (and, hence, osmotic pressure) with the changes in soil water content during the growing season, as influenced by seasonal rainfall and temperature, have implications on screening and selection of salt tolerant crops. First, the salt tolerance criteria developed using hydroponics or lysimeter may not be valid under field conditions, particularly under dryland cropping. Second, the tolerance mechanism also may change during the growing season, ionic effect being valid at high soil water contents and osmotic effect gaining prominence at low soil water contents. Third, salt tolerance for germination should be considered as a screening method in the selection of varieties suitable for soils affected by high salinity in topsoils at the beginning of the season as experienced in magnesia patches or soils affected by rising water table. 7.3 Seasonal changes in transient salinity Transient salinity in soils, characterised by a high concentration of salts in the subsoil, varies with depth and changes throughout the season in response to rainfall, surface evaporation, water use by vegetation and the leaching fraction (hydraulic conductivity) of the clay layer. Analyses of several profile soil samples in southern Australian dry land regions have revealed a high correlation between sodicity (reflecting the reduced leaching fraction) and the salt accumulation. This relationship varies with seasonal rainfall as shown in Figure Seasonal changes in rainfall pattern and evaporation are important factors in the accumulation of salts in soil layers. Transient salinity is greater in regions 44

45 7. Soil processes affecting crop production in salt-affected soils with lower rainfall. It is also greater in soils with higher levels of sodicity. Transient salinity generally occurs in soil layers above the sodic clay layers in sodic soils (Rengasamy 2002a). 7.4 Other soil processes in relation to crop production in salt-affected soils The use of salt tolerant varieties to overcome salinity effects is highly desirable. Efforts by plant breeders in different parts of the world have produced salt tolerant plant species, particularly irrigated crops and pastures, that are being adopted by farmers. In dry land farming, fluctuating soil moisture is a major constraint in developing suitable varieties. Genetic yield increases in dryland environment have historically challenged plant breeders with genetic gain in yield being low compared with that made in irrigated crops (Passioura 2004), due to complex physiology environment interactions. Water uptake in many soils is often limited by the presence of several subsoil factors, in addition to salinity, such as physical, chemical and biological constraints (Rengasamy et al. 2003). Recently, Tavakkoli et al. (2010) found that the effects of salinity on two barley varieties Clipper and Sahara differed between the hydroponic and soil systems at comparable salinity levels. Genetic differences in growth, tissue moisture content and ionic composition were not apparent in hydroponics, whereas significant differences occurred in experiments using soil. It is important, as a part of soil management to alleviate salinity, that salts are leached below the root zone. In the case of soil categories, sodic, acidic sodic and alkaline sodic, poor soil structure and physical properties hinder the leaching process. Even in saline sodic soils, when the salts are leached below a threshold level, soil structure deteriorates and leaching is minimised (Rengasamy and Olsson 1991). Gypsum application to these soils improves the soil structure facilitating leaching of salts, even under dry land conditions. Kelly and Rengasamy (2006), in their field experiments, successfully leached salts up to a depth of 40 cm within a period of 3 years by gypsum application (5 t ha -1 ) to a sodic duplex soil (dryland) in South Australia with an annual rainfall ~400mm. In irrigation regions, the quantity of water applied should include leaching requirements. In dry land cropping areas when the rainfall is average or above, crop selection should allow a fraction of the captured water move below the subsoil so that salts are leached. In areas affected by transient salinity where the water table is deep, species with high transpiration can concentrate more salt in the root zone and hinder their production as well as other plants. In saline areas where the water table is shallow (~2 m), the same species may help in deepening the groundwater levels (Rengasamy et al. 2003). However, the increasing accumulation of salts will decrease plant leaf area indices and their transpiration rates. Thus, soil processes specific to each types of salinity dictate the strategies for plant-based solutions to different forms of salinity. Although sodicity is a major problem in salt affected soils, several soils have multiple problems in different layers in their soil profiles (Rengasamy et al. 2003). For example, the topsoil can be alkaline sodic while the subsoil is saline sodic. When a salt tolerant wheat variety was grown in this type of sodic soil, the yield was similar to that of a less salt-tolerant variety. On further investigation it was found that topsoil sodicity and alkaline ph (9.6) prevented the roots from reaching the saline subsoil layer (Cooper 2004), salt tolerance character of the wheat variety being not utilised. Variations in soil characteristics in topsoil versus subsoil, such as acidic sodic, neutral alkaline sodic, sodic saline and alkaline sodic acidic, have been commonly found in southern Australia. Further, spatial variability, including horizontal and vertical variations, of the soil characteristics imposes constraints in the soil management. In alkaline saline soils, alkaline saline sodic soils and alkaline sodic soils, the dominance of bicarbonate and carbonate species induces toxicity effects on plants. Although additional osmotic effect may be prevalent in alkaline saline soils, the soil structural problem may induce water logging in alkaline sodic soils. Because of increased solubilities due to changes in ph and pe ( log [e - ]) associated with waterlogged soils, element toxicities during water logging include Mn, Fe, Na, Al and B (Setter et al. 2004). Sposito (2008) has detailed the different ionic species of these elements in relation to proton and electron concentrations in waterlogged or flooded soils. With alkaline sodicity being prevalent in subsoils in Australia, water saturation of poorly structured subsoils (i.e. water logging) provides toxic amounts of these elements in the root zone environment. Further, alkaline ph induces severe soil structural problems than neutral sodic soils at comparable sodicity (SAR) levels. Our investigations show also that chemistry of aluminium and carbonates in soils is completely different when the soil ph is above 9.5 compared with ph between 8.2 and

46 8. Diagnosis and management of salt-affected soils 8 DIAGNOSIS AND MANAGEMENT OF SALT-AFFECTED SOILS 8.1 Dispersive soils Land managers with paddocks that are prone to waterlogging, poor crop or pasture emergence, gully erosion or tunnel erosion may be experiencing the dispersive soils influenced by sodium and/or potassium. Currently used indices [exchangeable sodium percentage (ESP) or sodium adsorption ratio (SAR)] to identify these soils focus only on sodium. However, the newly developed cation ratio of soil structural stability (CROSS) is a better index to identify dispersive soils if significant amounts of potassium and magnesium in addition to sodium and calcium are present in the soils ( see section 6.8). Not all poorly structured soils are dispersive. Land managers need to distinguish between poorly structured non-dispersive soils and dispersive soils as management differs for each situation. Visual symptoms (some shown in Figures 8.1.1, and ) that are typical of dispersive problem in the rooting zone of plants (i.e. down to cm) include poor infiltration and drainage resulting in waterlogging, increased runoff and poor water storage, surface crusting, poor emergence of crops and pastures, problems with cultivation and different forms of erosion. Tunnel erosion (Figure 8.1.2) can be a problem where force of water moving down can wash out soil, leaving cavities which eventually collapse to form gullies. 8.2 Estimation of cations to calculate SAR or CROSS Soil extracts either from a soil saturation paste or 1:5 soil-water suspensions are used to estimate the cations present. Detailed methodology to obtain the extracts and determine the cations, as well as exchangeable cations, in the laboratory are given in several text books; the one being followed in Australia is by Rayment and Lyons (2011). Using the concentrations of cations (mmol L-1) in soil solutions, SAR or CROSS can be calculated as follows: SAR = [Na + ] / ([Ca 2+ ] + [ Mg 2+ ]) 0.5 (8.1) CROSS = ([Na + ] + [0.56K + ]) / ([Ca 2+ ] + [0.6Mg 2+ ]) 0.5 (8.2) While SAR measured in either saturation paste extract or 1:5 soil water extract, has been found to be highly related to exchangeable sodium percentage (ESP), Rengasamy and Marchuk (2011) showed that CROSS, measured in 1:5 soil water extracts, was strongly related to the exchangeable cations ratio (ECR). ECR is defined as: ECR = (exchangeable Na + K) / (exchangeable Ca + Mg) (8.3) FIGURE Symptoms of poor soil structure in the field caused by clay dispersion a) Soil crusting and b) waterlogging. a) b) 46

47 8. Diagnosis and management of salt-affected soils FIGURE Tunnels developed in a dispersive soil (derived from Permian mudstone), northern Bruny Island, Tasmania. PHOTO: Marcus Hardie CROSS is preferred to SAR, because it includes the quantitative effects of all cations involved. Although CROSS is related to exchangeable cations ratio, it is directly related to soil physical properties such clay dispersion, soil structural porosity and hydraulic conductivity (Marchuk et al. 2012). Hence, there is no need to convert CROSS values into exchangeable cations ratio. Critical values of SAR and ESP have been reported to identify sodic soils. But, as discussed in previous sections, these values differ because of several soil factors. Critical values of CROSS to describe dispersive soils have not been established. However, evaluation of dispersive potential for an individual soil using the methodology given in Rengasamy (2002 b) and relating it to CROSS will be useful in the management of dispersive soils Dispersion (surrogate measure of cationic effects) In soils with significant amounts of adsorbed monovalent cations, clays disperse spontaneously into smaller particles when wet. The amount of dispersed clay is also affected by soil clay content, mineralogy, soil solution constituents, and organic matter content. A measurement of the level of dispersion can be used as an indicator of the relative dispersivity of a soil. Simple methods that can be used by farmers to assess dispersion in a soil is described in section 8.5. Emerson dispersion test (Emerson 2002), usually done in laboratories, is also very useful in the preliminary assessment of dispersive soils. 8.3 Managing dispersive soils containing Na and K Dispersive soils having significant amounts of sodium and potassium can be improved by adding calcium and/ or organic matter. Usually calcium is added in the form of gypsum (calcium sulfate). Lime (calcium carbonate) can also be mixed with gypsum when the soil ph (Water, 1:5) is low (less than 5.5), as lime will both add calcium and raise the soil ph to desired levels (6-7). However, lime is not recommended for soils with a ph above 8, as lime is almost insoluble under these conditions. The calcium amendments displace adsorbed sodium and potassium from the exchange complex improving the soil structural stability (Figure 8.3.1). This increases the moisture retention and hydraulic conductivity of soil layers. This in general will lead to increased yields and profitability. 47

48 8. Diagnosis and management of salt-affected soils FIGURE Soil structural deterioration in the field of a dispersive soil affected by sodicity near Horsham, Victoria, after a rainfall event. Organic matter in broad acre agricultural soils can be built up over time through the adoption of stubble retention and minimum tillage practices. Increased soil organic matter has a range of benefits including: reduction of erosion; increased water holding capacity; and improved biological and nutrient status of soils. Gypsum is available as mined gypsum (from natural deposits) and as phosphogypsum, an industrial by-product. The purity of the product used should be checked for the percentage of calcium and other nutrients (e.g. sulphur) required per tonne, and the concentrations of unwanted components (e.g. Cadmium). Some products can contain significant concentrations of cadmium, posing a risk to food quality. In soils that already contain lime, phosphogypsum has the added advantage of dissolution of calcium carbonate, which may further assist in reducing adsorbed sodium and potassium. Gypsum is generally applied to soils in relatively large amounts (2-10t/ha) with broadcasting equipment. Granular gypsum products are available, at higher cost, and can be applied with airseeder spreaders and are incorporated into dispersive subsoils by deep tillage. 8.4 Management of saline soils A soil is defined as being saline when the level of salinity of soil water (concentration of ions) affects plant growth. However, plants have different susceptibilities to soil salinity and appropriate crop selection will be a part of a holistic management strategy for dryland salinity. Soil salinity classes as used in Australia are given in Table TABLE Saline soil classes based on different soil textures and EC 1:5. Salinity class EC e (ds m -1 ) Sand EC 1:5 (ds m -1 ) Sandy loam Clay loam Clay Low < Moderately low Moderate Moderately high > SOURCE: KELLY AND RENGASAMY,

49 8. Diagnosis and management of salt-affected soils FIGURE Mechanism of reclamation of dispersive soils (containing Na and K) by using gypsum. Ca Dispersive soil Na Sodium K Potassium Ca Calcium Clay particle Ca Gypsum -Na -Na -Na -K -K Ca Ca Ca Rain Ca -Ca Reclamation of saline soils is achieved by the leaching of salts from the root zone layers. In irrigation agriculture, this is possible by providing effective drainage and using irrigation water free of salts. In dryland cropping, one has to depend on the rain and also improve the drainage of soil layers by improving soil structure. Again, addition of gypsum and enhancement of soil organic matter generally improve percolation of water through soil profiles. Installation of artificial drainage network (e.g. tile drainage or mole drainage) will also facilitate salt removal. In irrigated soils, the amount of water needed to remove the salts from soil layer is known as leaching requirement (LR). The salt tolerance of the crop to be grown, salt content of the irrigation water and the soil characteristics determine the leaching requirement. Generally, LR is calculated as: LR = EC iw / EC dw (8.4) Where EC iw is the electrical conductivity of the irrigation water and EC dw is that of drainage water. EC dw will depend on the salt levels tolerated by the growing crops. The salt levels and composition of the drainage water will also affect the environment where it is disposed Measurement of soil salinity Soil salinity is commonly measured as the Electrical Conductivity (EC) of soil water. EC is commonly given in units of ds m -1 (deci-siemens per metre) and is measured in soil using two methods: Saturated paste extracts (EC e ) or 1:5 soil: water extract (EC 1:5 ). It is important to know which measurement was taken as the different methods give different results. Much of the theoretical information relating to plants and soils is based on EC e, but EC 1:5 is a much easier measurement to make and is the method used commonly in Australia. Salt tolerance of crops and plants found in the literature is based on EC e. The factors for the conversion of -Ca -Ca Na K Na K Na Ca Ca Ca Non-dispersive soil Na, K and Ca ions are washed through the soil leaving calcium adsorbed to clay particles. ECbased on ECe. The factors for the conversion of EC1:5 to EC e, in to EC e, in soils of different texture, are given in Table , but the factors vary as reported in the literature. Other methods used to measure soil salinity include measuring total dissolved solids (TDS) in water or the soil water extracts (for Na salts, TDS is approximately equal to 640 x EC 1:5 ), apparent conductivity of bulk soil sensed by metal electrodes in soil (EC a ) and electromagnetic induction (EM) of an electric current using surface transmitter and receiving coils (EC* a ). These measurements have to be calibrated for each soil to convert these values to laboratory measured EC values. Modern instrumentation, now available, allows field measurement of bulk soil salinity with facilities of geopositioning system (GPS) making provision for soil salinity mapping of individual paddocks. EC a measured in the field is not only related to the salinity of the soil solution, but also to soil water content, texture and other soil properties that influence electrical conductivity. TABLE Approximate relationship between EC e and EC 1:5 for different soil textures. Texture EC e: EC 1:5 relationship Sand EC e = 15 x EC 1:5 Sandy loam EC e = 12 x EC 1:5 Clay loam EC e = 9 x EC 1:5 Clay EC e = 6 x EC 1:5 SOURCE: KELLY AND RENGASAMY, Calculation of Osmotic effect of salinity on water uptake by crops In field soils, crops have to take up water against both matric and osmotic potentials of soil water. Plants struggle to take up water when the total potential (matric and osmotic) of the soil solution exceeds -1000kPa and will permanently wilt at -1500kPa. Soil matric potential varies highly with soil texture. Estimation of matric potential is laborious and time-consuming. However, the electrical conductivity of soil solution can be used to calculate the osmotic pressure due to salinity. The soil salinity is estimated by measuring EC 1:5 (ds m -1 ) in the laboratory. Osmotic pressure of the soil solution in the field can be calculated by using this value and the gravimetric water content of the soil in the field, using the following equation: Osmotic pressure (kpa) in the field soil = [EC 1:5 (ds/m) x18000] % field soil water (wt/wt) (8.5) 49

50 8. Diagnosis and management of salt-affected soils TABLE Percentage of available soil water not taken up by plants in different soil types due to osmotic pressure of soil water salinity in relation to laboratory measured soil salinity and field soil moisture. Laboratory measured soil salinity Field soil moisture (%) below which osmotic pressure due to salinity is Percentage of available soil water not taken up by plants due to osmotic pressure (>1000 kpa) of soil water salinity EC 1:5 (ds/m) >1000 kpa >1500 kpa Sand Sandy loam Clay loam Clay Note: Field soil moisture is on the basis of gravimetric water content. The available soil water is calculated from the field capacity and wilting point for each soil type. It is assumed that soil salinity is due to highly soluble salts such as sodium chloride. These data are not valid when the salts present are sparingly soluble such as gypsum. SOURCE: KELLY AND RENGASAMY, 2006 The following Table gives the effect of osmotic pressure on soil water uptake by plants in relation to laboratory measured EC and field soil water content Gypsum and osmotic effects Gypsum is a salt of calcium and sulfate (CaSO 4.2 H 2 O) which can occur naturally in some soils. Farmers also apply gypsum to reclaim sodic soils and also as a fertiliser to supply sulphur and calcium to some crops. However, because of its low solubility in water, the osmotic pressure due to gypsum is always low. This is in contrast to the highly soluble sodium salts, which dissolve completely even in low soil moisture levels. As the soil moisture dries through the season, sodium salts are concentrated in soil water, whereas the solubility of gypsum is restricted. The osmotic pressure due to gypsum never exceeds 101 kpa (Table 8.4.4), irrespective of the amount of gypsum present in the soil. Hence, applying gypsum to soil, for example to amend sodicity, will have little impact on changing osmotic pressure and crop growth. TABLE Osmotic pressure caused by gypsum and sodium salts for different water contents Salt type Measured EC (ds m -1 ) Amount in soil (t/ha) Soil water content (%) Osmotic pressure of soil water (kpa) Sodium Gypsum SOURCE: KELLY AND RENGASAMY Assessing farm soils for salinity, dispersivity and alkalinity This section describes how farmers can make an assessment of: salinity; dispersivity (or sodicity); soil ph; and texture., based on the manual written by Kelly and Rengasamy (2006) and the readers are referred to this manual for detailed information. Farmers can sample and analyse soils with the procedures outlined in this section and can have confidence in the results obtained. Using this information, farmers will be able to make management decisions on their farms that will enhance productivity, profitability and improve long-term sustainability by limiting the effects of surface and subsoil constraints on their crops. There is a step-wise approach to the sampling, preparation and analysis of soil samples in the following order: soil sampling, air drying of soil, final preparation (crushing), soil water (1:5) suspension, measuring dispersion, measuring soil salinity, measuring soil ph and Identifying soil texture classes 50

51 8. Diagnosis and management of salt-affected soils Soil sampling and identification of transient salinity This soil sampling methodology has been established so farmers are able to undertake simple (on-farm) analysis of the soil to identify if there are any physico-chemical barriers limiting the crop s ability to exploit the soil for moisture and nutrients. It is important that care is taken when taking soil samples as cross contamination between different samples, depths and horizons down the soil profile (Figure 8.5.1) can influence results and hence, the selection of the best management strategies. Farmers should use their knowledge of a paddock to identify potential sites of concern for initial investigation. These concerns may include: low yield; patchy growth; water logging; disease prone areas. Growers can identify the presence of transient salinity by digging soil pits and taking soil samples at depth intervals of 0-10, 10-20, 20-40, and cm. Analysing these samples for EC will identify if there are salinity levels that are likely to be a problem for crops (Appendix 2). The presence of wet subsoil at harvest, especially after a dry finish, is another indication of some types of subsoil constraints and this soil should be sampled and checked for salinity. These tests can be performed on samples taken for deep nitrogen level analysis. FIGURE Soil profile showing the upper 1 metre with differentiation of various horizons (soil layers). Dig a hole 1m deep to look at the soil profile. The hole needs to be large enough to enable examination of the profile and collection of samples. 1 metre Dig Sticks (soil samples. One-metre 'Dig Sticks) can be a useful tool for the collection of samples. Take samples using a small hand spade, screwdriver or other tool. Samples should be approximately 600g (allowing for drying) and separate samples taken at depth intervals of 0-10, 10-20, 20-40, and cm. If there is a non-uniform band of soil in the profile then take an additional sample of this soil for analysis. Samples from different depths are to be kept separate. Unlike other soil samples (composite samples) taken by farmers for nutrient management, root-zone constraint samples from individual sites must be kept separate as we are looking for site and depth specific information Soil preparation Clearly label samples with: location; depth; date; and other relevant information. This will ensure that mistakes and confusion are avoided. Soil samples need to be dried as water contained in the fi eld sample could affect the results of analysis. More than 500 grams of dry soil is required for analysis and texturing, so allow enough soil to allow for weight loss due to drying. This will ensure that enough soil is available in the event of a mistake or accident. Samples should be air dried (about three days) by placing them on a clean plastic sheet in a dry place. Care must be taken to ensure contamination does not occur during drying. Contamination can affect results. Air drying is the preferred method of soil drying as heat can affect the measurement of dispersion. Crush any large soil aggregates to <2 mm in size and then store these soil samples in sealed airtight bags prior to analysis. Screen through 2mm mesh if available Remove obvious rocks and plant residues. Note: the strength of aggregates when crushing during soil preparation may give some indication of problems associated with high soil strength and compaction as a constraint to root growth and distribution Soil: Water (1:5) suspension for analysis For the analysis of soil dispersion, ph and salinity, a suspension with a soil: water ratio of 1:5 is required. That means that for each unit weight of dry soil, 5 times that weight of water is added. For this procedure, use a 500ml clear plastic container with leak-proof lid (height 15cm and diameter 6.5cm). 1. Weigh 80gm of prepared dry soil into a clean container 2. Gently pour the rain or distilled water (400ml) down the side of the container to minimise the disturbance of the soil. (Weighing water can be a useful method of water measurement, 1ml of water will weigh 1gm. In this example, you would weigh 80gm of soil and then add 400gm of water). 51

52 8. Diagnosis and management of salt-affected soils 3. Fill container to line or add 400ml of distilled water or rainwater. Do not stir or agitate the soil at this point. If farmers do not have the type of jars used in this demonstration they can use transparent jars of their choice. It is important that a container of suitable height is used to enable sufficient depth to be able to use the dispersion meter. You will have to experiment with the jars to get the amounts of soil and water correct, but the ratio of soil: water must be 1:5. In the field, pickle jars have successfully been used with 100gm of soil and 500ml of water. If rain water is used in this test, check for contamination of salt in the water. This can be done by measuring the EC of the water which should be below 0.1dS/m. If EC exceeds 0.1dS/m then distilled water should be used. High: Highly dispersive (SAR or CROSS > 7). (The indicated SAR or CROSS values are only approximate and will depend on many soil factors) Measuring soil salinity (EC electrical conductivity) It is important to calibrate EC meters prior to measurement of soil salinity. Once calibrated, the meters FIGURE Measuring dispersivity of a soil in water using a dispersion meter Measuring dispersivity with a Dispersion Meter Use the 1:5 soil: water suspension prepared in the previous section. Put the lid on the jar and screw it tightly closed. Invert the jar, slowly and gently, once and then return to its original position (avoid any shaking). Then let stand for four hours, without vibrations or bumping. Using the Dispersion Meter, measure the level of dispersion in the jar after removing the lid. (Information on dispersion meter is given by Kelly and Rengasamy 2006). 1. Looking down from vertically above the jar. 2. Lower the Dispersion Meter into the jar to the point where the white disc is no longer visible. It is important to identify the point where the disc is just no longer visible. Move the Dispersion Meter up and down gently so as not to disturb the soil at the bottom of the suspension. 3. Place a finger over the end of the Dispersion Meter and remove from the 1:5 soil water suspension (this will trap water in the dispersion meter), (Figure 8.5.2). 4. Read and record the level of dispersion. This is indicated by the level of water trapped in the dispersion meter: None, Low, Medium, or High. Water level indicates high level of dispersion vthe following diagnosis can be made on the basis of the readings: None: Non-dispersive (SAR or CROSS < 3) Low: Low dispersive (SAR or CROSS 3-5) Medium: Moderately dispersive (SAR or CROSS 5-7) 52

53 8. Diagnosis and management of salt-affected soils can be used to measure several samples in a session. Calibration instructions are contained in the EC meter pack. 1. After replacing the lid on the jar, vigorously shake the 1:5 soil : water suspension 15 times and allow to settle for 10 minutes. 2. Rinse the EC Meter electrode with distilled or rain water before measuring soil salinity. 3. Immerse the EC electrode into the suspension (be careful not to immerse the meter too deep into the suspension causing damage) (Figure g). 4. Whilst in the suspension and when the EC reading has stabilised (stopped changing), record the reading. 5. Rinse the EC Meter electrode with rain or distilled water before measuring the next sample. Osmotic effect due to salinity from the EC readings, based on soil texture, is given in the following Table TABLE Salinity classes (based on EC 1:5, ds m -1 ) and soil gravimetric water content (%) below which water is not taken up by plants due to osmotic pressure > 1000 kpa. Salinity class EC Sand Sandy loam Clay loam Clay soil water EC soil water EC soil water EC soil water Low Moderately low Moderate Moderately high SOURCE: KELLY AND RENGASAMY Measuring soil ph (Alkalinity or Acidity) It is important to calibrate ph meters prior to measurement of soil alkalinity (or acidity). Once calibrated, the meters can be used to measure several samples in a session. Calibration instructions are contained in the ph meter pack. 1. After replacing the lid on the jar, vigorously shake the suspension 15 times and allow settling for 10 minutes. 2. Rinse the ph meter electrode with distilled or rain water before measuring soil ph. 3. Immerse the ph electrode into the suspension. 4. Whilst in the suspension and when the ph reading has stabilised (stopped changing), record the reading. 5. Rinse the ph meter electrode with rain or distilled water before measuring the next sample. Soils are classified on the basis of the readings as acidic (ph < 5.5), neutral (ph ), alkaline (ph ) and strongly alkaline (ph > 9.0) Measurement of Soil Water Content There are two methods of soil water content measurement: Volumetric Water Content and Gravimetric Water Content. Gravimetric water content is the easier method for farmers to measure and the water content is expressed as a percentage or in units of g/g (wt/wt). The gravimetric water content will be expressed as a percentage (%): Gravimetric water content (%) = Mass of water (M w ) Mass of oven-dried soil (M s ) (8.6) Collect a sample of soil (approximately 100gm) from the field. This sample could be taken at the time sampling was undertaken for the assessment of transient salinity. This could provide information about the soil moisture content through the soil profile. Soils should be bagged in airtight plastic bags so moisture is not lost prior to measurement and oven drying should be undertaken as soon as possible. The soil should be placed in an oven proof pot and weighed. The pot should be of suitable size to hold soil sample and be able to withstand oven heat (e.g. small aluminium pie tray or ceramic bowl). Tare weight of the pot should be measured and recorded so that weight of wet and dry soil can be calculated. The sample should be dried in an oven at 105oC. Generally overnight will remove most of the water, however, with heavy clay soils it could take longer. The best method is to remove the pot containing soil from the oven and weigh. Return to the oven then repeat the weighing step in ½ hour. The soil has been dried adequately when there is no longer variation in weight between weigh-ins. Mass of oven-dried soil (M s ) = (Mass of oven dry soil and pot)- (mass of pot) (8.7) Mass of water (M w ) = (Mass of oven-dried soil) (mass of original soil) (8.8) Identifying soil texture classes by hand texture method Methods for identifying soil texture include wet-sieving the soil to divide it into different particle sizes, from which soil texture can be ascertained. However, there is a hand texture method (Figure 8.5.3) which gives results of sufficient accuracy for the management and amelioration of soils. The more practice you get hand texturing soils the more accurate the results. 53

54 8. Diagnosis and management of salt-affected soils FIGURE Identifying soil texture by hand texture method. 1. Add a small amount of water to a small amount of soil in your palm to make a ball about 4cm in diameter. 2. Keep adding small amounts of water until the ball starts to stick to your hand. Note the feel of the soil as you are working it: sandy, smooth, etc. 3. Make a ribbon by squeezing the ball between your thumb and fingers, trying to keep the sample in one flat strip. Note the length of the ribbon when it breaks. 4. You can estimate the soil texture from the table below to classify your sample into one of the following: Texture class Sand Sandy loam Clay loam Clay Properties of moist ball of soil Loose and single-grained with gritty feeling when moist. Not sticky and will not form a ribbon when pressed between thumb and index finger. Contains sufficient silt and clay to give coherence to the moistened soil, forms a fragile ball. Feels gritty and also slightly sticky. Will not form a ribbon. Has sufficient clay to form a smooth, spongy ball. Forms short ribbons of less than 3cm long. Extremely sticky and plastic when moist, feels a bit like plasticine. Easily forms a ribbon longer than 3cm. 54