4.0 CROP CHALLENGE Understanding bleached subsurface soil horizons and potential management options in South West Victoria ARTICLES OF INTEREST 9.2 Understanding bleached subsurface soil horizons and potential management options in South West Victoria Christian Bannan¹, Simon Falkiner², Zoe Creelman³ and Jon Midwood³ 1South East Soil and Water 2Falkiner Ag 3Southern Farming Systems (SFS) KEY MESSAGES It is important to understand physical and chemical characteristics of soils across a property to help in selecting appropriate soil amelioration options and management methods The bleached soil layer between the A and B horizons is common across the Western District, impeding crop root development and influencing the water use efficiency from Agronomic management and drainage provide opportunities to better utilise water where it lands and minimise waterlogging Opportunities for improving yield potential and utilising additional subsoil moisture exist, including subsurface structural improvement, increasing crop root mass, greater soil moisture drawdown and drying, movement of topsoil, nutrients and organic matter to depth, utilising deep rooted species such as Lucerne to encourage this process and the incorporation of various forms of organic matter. INTRODUCTION SFS has been heavily involved in the research and development of the remediation of subsurface layers and their constraints in the Western Districts of Victoria. Much of this work has been previously targeted towards the treatment of sodic and heavy clay subsoils with the assumption that sodic clays provide constraints to plant growth. This assumption is based on standard soil classification and the assumption that sodic subsoils are a constraint to cropping systems (Isbell, 1992). There have been mixed results by site and across different seasons in South West Victoria with this subsoil amelioration trial work. Subsoil manuring is one of the treatments promoted for improving physical and chemical properties of sodic and heavy clay subsoils. The results are not always consistent (see report 8.4 in the 2015 SFS Trial Result Manual) which has raised our awareness of the need to become more site specific and apply a higher level of identification of a soil profiles physical and chemical characteristics, when deciding on where and when to apply this type of treatment. We also recognise that organic matter increases soil water holding capacity, often in cases where we are looking for improved internal profile drainage. The challenges with the use of organic amendments centre on supply, price and logistical issues which has forced us to look for alternative sources of organic amendments. Some of these include piggery eco-shelter litter, various types of compost and bio-solids, just to name a few. Opportunities for growing organic plant material and using this to ameliorant subsoils have been recently trialled. This process works on growing the material in situ, then harvesting and depositing the material within subsurface soil horizons. This report aims to touch on these developments, SFS s understanding of the soil issues that must be considered and evaluated in the process of deciding if a sub-surface ameliorant is suitable for a site and provide scope surrounding the practical and economic viability of such systems in South West Victoria. With growers asking questions on how to increase yield and water use efficiency in a changing climate, further investigation is warranted. SOILS IN THE HIGH RAINFALL ZONE OF SOUTH WEST VICTORIA Based on current soil classification we recognise that the soils in the Western Districts of Victoria are classified using The Australian Soil Classification system (Isbell, 1992). Classifications including as Sodosols, Ferrosols, Chromosols, Vertosols and others including Tenosols are found in this region. Research priorities are often based on these soil classifications and we assume soils within the classification system have a reasonable degree of uniformity. Testimonial evidence shows that a more specific understanding of individual soils is required in order to inform growers of site characteristics and variability in order to apply better management. Use of some older methods of identification and classification 158 Southern Farming Systems Trial Results Book 2015
compliment current standards for describing key components of soil variability which influence agricultural productivity (Northcote, 1992). For example, a focus on sodic soils is important and based on the current classification system, but of greater importance are the key characteristics of a particular sodic soil and how these can be improved for enhancing agricultural production. Extensive and large scale projects characterising soils and landscapes have occurred in the past by the likes of Gibbons and Downes (1964), Martin and Maher (1987) and Pitt (1981) and is still highly relevant for agricultural production across the region. This work helps us to put more modern soil classification into perspective for sub-regions. There are also many other technical reports both published and unpublished from this era that contain a high level of site detail and can aid decision making. Referring to a range of information sources both current and dated helps give a broader understanding of the soil types in southwest Victoria and their characteristics. While the Isbell soil classification system helpfully describes soil profiles based on their key physical and chemical properties, it is up to the individual to characterise a specific soil profile, define the magnitude and extent of these constraints and highlight the range of options available to the grower for potential improvements to productivity based on all resources available. We should question ourselves when we assess soil profiles to avoid a misunderstanding of a soils potential for cropping. Some examples include: Sodic soils and dispersive soils. There are many sodic soils, particularly sodic topsoils around the region which are nondispersive due to the retention of organic matter. There are also many dispersive soils which are non-sodic. The emphasis on soil chemistry in farming systems leads us to believe that the cation balance is the main issue however interactions between soil chemistry, organic matter and physical properties including texture are very important in the process of evaluation. The hostility of the bleached layer and the chemical and physical characteristics of this layer. For example, what is the percentage of buckshot in a bleached layer, what is the particle sizing range of the buckshot layer, what percentage of soil remains? Clay dominant subsoils. Based on soil tests, is the subsoil actually hostile just because it tests up as sodic? If we can dewater the subsoil (partial drying), shrink clay and create vertical cracking, can we achieve movement of topsoil, organic matter and nutrients downwards through cracks and line the skins of clay peds with soil material from above to buffer the plant root system and encourage root elongation? Complete identification of a soils physical and chemical properties is required in order to determine the soils capacity to grow crops under variable rainfall and soil moisture scenario s. Soil amelioration starts from the top-down as opposed to aiming too deep and missing problems in the surface horizons. We cannot expect to effectively change and improve the subsoil if we have not addressed the issues in the immediate A1 horizon topsoil and A2 horizon bleached layer. 18% 11% 5% Figure 1. Proportion of South West Victoria land area under dominant soil profile classes (Maher and Martin, 1987). 9% 4% 15% Hard, mottled - yellow duplex soils, prominent subsurface bleach Hard, mottled - yellow duplex soils, partial subsurface bleach Complex unit of Aa and Ab Hard, mottled - black duplex soils, partial subsurface bleach Soft - Firm, mottled - yellow duplex soils, prominent subsurface bleach Shallow friable loams 9.2 ARTICLES OF INTEREST Understanding bleached subsurface soil horizons and potential management options in South West Victoria Southern Farming Systems Trial Results Book 2015 159
4.0 CROP CHALLENGE Understanding bleached subsurface soil horizons and potential management options in South West Victoria ARTICLES OF INTEREST 9.2 SOIL ISSUES TO CONSIDER Based on SFS field observations, grower experience and published literature, the primary soil problems in our region influencing yield and water use efficiency include: 1. Drainage profile and surface 2. Waterlogging causes and relationship with soil physics and chemistry 3. Bleached soil layers 4. Clay dominant subsoil, infiltration and water holding characteristics 5. A horizon texture 6. Physical and chemical characteristics of the upper profile A starting point must be set which is site specific and not reliant on broad scale soil classification. The points above are just some of those that must be evaluated. Inspection of a series of sites also allows variability within the abovementioned parameters to be understood. In this paper the soil problem being discussed relates to point three, bleached soil layers BLEACHED LAYERS How are bleached layers formed? Bleached soil layers found in many soil profiles in South Western Victoria reflect areas where the hydrological balance is not in equilibrium with the rainfall of a set region. Much of this process is a naturally occurring phenomenon but has been exacerbated by the dominance of annual cropping and pasture based systems as opposed to perennial systems that once existed. Removal of native vegetation, cropping of annual species and the addition of irrigation water for extended months of the year cause ponding of water above subsoil clay, causing structural deterioration, deoxygenation and bleaching. A bleached soil horizon is one which exhibits a greyish colouration which can be associated with the presence of waterlogging. The bleach is commonly positioned in the lower A horizon where water remains perched when water which infiltrates rapidly throughout the topsoil rests above a subsoil clay with low permeability. The presence of a saturated zone and a low oxygen environment influences the formation of ironstone buckshot as a result of a reducing condition. Prolonged periods of saturation combined with soil chemical constraints such as high sodium and magnesium results in a poorly structured medium of high density. An example of the position of the bleach in the soil profile is shown in figure 2. SOIL CHARACTERISTICS A1 horizon Topsoil or tillage layer. High organic matter and plant root mass A2/A3 horizon Bleached layer. Varying levels of bleaching depending on texture of layers B horizon subsoil Medium and heavy clays. Yellowish and greyish, poorly drained WATER PERMEABILITY Moderate to high permeability depending on texture, structure and chemistry Variable permeability depending on texture, density, buckshot, chemical properties etc. Low permeability due to medium to heavy clay subsoils Figure 2. The typical position of a bleached A2 horizon in the Victorian Western Districts. 160 Southern Farming Systems Trial Results Book 2015
The soil profile in figure 2 is from the Inverleigh trial site, showing the bleached, buckshot layer between the topsoil but overlying medium clay subsoil. The bleached layer has set like concrete and has very limited plant roots and organic matter. In dry years capillary rise from the deeper clay is poor because there are minimal plant roots to extract moisture from the bleached layer. A soil pit from the Westmere Trial Site in 2015, found over 90% of the bleached layer to be ironstone buckshot and the remaining fraction consisted of heavy clay fines. Minimal soil within this zone ultimately places emphasis on what amelioration strategies are available in order to achieve best utilisation and higher WUE from a layer which is nearly all ironstone buckshot. Many of the strategies come down to sound agronomic management. How do bleached subsurface layers influence crop production - What does this mean for farmers? Bleached layers influence crop production by the following processes: The layer below the bleach is usually of poorly drained, restricting deep percolation of rainfall and an accumulation of moisture in the layer below the surface The bleach tells us where the soil will suffer from waterlogging on an average year The bleach accumulates cations by illuviation, these include sodium. Most bleached layers are of high density, causing limited pore space for plant roots to explore In wet seasons, saturation of the bleached zone restricts root elongation due to a low oxygen and limited pore space for roots to extend In dry seasons, high soil density and dry material in the bleached zone restricts the bulk of the crops root system to the surface topsoil layer above the bleach Ironstone buckshot and presence of iron oxides, an acidic environment and high exchangeable aluminium have a strong relationship with phosphorus tie-up Limited root growth into the bleached layer discourages the growth and deposition of roots and organic matter, which is often a key tool for buffer plants from adverse soil conditions The bleach is found in many other areas of south-eastern Australia and is not limited to the Western Districts of Victoria. The high rainfall zones are the most prominent areas for its occurrence and these include Gippsland, Tasmania, parts of the central Victorian Highlands and in north-east Victoria. Further north in Victoria and New South Wales bleached soil layers are usually found in the medium and low rainfall zones where deeper topsoil overlies clay of low permeability or in local drainage lines where water accumulates. Opportunities for correcting bleached layers and subsoil of low permeability A first step to the remediation of a clay-dominant subsoil should always address opportunities to de-water clay subsoil, shrink clay as it dries, create cracks and promote movement of topsoil from above to depth. This is normally recognised as self-mulching within a self-mulching clay soil type but the mechanism of topsoil movement deeper in the profile is not limited to this soil type only. The movement of nutrients and organic matter downwards is an important factor in improving subsoil conditions and this occurs with the movement of topsoil. The opportunity to dry and shrink clay may not be achievable if the bleached layer above restricts root elongation. Figure 3 is an example of heavy clay subsoil from Lake Bolac that shows signs of selfmulching after a lucerne phase in which the soil was de-watered and lucerne roots created channels through the dense layer. Figure 3. A block of dense subsoil at Lake Bolac showing signs of movement of soil from upper levels and root channels that were used by the following canola crop to reach into the subsoil Physical amelioration should always be a second approach because it occurs a high energy and labour input. However, if the capacity to dry a soil naturally is compromised by existing conditions the options of physical amelioration become viable options for long term soil remediation. Physical amelioration encourages an improvement in soil structure which can change the dynamics of the plants root system. Changing the wetting and drying process in a soil after physical amelioration are not instantaneous and may take a number of years. During dry seasons, plant roots will explore more soil if the hard pan is shattered, the bleached layer is modified and the number of roots exploring the deeper subsoil increase. These processes are important for changing the rate of infiltration and capillary rise into and out of deeper subsoil clay. 9.2 ARTICLES OF INTEREST Understanding bleached subsurface soil horizons and potential management options in South West Victoria Southern Farming Systems Trial Results Book 2015 161
4.0 CROP CHALLENGE Understanding bleached subsurface soil horizons and potential management options in South West Victoria ARTICLES OF INTEREST 9.2 In order to achieve improvements in soil moisture movement and root development from the surface to depth, soil horizons above the subsoil must be adequately ameliorated in order to improve water use and root development. A higher existence of plant roots in a bleached layer will allow for better moisture extraction, clay shrinkage and a greater capacity to refill the subsoil with rainfall. Soil structural improvement above subsoil clay is a key research priority for SFS in order to achieve improved use and productivity from increasing the bucket size our crops draw moisture from in difficult soils. Amelioration priorities we intend to address within soil types with bleached layers include: Mixing of organic matter to buffer adverse soil conditions and stabilise soil aggregates, which includes mixing of crop residues as well as exploring techniques for placement of organic matter Shattering aggregates to improve structure by increasing tillage depth Gypsum application for stabilising soil aggregates Lime application and mixing to improve ph conditions throughout the bleach, if acidic Increasing the volume of plant roots in the bleached layer, increasing the drying capacity to depth by the crop and utilising more subsoil moisture and allowing for a greater refill volume from rainfall Combinations of the above techniques. SFS intends to explore these opportunities moving forward and ensure there is compatibility between the amelioration technique and in-field soil conditions. Much of this work will result from precise field identification of the problems and best use of equipment to undertake work. A possible approach to placing organic material into the bleached layer Where a bleached soil material exists above subsoil clay the material tends to be poorly structured and contains minimal plant root mass. This restriction appears to be limiting root movement into the B1 horizon. SFS has been researching methods of encouraging root development through the bleached layer and into the subsoil. Part of the GRDC funded Pastures in Crop Sequences project was around using organic matter grown in situ as a soil amendment. Figure 4 shows the pea/oat mix that was grown, cut and buried in a trench at depth. Figure 4. Standing pea/oat mix that was later slashed and incorporated at depth In the subsequent crop, we observed roots growing towards the amendment strips (figure 5). It cannot be said exactly why this occurred, whether it was because there was more moisture in the amendment chamber, or a nutritional difference, or it was the path of least resistance because of the physical impact of trench incorporation. Most likely it was a result of a combination of factors. If a better connection can be made between the soil horizons, it is hoped that we will increase the bucket size and gain access to a larger volume of stored soil moisture. By placing organic amendments in the interface between the two horizons it is thought that the crop may now have channels for root movement that are not bounded by strongly differentiated soil horizons, with the aim that subsoils can be explored by plant roots. Figure 5 shows the root system of the subsequent crop, where plants appeared to actively track towards the amendment. 162 Southern Farming Systems Trial Results Book 2015
Figure 5. Trial plot showing the condition of organic amendment applied in 2014. Plant roots are migrating towards the organic materials placed at 30-40 cm. While figure 5 showed root systems where organic amendments were mechanically placed at depth, similar observations have been made where previous crops have biologically drilled through the profile. Figure 6 is photograph showing the roots of a barley plant using decaying lucerne roots to bridge the gap between the surface layer and subsoil clay. Deep rooted crops also extract water from depth which promotes de-watering of subsoils and increases the likelihood of cracking and self-mulching of soils. Figure 6. Decaying lucerne root channel being explored by the roots of a barley plant. The lucerne channel is highlighted with the white line. Amendment REFERENCES AND FURTHER READING The aim of this work has been to develop agronomic management strategies to promote drainage and smoother soil texture transitions. The objective being to improve the bleached B horizon that is widespread across South West Victoria. Guide to evaluating soils We started by acknowledging the importance of understanding your soils to understand agricultural potential and tailor management strategies. To assist in this process, the following questions have been raised based upon the review of work by Martin and Maher (1987) and their map of the dominant soil profile classes. Key soil profile features to consider in evaluating soils include: Is the soil duplex or not and/or do variable soil horizons exist? What soil textures are associated with duplex conditions? What is the depth, thickness and chemical characteristics of each layer? If a bleached subsurface layer exists, what are the physical and chemical properties of this layer? What are the colour and drainage characteristics of the B horizon subsoil? Are there issues with soil hardness and associated density? Amendment The widespread issue of a bleached layer in Western District soils highlights that existing classification systems do not replace assessments of individual physical and chemical soil characteristics. SFS is working to better understand soil constraints to agricultural production across the region with small plot trials and farm demonstrations. The objective of which is to investigate the potential for remediation of the bleached layer to promote better root development and self-mulching of heavy subsoils. Gibbons FR and Downes RG (1964), A Study of the Land in South-Western Victoria, In: Technical Communication 3, Soil Conservation Authority, Victoria. Martin JJ and Maher JM (1987), Soils and Landforms of South Western Victoria. Part 1. Inventory of Soils and their Associated Landscapes, Department of Agriculture and Rural Affairs, Victoria. Pitt AJ (1981), A Study of the Land in the Catchments of the Otway Range and Adjacent Plains, In: Technical Communication 14, Soil Conservation Authority, Victoria. Isbell RF (1996), The Australian Soil Classification, CSIRO Publishing, Victoria. Northcote KH (1992), A Factual Key for the Recognition of Australian Soils, Rellim Technical Publications Pty Ltd, Coffs Harbour, New South Wales. 9.2 ARTICLES OF INTEREST Understanding bleached subsurface soil horizons and potential management options in South West Victoria Southern Farming Systems Trial Results Book 2015 163