Chapter 19 Soil Fertility and Sustainability

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1 Chapter 19 Soil Fertility and Sustainability Introduction Soil properties can be conveniently, though arbitrarily, classified into physical, chemical and biological classes. However, in the real world, there is much interaction between the three. Soil properties result from past and present soil-forming processes (pedogenic processes) which in turn are governed by prevailing soil-forming factors (pedogenetic factors). This chapter looks in turn at key physical, chemical and biological soil properties. These properties can generally be recognised and studied in both the field and the laboratory. Physical, and to a less extent, biological properties are very obvious in the field. However, chemical properties require laboratory work, or at least reference to analytical tables in soil survey reports. Although each property is treated separately, a good discussion topic would be to look at interconnections between them. For example, the influences of soil organic matter on soil structure and soil water properties are obvious cases. A more indirect relationship is the effect of soil texture on soil ph via the effect on water movement through the soil and hence leaching. Exploration of these relationships also results in a better appreciation of the properties themselves. Chapter summary Physical properties: texture Soil texture is the property which describes the particle-size distribution of mineral particles. Three main size-groups are sand, silt and clay, together with subdivisions of the sand size-group. Textural name or class of a soil is determined by field inspection or particle-size analysis in the laboratory. Texture results from inheritance from the parent rock or parent material (sandstone versus clay, for example) plus modification by processes of weathering and soil formation (clay formation). Texture is a fundamental soil property which greatly affects many other basic physical (structure, water properties, temperature) and chemical characteristics (leaching, cation exchange capacity). Physical properties: structure Structure describes the state of aggregation of the solid material in soils in their arrangement into what are called either aggregates, structural units or peds. Unlike texture, structure involves, and is indeed much influenced by, the organic matter content of the soil.

2 The morphology of the aggregates formed depends very much upon prevailing processes of soil formation, and includes shapes described as: granular, crumb, platy, prismatic, columnar and blocky. The condition of no structure or structureless is used where there is no aggregation between individual soil particles, as in the case of massive and single grain. The Emerson model of the close association between humic colloids, clay colloids and quartz particles is the most comprehensive model of structure formation yet produced. Physical properties: water Soil water is held in, and flows through, the air space or porosity of soils. The classification of soil water into three main categories gravitational, capillary, hygroscopic is based upon the energy with which each is held by the soil solids, which in turn governs their behaviour and availability to plants. Soil water held at energy levels greater that the wilting point, i.e. greater than the energy with which plant roots can absorb it, is clearly unavailable to the plant. Soil water held at energy levels less than field capacity quickly drains from soil through the larger transmission pores. The ability of the soil to provide water for plants is an important fertility characteristic, and depends upon texture, structure, porosity and organic matter content. Physical properties: drainage and infiltration Pores of diameter greater than 0.05 mm allow excess water to drain away. Permeability describes the ability of the soil to allow water to pass through, and is governed by texture and structure, which in turn control the number of larger pores. The infiltration rate describes the speed at which water is able to enter the soil via the surface. Infiltration depends upon surface structure, especially cracking and crusting, and also upon the water content of the soil. An infiltration graph depicting the cumulative rate of infiltration over time can be constructed, using simple infiltration equipment in the field. Colloidal properties of soils: clay minerals Clay minerals are mineral colloids, less than two microns diameter, which are newly formed in soils by weathering processes. The basic building blocks of clay minerals are silica tetrahedra and alumina octahedra, which form sheets and hence the platy structure of clay minerals.

3 Substitution of the silicon (Si 4+ ) and aluminium (Al 4+ ) atoms by other cations leads to isomorphous substitution in the clay mineral lattice. Different species of clay minerals kaolinite, illite, montmorillonite, vermiculite, chlorite result from different arrangements of the silica and alumina sheets, and variations in isomorphous substitution. Clay minerals have large surface areas per unit weight, and can greatly affect soil properties by their swelling shrinkage characteristics. Colloidal properties of soils: cation exchange Clay minerals carry a net negative charge owing to a combination of isomorphous substitution in the lattice and broken bonds at the edges of the mineral. The net negative charge is able to attract positively charged cations, a property known as the cation exchange capacity. The process of cations being attracted and held at colloidal surfaces is called adsorption. The cation exchange capacity is expressed as milliequivalents per 100 g soil and is measured at ph 7. Above ph 7, hydroxyl groups on the edges of clay minerals become ionized, thus increasing the cation exchange capacity. Colloidal properties of soils: organic colloids Soil humus has a higher cation exchange capacity than clay minerals, owing to the reactivity of phenolic and carboxyl groups. As with clay minerals, the cation exchange capacity increases with increasing ph values. The structure of organic matter in soil is complex and varies from situation to situation. Acid organic matter or mor is little decomposed, whereas mull consists of well humified organic colloids. In addition to colloidal properties, humus is also a major nutrient in its own right, providing nitrogen, phosphorus and many trace elements. ph and soil reaction ph values in soil range from about 3.0 to 9.5, the higher values indicating increasing alkalinity and the lower values increasing acidity. ph has a great effect on the soil fauna and soil micro-organisms, and hence indirectly on many of the physical properties of soil. ph is very influential on the availability of plant nutrients, with the ph range being optimal for most plant nutrients.

4 ph values generally reflect the proportion of the colloid complex occupied by base cations rather than hydrogen and aluminum (the base saturation). Liming is employed in most farming in the temperate zone to counteract leaching and a lowering of ph. Soil fertility The fertility of a soil is its ability to support high yields of good-quality crops. Plant roots absorb nutrient ions from the soil solution, which immediately receives ions from the colloid complex to maintain equilibrium. Available nutrients are those which the plant root can easily absorb and include those nutrients in the soil solution and on the cation exchange sites. Unavailable nutrients are held in forms which the plant root cannot absorb and include nutrients in mineral structures, in insoluble chemicals and in complex organic molecules. Deficiencies and toxicities of nutrients cause poor growth, yellowing of the leaves and possibly the death of the plant. CASE STUDY Soil Hydrophobicity: the Ability of a Soil to Repel Water We have seen that the soil is a porous material and capable of holding almost its own weight of water in small pores or capillaries by strong capillary forces. This is achieved when the attraction between a water molecule and the soil is greater than the attraction between individual water molecules. Water is necessary for the growth of plants, and therefore the ability to allow water to infiltrate and be held against gravity is crucial for soil fertility. However, we now know that there are situations when the soil repels water. For example, peats in Ireland, when dried and comminuted for use in thermal power stations, are impossible to rewet, even with a hose! In hydrophobic soils, the soil surface is repelling water. (Non-soil examples include bird-feathers, grass, insects, and the human skin under some circumstances. Magic Sand is a play-room example!) Hydrophobicity is caused by organic coatings around soil minerals. (These are the same coatings which we have seen to be responsible for a stable soil structure.) Stefan Doerr (2007) has studied hydrophobicity in a variety of different soils. Studies in southern England and Wales show that many soil types are hydrophobic under a range of vegetation such as grass, heather, bracken and coniferous woodland. There appear to be two reasons to explain why hydrophobicity has only recently been recognised. First, most UK soils are covered by dense vegetation or crops, making it difficult to spot. Secondly, soil hydrophobicity appears not to be a permanent condition. Prolonged wet periods cause it to disappear, only to reappear during short dry spells. It becomes significant when soil moisture levels fall below a critical threshold, and the soil system flips from a wettable to a hydrophobic state. Medium textured soils such as loams switch over when the water content is 20-30% by volume. Using nano-microscopy, Duerr has discovered that the organic coatings are not continuous but form globules that expand and shrink

5 depending on the soil moisture content, a property which probably explains the switches between hydrophobic and wettable soil conditions. Hydrophobic soils cause three main problems. First, enhanced runoff and erosion can lead to flooding, even when the soils are dry and theoretically capable of storing more water. A special case is when wildfires produce a burnt landscape producing runoff and flooding, not only because of the loose ash surface, but also because of coatings of carbonised organic matter on mineral particles. Predicting when a soil falls below the critical moisture level that makes it hydrophobic is important for flood-forecast models. Secondly, for reasons not entirely understood, areas of preferential water flow can form within hydrophobic soils, and can transmit agricultural chemicals, i.e. pollution, to the groundwater. Thirdly, dry surfaces give reduced plant growth. Not all effects of hydrophobicity are negative, however. It can reduce surface evaporation, thus maintaining the subsoil in a moister state, and it can prevent the germination of plants that might compete with vegetation or crops. Reducing hydrophobicity is an economic and environmental necessity in some situations. Because in a hydrophobic soil water droplets sit on sand grains and organic hairs, like a bed of nails, the use of detergents such as non-ionic surfactants will lower the surface tension of the water drops, allowing them to penetrate the soil. Soil surfactants are used in horticulture to eliminate hydrophobicity from composts, in amenity land-uses like golf courses and sports fields, and in regions of the world prone to wildfire. Hydrophobicity will become more of a problem in the UK if more droughts cause soil moisture contents to fall below the critical threshold, and if the drought periods end with intense rainstorms. Reference Doerr, S. (2007) Fear of water: why do some soils repel water? Planet Earth Summer 2007, Essay questions 1. Distinguish between soil texture and soil structure. How do both influence the water-holding properties of soils? 2. Explain why different types of soil colloids (different types of clay minerals and of humus) have different cation exchange capacities. 3. Discuss the factors which control the infiltration and retention of water into soils. It might be useful to compare and contrast sand and clay soils. 4. What do you understand by structural stability when applied to soils? Under what conditions would you expect (a) soil blowing and wind erosion, (b) soil 5. crusting and baking? Discussion topics 1. Discuss how use of the soil for (i) farming and (ii) forestry can influence the soil reaction (ph). What remedies can be used to rectify any problems?

6 2. Discuss the advantages for soil fertility of farming systems which add organic matter to soils compared to those which do not. 3. Soil erosion - the rape of the Earth. Discuss how it arises, and how can it be controlled and prevented. 4. Mankind s most precious natural resource the soil. Discuss. Further reading Ashman, M.R. and Puri, G. (2002) Essential soil science, Oxford: Blackwell. A clearly written introduction to soil science which explains essential concepts by the use of innovative, everyday analogies in the text and illustrations. Brady, N. C., and Weil, R. R. (2004) Elements of the nature and properties of soils, Upper Saddle River, NJ: Prentice Hall. The latest version of this famous US textbook which ran into 13 editions! A very comprehensive and popular treatment. Clear exposition and student-friendly, although most examples are American. Fullen, M.A., and Catt, J.A. (2004) Soil management: problems and solutions, London: Hodder. A detailed yet accessible treatment of the latest research on soil management issues, including climate change and human health. Lampkin,N. (2002) Organic farming, Ipswich: Old Pond. The organic farmer s bible! Rowell, D.L. (1994) Soil Science: Methods and applications, London: Longman. A detailed discussion of soil physical and chemical properties. Many examples of practical work in the field and the laboratory are given. Royal Commission on Environmental Pollution (1996) Sustainable Use of Soil, London: HMSO. A select committee report on the impact of development on soils, on soils in relation to land use, and on soil pollution. Up-to-date, with many examples from the United Kingdom. White, R.E. (1987) Introduction to the Principles and Practice of Soil Science, second edition, Oxford: Blackwell. A clearly written text for the student with little background knowledge of soils. Wilde, A., ed. (1987) Russell s Soil Conditions and Plant Growth, eleventh edition, London: Longman. The classic work covers all aspects of soil science in an advanced manner. Web resources As well as being involved in mapping soils (see Chapter 18), the National Soil Resources Institute (NSRI), Cranfield University, has been active in many applied studies of soil use and sustainability, e.g. soil acidification, soil carbon balances, soil reclamation.

7 The website of the Department of Environment, Food and Rural Affairs (DEFRA) which gives advice to organic farmers on matters of techniques, husbandry, economics, and subsidies. As well as being involved in pedological studies (see Chapter 18), the Macaulay Land Use Research Institute (MLURI), Aberdeen, actively researches, and produces policy documents on, the sustainable use of soils for agriculture, forestry, wildlife conservation and recreation. The Soil Association is the foremost Non Governmental Organisation (NGO) which researches into, and promotes, organic farming in UK. It inspects farms and awards its reputable kite mark.

Soil Texture and Structure. Chris Thoreau February 24, 2012

Soil Texture and Structure. Chris Thoreau February 24, 2012 Soil Texture and Structure Chris Thoreau February 24, 2012 Soil texture refers to the relative amount of sand, silt, and clay found in a soil The mixture of these components affects the feel of the soil