- Nature of Structural Deterioration
- Organic matter breakdown
- Nature and role of organic matter
- Consequences of its breakdown
What Is Soil Structure?
Soil structure has been described as the "architecture" of the soil. The primary building materials of soil consist of mineral particles ranging in size from stones to sand and clay. Organic matter and clay bond the larger particles together and, in some soils, calcium or iron and aluminium compounds also act as bonding agents. Structure describes the size, shape and stability of the solid soil material and the size, shape and continuity of the spaces (pores) between the soil solids. The strength of the bonds holding the solid particles together determines the stability of soil structure and its potential to withstand the effects of external forces. The stability of soil structure can change due to increases or decreases in the quantity or quality of binding agents such as organic matter.
The relevance of structure to good agricultural management lies in the key role played by the soil pores. It is here that drainage of water, aeration and growth of crop roots occurs. The pores are also where soil organisms live. These organisms range from microscopic bacteria and fungi to earthworms and beetles. Soil pores are created by various processes including:
- shrinkage and cracking of soil caused by drying and rewetting and freezing and thawing cycles,
- death and decay of ramifying roots and other organic debris, and
- the burrowing activities of soil fauna such as earthworms.
In relation to agricultural use, the strength of soil structure is an important consideration. Soil strength describes the ability of a soil to withstand a force, or load, without collapsing or deforming. It is an important physical property since it determines the amount of soil compression (compaction) that occurs when surface loads (e.g. tractor traffic or grazing animals) are applied. Structural strength is greatly affected by soil water content. A soil can be malleable and plastic when wet and can, therefore, change shape when compressed. When dry, a soil can be very strong and difficult to fracture or shape.
Nature of Structural Deterioration
Under natural conditions, deterioration of soil structure is rare. It occurs along tracks used regularly by foraging animals, around drinking holes and where plant cover is removed, for example by fire. Where agriculture is practised, changes in soil structure are common. They can be divided into two main categories: (1) those associated with a net reduction in soil organic matter in the topsoil, and (2) those changes resulting directly from the reaction of the soil to an applied force (e.g. tractor traffic).
Organic matter breakdown
Nature and role of organic matter
Organic matter in soils is derived mainly from dead plant material. During microbial decomposition of this material some of it is converted into resistant humus material, which forms the bulk of soil organic matter. Soil humus binds strongly with soil mineral material and helps form a stable structure. In addition, some groups of carbohydrates in soils, known as polysaccharides, are involved in bonding particles into soil structural aggregates. These organic gums are formed by soil microorganisms during the decomposition of plant material. They are also formed by the soil microorganisms that congregate in the soil around the roots of plants (the rhizosphere) and by growing roots themselves. Filamentous fungi and fine roots can also enmesh soil particles and further enhance aggregate formation.
Soil organic matter content of soils in their natural state can vary greatly depending upon environmental conditions such as temperature, water content, pH, aeration and soil texture. Climate and soil texture are the most dominant influences. Organic matter decomposition increases up to about 30°C so that soils from cooler climates can have higher organic matter levels than those in warmer localities. Wetness is also important. For example, where soil remains wet and poorly aerated throughout the year decomposition is slow and peat may form. Soil texture has an important effect on organic matter content. As the clay content of soils increases, so too does organic matter. This is because some of the soil humus combines with clay to form stable complex structures where organic matter is protected from microbial breakdown.
Consequences of its breakdown
Compared with soils that support pasture, soils underlying arable land under a similar climate invariably have a lower soil organic matter content in the topsoil. There are two main reasons for this. Firstly, cultivation of the soil causes physical fracturing and mixing of soil, increased aeration, and, therefore, stimulates breakdown of soil organic matter. Secondly, less organic material is returned to the soil in the form of decaying root and shoot material under arable cropping regimes. This is because when annual crops are grown there are one or two periods of the year when there is no production of organic matter derived from plant growth. One period is during seedbed preparation and the other is after harvest. In addition, much of the plant dry matter is removed with arable crops through harvest of grain and burning of stubble.
As already noted, soil organic matter has an important role in binding soil aggregates together. When soil organic matter content declines there is also a breakdown in soil structure. A salient point is that when a regular sequence of crops is followed for some time, the organic matter content of a particular soil will settle to a steady value. A problem occurs when this steady state organic matter (and aggregate stability) level is so low that adverse soil physical conditions make it difficult or impossible to continue to grow arable crops.
A deterioration of soil structural stability may be observed in the field as a splitting-off of individual particles of soils from aggregates. This occurs because the bonds holding aggregates together become so weak (because of the low organic matter content) that they can no longer withstand disruptive forces (e.g. wetting and drying or raindrop impact). When exposed to heavy rain these soils may form a surface crust that impedes movement of water into the soil and/or emergence of seedlings through it.
Particularly under intensive horticultural management where soil organic matter content has run-down too far, "slumping" and "hard setting" can be problems. Where soils have very unstable aggregates they may slump back after ploughing and cultivation to the same density as before. This condition can be followed by hard-setting where the slumped soil runs together in a wet state and then dries as one complete dense mass. Where this occurs, it is difficult to prepare a good seedbed and crop development is often inhibited by poor root growth.
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