Soil structure

Soil structure is the arrangement of the particles in the soil. It is determined by the way in which individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them. Soil structure has a major influence on water and air movement, biological activity, root growth and seedling emergence. Soil structure changes over time and is non-uniform in space. Soil structure controls movement of water, ions, solutes, clay and anthropogenic compounds.

Overview

The network of pores within and between aggregates greatly influences the movement of air and water, the growth of plant roots, and the activities of soil organisms, including the accumulation and breakdown of organic matter.

Soil structure describes the arrangement of the solid parts of the soil and of the pore space located between them (Marshall & Holmes, 1979). The structure depends on what the soil developed from. The practices that influence soil structure will decline under most forms of cultivation—the associated mechanical mixing of the soil compacts and shears aggregates and fills pore spaces; it also exposes organic matter to a greater rate of decay and oxidation (Young & Young, 2001). A further consequence of continued cultivation and traffic is the development of compacted, impermeable layers or pans within the profile.

Soil structure decline under irrigation is usually related to the breakdown of aggregates and dispersion of clay material as a result of rapid wetting. This is particularly so if soils are sodic; that is, having a high exchangeable sodium percentage (ESP) of the cations attached to the clays. High sodium levels (compared to high calcium levels) cause particles to repel one another when wet and for the associated aggregates to disaggregate and disperse. The ESP will increase if irrigation causes salty water (even of low concentration) to gain access to the soil.

A wide range of practices are undertaken to preserve and improve soil structure. For example, the NSW Department of Land and Water Conservation, (1991) advocates: increasing organic content by incorporating pasture phases into cropping rotations; reducing or eliminating tillage and cultivation in cropping and pasture activities; avoiding soil disturbance during periods of excessive dry or wet when soils may accordingly tend to shatter or smear, and; ensuring sufficient ground cover to protect the soil from raindrop impact. In irrigated agriculture, it may be recommended to: apply gypsum (calcium sulfate) to displace sodium cations with calcium and so reduce ESP or sodicity; avoid rapid wetting, and; avoid disturbing soils when too wet or dry.

Three Broad Categories of Soil Structure

1. Single-grained: Soil is broken into individual particles that do not stick together. Stimulates rapid water infiltration.

2. Massive: Soil has no visible structure, is hard to break apart and appears in very large clods. Clods are the compressed, cohesive chunks of soil that form artificially when wet soil is plowed or excavated. Stimulates slow water infiltration.

3. Aggregated: Soil particles are associated in peds (aggregates). Stimulates slow water infiltration.

Types of Soil Structure

Granular: <1.5 cm in ∅. Usually in surface horizons with roots. Stimulates rapid water infiltration.
Prismatic/columnar: vertical columns (up to 15 cm). In B horizons. Stimulates moderate water infiltration.
Platy: Thin flat plates that lie horizontally. Usually in compacted soils, or clay deposits. Stimulates slow water infiltration.
Blocky: irregular blocks of 1.5-10.0 cm in ∅. Stimulates moderate water infiltration

Effect of soil structure

The graph below shows the generalized relationship between the soil water potential curve of a representative compacted soil (continuous line) and aggregated soil (dashed line).

Impacts of improving soil structure

The benefits of improving soil structure for the growth of plants, particularly in an agricultural setting include: reduced erosion due to greater soil aggregate strength and decreased overland flow; improved root penetration and access to soil moisture and nutrients; improved emergence of seedlings due to reduced crusting of the surface and; greater water infiltration, retention and availability due to improved porosity.

It has been estimated that productivity from irrigated perennial horticulture could be increased by two to three times the present level by improving soil structure, because of the resulting access by plants to available soil water and nutrients (Cockroft & Olsson, 2000, cited in Land and Water Australia 2007). The NSW Department of Land and Water Conservation (1991) infers that in cropping systems, for every millimetre of rain that is able to infiltrate, as maximised by good soil structure, wheat yields can be increased by 10 kg/ha.

Formation of Soil aggregates

Both biological and physical-chemical (abiotic) processes are involved in the formation of soil aggregates. The physical-chemical processes of aggregate formation are associated mainly with clays and, hence, tend to be of greater importance in finer-textured soils. In sandy soils that have little clay, aggregation is almost entirely dependent on biological processes. Most important among the physical-chemical processes are (1) flocculation, the mutual attraction among clay and organic molecules; and (2) the swelling and shrinking of clay masses. Except in very sandy soils that are almost devoid of clay, aggregation begins with the flocculation of clay particles into microscopic clumps, or floccules.

References

This article incorporates text from this source, which is in the public domain: http://soils.usda.gov/technical/manual/contents/chapter3.html

Cockroft, B & Olsson, KA 2000, Degradation of soil structure due to coalescence of aggregates in no-till, no-traffic beds in irrigated crops,
Australian Journal of Soil Research, 38(1) 61 – 70. Cited in: Land and Water Australia 2007, ways to improve soil structure and improve the productivity of irrigated agriculture, viewed May 2007, <http://www.npsi.gov.au/>
Department of Land and Water Conservation 1991, "Field indicators of soil structure decline", viewed May 2007
Leeper, GW & Uren, NC 1993, 5th edn, Soil science, an introduction, Melbourne University Press, Melbourne
Marshall, TJ & Holmes JW, 1979, Soil Physics, Cambridge University Press
Soil Survey Division Staff (1993). "Examination and Description of Soils". Handbook 18. Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture. Retrieved 2006-04-11.
Young, A & Young R 2001, Soils in the Australian landscape, Oxford University Press, Melbourne.
Charman, PEV & Murphy, BW 1998, 5th edn, Soils, their properties and management, Oxford University Press, Melbourne
Firuziaan, M. and Estorff, O., (2002), "Simulation of the Dynamic Behavior of Bedding-Foundation-Soil in the Time Domain", Springer Verlag.

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