Abstract

Soil structure is defined as the arrangement of particles and associated pores in soils across the size range from nanometres to centimetres. Biologic influences can be demonstrated in the formation and stabilization of aggregates but it is necessary to distinguish clearly between those forces or agencies which create aggregations of particles and those which stabilize or degrade such aggregations. The formation of soil structure involves the physical forces of shrinking and swelling created by changes in water status of soils, freezing and thawing, tillage, or by movement of the larger biota in soils. Expansive properties of soils are controlled by the clay content. Thus changes of structural organisation are minimal in sands and maximal in clays. Plant roots, earthworms and other macrofauna large enough to move soil particles create pores recognisable by cylindrical shapes and smooth curved surfaces. Various visual and microscopic techniques aided by dyes are available to demonstrate the extent of biovoids in soils. Biology plays a major role in stabilization of soil structure. The major factors vary depending on the scale of soil structure. At larger scales plant roots and associated hyphae can be seen to enmesh soil particles by acting as a "sticky string bag". At the microscale the influence of mucilages from roots, hyphae, bacteria and fauna such as earthworms can be shown by a range of microscopic techniques to be involved in stabilizing smaller aggregates and the linings of biopores. Techniques include optical and fluorescence microscopy, scanning electron microscopy including EDAX, transmission electron microscopy using heavy metals or other electron dense staining techniques for specific chemical compounds, and computer aided tomography. The microscopic techniques can be used on individual aggregates, stabilized soils, sections or separates of soils. Both microflora and fauna are involved in the degradation of stabilizing agents. Fauna may comminute roots and hyphae which stabilized larger aggregates and microorganisms utilize mucilaginous stabilizing agents as an energy source resulting in a slow breakdown of structural stability. Such effects can be established by combinations of studies of aggregation including microscopy. Further destruction of structure is caused by tillage and compaction by vehicles and animals.