A GUIDE TO BETTER SOIL STRUCTURE
Many people tend to confuse a soil structure with texture. A soil’s texture is the bricks (a mix of sand, silt and clay), which when stuck together with organic matter and other natural “mortar” make up the larger all-important structural blocks. The structure of the soil is the arrangement of blocks around which the roots grow and air and water move. Just like our houses, a soil is made up of a number of different ‘building’ blocks, which are described according to their shape and size using fairly easily defined terms such as blocky or granular, fine or medium. While there is little farmers can do to modify the texture of the soil, they can influence the way the soil is structured.
Soil structure changes naturally due to weather related factors such as wetting and drying and freezing and thawing. Thus the different composition and orientation of the structures within a soil vary with depth and also crop stage during the season. Recent research by Cranfield University at Silsoe shows that the extent of a soil’s natural deterioration during arable cultivation over the season is largely governed by its texture. The results suggest that texture is as important as organic matter and root exudates (natural sticking compounds). If a soil has a high clay content, the structures are more tightly formed and thus less likely to be broken apart and to slump. Natural processes, such as freezing and thawing in the topsoil, can also help these soils to recover. Sandy and silty soils have less well defined structures and are more likely to slump, especially if they have been subject to excessive cultivation and have been weakened.
Structure stability can be defined as the resistance of the soil structure to external factors such as water. Soils that naturally have a good structure in the long term have a ‘stable’ soil structure; those that would naturally lose all aggregation have an ‘unstable’ structure. Maps prepared by SSLRC show the distribution of stability in topsoils based on a classification of topsoils according to their risk of slumping.