Soil structure

Many people confuse a soil's structure with its 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 that farmers and gardeners can do to modify the texture of the soil, they can influence the way the soil is structured for better or worse.

Structure names describe the size (fine, medium, large), shape (crumb, blocky, platy) and stability (weakly, moderately well developed) of the units into which the individual particles in a soil are combined and to the spaces between and within them - the architecture of the soil. The importance of soil structure is because it forms the plumbing system for the soil which controls water and air flows. It provides space and a protected home for roots, germinating seeds and soil fauna. Structural development affects farming operations - trafficability of machinery, response of the soil to tillage and stocking and ease of cultivation. It affects the impact of land use on the environment through the amount of run-off and erosion, the amount of nutrients lost in drainage, run-off and the amount of pollutants lost in erosion, run-off and drainage.The structure of a soil is the result of a number of different processes, such as freezing and thawing, wetting and drying, chemical processes, earthworms, plants, drainage, manure and liming, machinery and cultivation. The result of all these processes is a soil that often has finer aggregates at the surface and coarser ones at depth. All the processes have an effect on the topsoil while their effect at depth varies.Traditional agricultural practices have generally caused changes in soil structure which have compromised aggregation and porosity. Soil structure will become less porous and more aggregated under most forms of cultivation - the associated mechanical mixing of the soil compacts and sheers aggregates and fills pore spaces; it also exposes organic matter to a greater rate of decay and oxidation. A further consequence of continued cultivation and traffic is the development of compacted, impermeable layers or pans within the profile. Soil structure decline when waterlogged is usually related to the breakdown of aggregates and dispersion of clay material.

A wide range of practices is undertaken to preserve and improve soil structure. For example, increasing organic content by incorporating grassland into cropping rotations, reducing or eliminating cultivation in cropping and pasture activities, avoiding soil disturbance during periods of excessive dry or wet and ensuring sufficient ground cover to protect the soil from water impact. The benefits of increasing soil structure to 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 associated access by plants to soil moisture; improved emergence of seedlings due to reduced crusting of the surface, and; greater infiltration and associated available soil moisture due to improved porosity.