Soil compaction – background

Page last updated: Tuesday, 13 March 2018 - 8:25am

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Subsurface compaction is a widespread constraint in Western Australian cropping areas. Estimates suggest that of 18.8 million hectares of agricultural land, 14 million hectares have a moderate to high risk of subsoil compaction.

Most of the common agricultural soils of the WA wheatbelt are at risk of subsurface compaction. Subsurface compaction is particularly common in the loamy and clayey sand textured Wheatbelt soils which have a wide range of particle sizes. In these soils, a tightly packed (compacted) layer at 10-40cm may develop as a result of agricultural practices, particularly if they are carried out when the soil is too wet. Deep sands along the south coast are also susceptible to compaction.

Good soil structure and higher levels of organic matter may reduce the degree of compaction. Higher levels of organic matter are only present in the surface soil and have little influence on subsurface compaction. Medium to fine textured soils are susceptible to plough pans, which occur just below cultivation depth. These may occur in addition to traffic pans which are deeper and occur in medium to coarse textured soils.

Soils with high susceptibility to compaction are characterised by having: Soils that are less prone to compaction include:
A good representation of a wide range of soil particle sizes - common in loamy and clayey sands and sandy loams High gravel content soils where the gravel helps support the weight of machinery
Soil particles that are rounded - coarser sand particles are more likely to be rounded Cracking clays that are only prone to compaction when wet and are self-repairing
Low organic matter (usually less than one percent organic carbon) in the topsoil - therefore likely to be a sandy soil Non-cracking clays are only susceptible to compaction when wet
Poor soil structure Shallow soils that are overlying rock or cemented ironstone gravels (ferricrete) at 30cm or less (this layer supports machinery but will also restrict root growth)
roots restricted in compacted sand
Roots restricted in compacted sand