Soil Structure

Yesterday, we looked at the texture of a soil – the sand, silt and clay particles in a soil. Now we turn towards STRUCTURE.

Soil structure can be regarded as the architecture of soil (the way it is built). By adding organic matter and water to the sand, silt and clay (and allowing for a good deal of time) ‘texture’ will develop into a larger soil structure called aggregates (or peds). These aggregates consist of many smaller particles that are held together. Two soils with the same texture can behave very differently depending on their structure.

Structure helps to control water movement, allowing excess to drain away and some to be retained within the pore spaces. Further, a good crumb structure is more stable against erosion by wind action during drought, because the fine particles are anchored by finer aggregates that are more bulky.

Soil structure is partly influence by particles being pushed closer together by freezing and thawing, wetting and drying, and by roots pushing through the soil as they grow in length and width.

Soil structure is also affected by the activities of soil organisms, especially earthworms. Roots and the hyphae of fungi and actinomycetes will also mesh particles together.

Soil structure can be blocky, platy, prismatic, columnar, angular blocky, sub angular blocky, granular.

Some loamy soils have aggregates that are small and crumb like. Some clay soils on the other hand have aggregates that are large and prism like.

Soil structure can deteriorate through:

• Cultivation
• Compaction
• Removal of vegetation
• Overmovement/working of soils

A good soil structure relies on humus…which will be the topic for tomorrow.

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