Soil structure

The structure of the soil relates to how the mineral particles in the soil (the size of which defines the soil texture) are arranged, or stuck together. Lots of mineral particles stuck together in a clump are called ‘aggregates’ or ‘peds’. The mineral particles are stuck together with very small clay particles, or humus (which is derived from the organic matter in the soil). If we take two extremes of soil types, we can illustrate the range of different types of aggregates:

Very sandy soil

• If a soil is made up of lots of sand particles and little or no clay or humus particles then it is described as ’single grain’. This means that there’s nothing to stick the sand particles together in clumps, so they are all separate.

Very clay soil

• On the other hand, a soil which has a lot of clay in it and few larger particles (such as sand) is described as ‘massive’. Basically it exists as one big lump with few gaps in the soil.

Most soils are somewhere between the two extremes.

Why is structure important?

The structure of the soil, ie the arrangement of the soil particles into aggregates, is important in many ways:

Stability of the soil

If the soil is sandy or silty with few clay or humus particles it will have poor stability and therefore be easily eroded by wind or rain.

Ease of working the soil

Sandy soils are easily broken down into clumps of the right size, however it requires much harder work to break down clay soils (these are often partly cultivated in the autumn then left over the winter for the frost to break up the large clumps).

Capping

Finely structured soils (eg silt or sand) are prone to capping. The particles on the surface are easily broken down by rain (because they do not form strong aggregates) and the particles fall into any gaps in the soil surface and block them, causing an impenetrable cap to form on the top of the soil (which stops water getting into the soil and seedlings getting out).

Panning

Where the soil is clayey this can cause panning as the closely packed clay particles are smeared. The smeared particles are grouped so closely together that they stop water or roots penetrating below them. This can be caused by repeatedly digging to the same depth. Panning can also occur on very sandy soil if the particles are packed closely together underground.

Drainage

With clay soils which are packed closely together, drainage can be a problem. This can be further aggravated by panning or capping of the soil.

Water holding

Sandy soils, particularly those with poor structure (eg single grain), can drain very freely and therefore be vulnerable to drought.