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April's issue of The Irish Garden








Soils : Soil structure

Improving soil structure

The bonding action of humus and clay on the other soil constituents causes soil ‘crumbs’ to form. These are constantly forming, breaking up, and reforming because the bond, though strong, is not perfect.

Well-structured soil is easy to dig

Well-structured soil is easy to dig

The crumbs loosely bond to neighbouring crumbs but because they do not fit tightly together, spaces exist in between. These spaces, or pores, vary in size – the smaller ones fill with water, which sticks to the crumbs and provides a reservoir for plant roots.
The larger pores initially fill with water too, but this drains away and is replaced by air.

Good structure exists when there is extensive crumb formation, and the crumbs are arranged so that there is plenty of pore space for water and air. Poor structure exists in a soil where the soil constituents have not formed into crumbs, or have formed into very small, tight crumbs.

In poorly structured soil, there is little or no pore space and, though there is probably plenty of water, there will be very little air. The result is a heavy, wet, hard and lumpy soil.

Improving soil structure

Practically any soil type can have good or bad structure, depending on the presence or absence of good crumb formation. Certain soils however can have good structure even without crumbs.

In very sandy soils, the sand particles are large and about the same size as soil crumbs. They fill the function of crumbs, creating plenty of pore space. However, these are nearly all large pores, which drain quickly and, because of the absence of small pores, there is no reservoir of water for plants.

These soils tend to dry out very quickly. The addition of humus-forming organic material will create small pore space and improve their moisture-holding capacity.

Very peaty soils are composed almost purely of organic material. The fibrous nature of the peat creates both large pore space and small pore space, resulting in soil that is well structured and retains adequate moisture.

Heavy soils with poor structure can be improved by adding sand or organic material. Sand is not nearly as efficient as organic material because it works only by taking the place of soil crumbs. This also means that large quantities of sand must be added to achieve the same structural effect.

Organic material – farmyard manure, garden compost, straw, peat – breaking down into humus, is about ten times as efficient a soil improver as sand, on a weight for weight basis. However, sand has a permanent improving effect whereas organic material eventually decays.

Good soil structure develops naturally in well-drained soil by the action of roots, and the build-up of natural humus. Improving the drainage of wet soil is an important step towards better soil structure.

Removing plant material without adding organic matter back leads to progressive deterioration of soil structure. Walking on wet soil, and cultivating it when wet, quickly destroys the soil crumbs.

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