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Horticulture 1000
HT1031 - Soils 1 - Soil structure
Activity 4 - Soil structure
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Soil structure

You have seen that soils can be classified by their particle size. Now you will explore another way horticulturists describe soils.

The proportion of sand, silt and clay describes a soil’s texture. However, soil particles don’t always exist as separate tiny units. More often they are found clumped together in crumbs.

The size of its crumbs gives a soil its structure. Crumbs vary in size, but anything from 1 mm to 5 mm is regarded as a suitable soil structure.

Crumbs of this size have big air spaces between them. These spaces are called pores. Pore size or porosity is another important part of soil structure.

The pore spaces in soils with crumbs between 1–5 mm diameter are large enough to leave room for air and to allow excess water to drain away. Plant roots and beneficial soil animals can also move easily through a soil with a crumbly structure and they also have access to adequate amounts of air and water.

A well-structured soil with a crumbly structure is called friable.

The diagram below shows how the soil particles are bound together to form the crumbs.

Diagram showing how the soil particles are bound together to form the crumbs.

Clay and/or organic matter hold sand and silt particles together in crumbs. When lime or humus is added to a clay soil the clay particles group themselves around the lime or humus particles. They clump together to form crumbs. This creates air spaces so the soil drains better.

The table shows the advantages and disadvantages of different crumb sizes.

Crumbs smaller than 1 mm Crumbs 1–5 mm Crumbs larger than 5 mm

Pores are too small and there is not enough air around the plant roots.

Excess water does not drain freely.

The plant roots don’t grow easily through it.

Pores give enough air and excess water drains freely.

The plant roots grow easily through it.

Pore spaces are very large and excess water drains readily.

Drying out is a problem because there is too much air around plant roots.

The photo below shows what a soil with a good structure looks like when you dig it and take a handful.

Soil with a good structure looks like this you dig it and take a handful. A soil with an ideal structure has properties midway between a sandy soil and a clay soil.

Effect of soil structure on erosion

Soils with a well developed soil structure are less likely to be eroded by wind and water. Sandy soils that have no structure are easily moved by wind and water. Their surface structure moves easily.

Good sized pore spaces allow water to move through the soil easily, and that water is available to plant roots. This avoids problems of surface erosion.

Ideal soil structure

A soil with an ideal structure has properties midway between a sandy soil and a clay soil. It has groups of crumbs about 1mm to 5 mm in size. This occurs naturally in some soils, especially loams that contain the three particles sand, silt and clay.

Classes of soils

Soils can be put into classes depending on the structure of the soil.

Single grains Single grains e.g. sand Loose particles that are not stuck together (sand is an example).
Crumbs Like biscuit crumbs and usually less than 5 mm in diameter Like biscuit crumbs and usually less than 5 mm in diameter.
Blocky Irregular blocks that are usually 1.5–5 cm in diameter Irregular blocks that are usually 1.5–5 cm in diameter.
Clods Hard to see particles that appear in large clods. Common in clay soils. Hard to see particles that appear in large clods. Common in clay soils.

Complete Activity 4A in your workbook

Key points   Key points

  • Soil texture and soil structure are two different but related features of soils.
  • Soil structure is how the soil particles join together and the resulting pore spaces.
  • Often the particles are clumped together into crumbs.
  • The size of the crumbs is very important in how much air, water and nutrients a soil contains.
  • Crumbs between 1–5 mm in diameter give a good soil structure.

What's next?

Go to: 5 Alternative growing media.

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