Soil Texture
Soil is composed of mineral particles such as clays, silts, sands and gravels (see table) and organic materials.

Textural Group Particle size (mm)
Clay < 0.002
Silt 0.002 - 0.02 
Sand > 0.02

Any soil can be placed within the Soil Textural Triangle, once the relative proportions of clay, silt and sand are known.


The texture of a soil influences how the soil responds to different stresses. Sandy soils are much better drained than clay soils. In a heavy rain, sandy soils allow the water to freely enter and wash through. But clay soils can be poorly drained and water might start accumulating on the surface. However, sandy soils dry out much quicker than clay soils. For most agricultural plants, loamy soils are generally the best because they are well drained, but still retain water longer than sandy soils.

Soil Organisms
Soils also contain an incredible number of organisms. The range of different species of animals and microorganisms in soils is similar to the biodiversity found in rainforests and tropical reefs. These soil organisms range in size from tiny viruses and bacteria up to earthworms as large as 2m in length


Current estimates of the number of species of some groups include:
  • bacteria (30,000)
  • fungi (1,500,000)
  • algae (60,000)
  • protozoa (100,000)
  • nematodes (500,000)
  • earthworms (3,000)

These soil organisms are responsible for decomposing all the organic matter which enters the soil (ie leaves falling on the surface, roots dying underneath the surface) and recycling the nutrients contained in it for further plant production. Larger soil animals chew the organic material into smaller pieces and make burrows, which aerates the soil and provides channels for water movement. Tiny microorganisms are responsible for most of the decomposition of organic materials and also produce special glues which stick soil particles together, making the soil less prone to wind erosion. Some microorganisms can even produce plant growth hormones which speed up the growth of plants. Without these millions of creatures, the soil is dead and produces nothing.
Putting the Pieces Together
The arrangement of the particles and spaces between the particles (pores) is called the structure of a soil. What makes a good structure is dependent on what you want to use the soil for. A good structure for growing tomatoes is like a sponge, with large soil particles and large air spaces for the free movement of water and air.

Rice on the other hand needs a soil structure with few air spaces between the particles. Buildings require very compact soils which do not crack or swell.


Small particles of soil can stick together to form bigger particles. Soil particles are stuck together by chemical forces between
clays and by glues made by soil microorganisms.
A soil aggregate, made up of many smaller parts

The strength of the bonding between soil particles is called the soil stability. Stable soils have strong bonds and maintain their structure during wetting/drying cycles. Unstable soils lose their structure during wetting cycles or wind storms and are subject to high rates of erosion

Some soils are naturally more stable than others because of their chemical properties. However, many Australian soils are unstable due to poor soil management.


Leaving soils bare leaves them prone to rainfall impact, which smashes up the soil particles on the surface. 

Constant ploughing also breaks up the soil surface.
Organic matter can help bind soil particles together and not returning organic material to the soil every year will result in a reduction in organic bonding between soil particles and a less stable soil.

     


These paddocks are close to each other and the photos were taken on the same day after a rainstorm. The paddock on the left has stable soils, because the farmer keeps a cover of mulch on the surface and does not continually cultivate his land. The one on the right has unstable soils because the farmer removed all the mulch and cultivates every year. When it rains, the water breaks up the unstable soil and all the soil pores are clogged with the tiny pieces of soil and water movement through the soil is restricted.

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