Biological properties

Biological  properties include:

• organic matter
• soil organisms
• the presence of disease-causing organisms.

Organic matter

Two main parts  of soil are inorganic matter and organic matter. The inorganic part is the  non-living part: the sand, silt and clay particles. Organic matter includes all  the decomposing plant and animal material in the soil. It makes an important  contribution to the chemical and physical properties of the soil.

Decomposition

The breakdown or decomposing of organic matter can help the soil structure. It also gets rid  of dead things and helps recycle nutrients. Micro-organisms and soil animals do  the decomposing. They turn organic matter into humus.

Humus

When plants and animals have completely decomposed and lost their structure, the material formed is called humus.

It is a dark, spongy material, which forms a coating on the inorganic soil particles. All organic matter in the soil will eventually decay to humus.

Clay plus humus attract nutrient ions and hold them on its surface. This prevents leaching and makes the nutrients available for plant roots.

The clay-humus combination means soil can hold more water during dry periods.

Advantages of organic matter

The advantages  of organic matter in the soil are:

• increased  supply of available nutrients for plants as it is broken down
• increased  water-holding capacity in sandy soil, because organic matter holds water
• a greater  amount of air in the soil, because the organic matter opens up pore spaces by  forming crumbs (especially important in clay soils)
• raised soil  temperature as the dark colour of the organic matter absorbs and holds heat.

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What's next?

Go to: 4 Amount of organic matter in soil.