The raw material consists of plant and animal remains and animal excreta. It is broken down in the soil by soil organisms: beetles, mites, earthworms, fungi and bacteria. These are present in vast numbers in a fertile soil; e.g. worms can number 2 million per hectare and microorganisms can weigh 2.5 t/ha. Organic matter is composed of:

• Sugars, starches, cellulose, nitrogenous compounds
• Lignins and mineral matter

The soil organisms rapidly break down the sugars, starches, nitrogenous compounds and some of the cellulose. This process is called mineralisation. The remaining matter is decomposed much more slowly and forms a black or brown mixture called humus.This humus has several beneficial effects on the soil:

• Maintenance of soil structure
• Retention of available nutrients
• High water-holding capacity
• Low plasticity and cohesion
• Gradual release of available nutrients
• Darkening of colour

Losses of organic matter from arable soils will be increased by any activity that increases microbial activity, such as cultivations and nitrogen fertiliser applications. Permanent pasture and minimally cultivated soils maintain their organic matter content better than cultivated soils.