Soil Organic Matter
Organic matter is the partially decomposed remains of soil organisms, animals and plant life. It makes up only a small fraction of the soil (normally 2 to 10%), but is essential for soil aggregation which allow air and water to move through the soil. Organic matter retains moisture (humus holds up to 90% of its weight in water), and is able to absorb and store nutrients. Importantly, organic matter is the primary food source for microorganisms and other forms of soil life in dryland agriculture. Organic matter quality is important, as incorporating large amounts of high-carbon material (i.e. wheat stubble) can deprive plants of soil derived nitrogen in the short term.
Organic matter contributes to the development of the darker friable topsoil that retains moisture and cycles nutrients for plant growth. The value of organic matter in soil health is hard to overestimate; providing ground cover, increasing soil stability, cycling nutrients, habitat and erosion control. Farming practices that reduce soil organic matter such as burning, tillage, overgrazing and continuous cropping run the risk of contributing to a decline in soil condition which may not become evident for many years.