Soil organic matter is an accumulation of plant and animal residue in varying stages of decay. These materials are continually being broken down by soil microorganisms. Consequently, organic matter is a rather transitory soil component, and must be renewed constantly through the addition of new plant or animal residues.
Even though the percentage of organic matter in soil is relatively small, normally varying from 2% to 6% by weight, its influence on soil properties and plant growth is far greater than the low percentage would indicate. The following are ways in which organic matter functions in soils:
1) As a granulator of mineral particles.
2) As a major source of the mineral elements phosphorus, sulfur and nitrogen.
3) Increases water holding capacity.
4) As the main source of energy for soil organisms.
The more soluble organic substances, such as sugars and starches, are utilized rapidly. Less soluble organic substances, such as the natural carbohydrates hemicellulose and lignins, are decomposed more slowly. Soil organic matter consists of two general groups:
1) Original tissue (undecomposed roots and tops of plants, and animal remains).
2) Humus (decomposed roots and tops of plants and animal remains.)
Humus is usually black or brown in color, and is colloidal in nature. (Colloidal means it will stay suspended in soil solutions.) It’s capacity to hold water and nutrients far exceeds that of clay, it’s inorganic counterpart. Thus, it takes only small amounts of humus to greatly enhance the soil’s water holding and plant production capacities.