The organic matter in soil derives from plants, animals and microorganisms. In a forest, for example, leaf litter and woody material falls to the forest floor. This is sometimes referred to as organic material. When it decays to the point in which it is no longer recognizable, it is called soil organic matter. When the organic matter has broken down into a stable substance that resist further decomposition it is called humus. Thus soil organic matter comprises all of the organic matter in the soil exclusive of the material that has not decayed.
An important property of soil organic matter is that it improves the capacity of a soil to hold water and nutrients, and allows their slow release, thereby improving the conditions for plant growth.  Another advantage of humus is that it helps the soil to stick together which allows nematodes, or microscopic bacteria, to easily decay the nutrients in the soil.
There are several ways to quickly increase the amount of humus. Combining compost, plant or animal materials/waste, or green manure with soil will increase the amount of humus in the soil.
# Compost: decomposed organic material.
# Plant and animal material and waste:  dead plants or plant waste such as leaves or bush and tree trimmings, or animal manure.
# Green manure:  plants or plant material that is grown for the sole purpose of being incorporated with soil.
These three materials supply nematodes and bacteria with nutrients for them to thrive and produce more humus, which will give plants enough nutrients to survive and grow.
Soil organic matter is crucial to all ecology and to all agriculture, but it is especially emphasized in organic farming, where it is relied upon especially heavily.