One of the major effects of organic matter is to improve soil structure,
Plant roots, earthworms, bacteria, fungi and other micro-organisms release organic compounds which help bind soil particles together to produce stable aggregates. this improves aeration and increases permeability which in turn makes the soil less susceptible to erosion. Stable aggregates also resist compaction caused by ploughing, and vehicle and animal traffic.
Organic substances have been shown to hold up to five times their own weight of water. This contributes to improving the available water-holding capacity of the soil, making growing plants less prone to short term droughts.
The dark colour imparted by organic matter in the topsoil increases the absorption of solar radiation. This may facilitate soil warming in spring and consequently seed germination and plant growth.
See other functions of organic matter