Organic matter is very important in soil fertility management because it has many properties that help increase soil fertility and improve the soil structure. Organic matter has a great capacity to retain nutrients this is especially important in sandy soils, which retain very few nutri- ents. Organic matter can also retain a lot of water, which means that in dry periods more water is available for the plants for a longer time. This is especially important in sandy soils, which retain little water. Organic matter can improve the soil structure. This is important for both sandy and clay soils, because they have a poor structure. Finally, organic matter stimulates the growth of soil organisms, which help make the nutrients in the organic matter available to the plants.

The organic matter in the soil consists of fresh organic material and humus. Fresh organic material is plant and animal waste that has not yet decomposed, such as roots, crop residues, animal excrement and cadavers. The fresh material is transformed by soil organisms into humus, which is also called organic soil matter. In the process, nutri- ents are released organic matter thus makes nutrients available to the plants. Humus, i.e. organic soil matter, is material that has been bro- ken down so far that the original fresh material is no longer distin- guishable. It gives the soil a dark colour. Humus itself is also broken down by the soil organisms, which releases even more nutrients, but this process takes much longer.

Crop husbandry that contributes to a positive balance of organic mat- ter is the basis for good soil fertility in the long term. The balance of organic matter must be even or positive, that is, the amount of organic matter that is added must be equal to or greater than the amount that is broken down and thereby lost. However a positive balance of organic matter is difficult to achieve. This means that if a lot of organic matter is lost (by erosion for example) it is difficult to increase the level of organic matter in the soil. Even in favorable conditions and with good crop management, this can take a number of decades, especially if during that time crops are grown that are almost completely removed with the harvest.