The soft moist amorphous black or dark brown organic matter in soil derived from decaying plant and animal remains (especially leaf litter) and animal excrement. The complex chemical reactions involved in the decomposition by soil organisms result in the formation of end products that can be used by plants. Humus has a very complex and variable chemical composition. Its colloidal properties enable it to retain water, making it a useful addition to sandy soils. It also helps the formation of soil crumbs, which improve aeration and drainage in clay soil and can also provide insulation in soil by reducing temperature fluctuations. There are two main types of humus. Mor humus (raw humus) is formed in acid conditions (pH 3-4.5) where the plant litter is low in bases. It is thus acid and this, together with the extremes of moisture or temperature that are often found in such habitats, inhibits the activity of the soil fauna. The slow decay is thus brought about by saprophytic fungi. Bogs, heaths, and pine-forest soils have thick layers of mor humus. Mull humus (mild humus) is formed in less acid or alkaline soils, which are richer in calcium. Animals, e.g. earthworms, can consume it and the bacteria that bring about decay flourish. It may be found in warm or mild humid climates in such habitats as deciduous forest or grassland. An intermediate form of humus is termed moder. See also soil. |
