Humus refers to decayed or partially decayed organic matter such as partially decomposed leaf litter, animal waste and animal remains. Humus has a number of desirable qualities that make gardeners seek it out. It retains water very well, is generally porous, contains minerals for plants and can effectively trap heavy metals and other poisons, preventing them from getting into plants.
Compost contains a high degree of humus, and can be almost entirely humus depending on how it is prepared. Compost consists of decayed vegetable scraps, animal waste, newspaper, egg shells and other organic wastes. The wastes are typically covered in a thin layer of soil and allowed to rot. They may be stirred up or exposed to worms to help them breakdown more quickly. The less soil used to cover compost, the higher the proportion of humus to other soil components will be.
Peaty or peat soil forms in wetlands such as swamps, bogs and marshes. In these areas, plant litter forms a thick layer and slowly decomposes. Peaty soil is acidic, which slows the rate at which the plants decompose and their nutrients become available for other plants. Because of this, peat is not as useful or nourishing as other forms of humus. It can, however, be improved by adding lime to reduce the acidity and adding fertilizer to improve the nutrient composition.
Organic Animal or Plant Fertilizer
Chemical fertilizer consists mostly of the concentrated nutrients nitrogen, phosphorous and potassium. Organic plant and animal-based fertilizers, by contrast, consist of organic matter with a lower concentration of nutrients. As additives such as compost, soybean meal, cottonseed meal and manure break down, they add more humus to the soil. Properly managed organic soil can continually build up humus while the quantities of sand, silt and clay remain rather static. Eventually, the humus can become the primary component of the soil.