HUMUS (a Latin word meaning the ground), a product of de composing organic matter. It is especially present in peat bogs, and also occurs in surface soils, to which it imparts a brown or black colour. It is one of the most important soil-constituents from the agricultural point of view; it modifies the properties of the soil by increasing its water-holding capacity and altering its colloidal nature. The chemical composition is very complex and has been much disputed.