1911 Encyclopædia Britannica/Humus

​HUMUS (a Latin word meaning the ground), a product of decomposing 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 is the chief source of nitrogenous food for plants, and modifies the properties of the soil by increasing its water-holding capacity and diminishing its tenacity. Little is known with regard to its chemical composition. By treating with a dilute acid to remove the bases present, and then acting on the residue with ammonia, a solution is obtained from which a mineral acid precipitates humic acid; the residue from the ammonia extraction is termed humin. Both the humic acid and humin are mixtures, and several constituents have been separated; ulmic acid and ulmin, in addition to humic acid and humin, are perhaps the best characterized.