Of Plant Humus

Abstract

Humus is the black substance into which dead plants are converted by the combined action of oxygen and water. Oxygen gas penetrates humus and combines with its carbon to produce carbon dioxide gas, which is released, thereby becoming a nutrient for plants. Carbonization of 47 plants, plant parts, or plant substances showed that humus contains more carbon per unit weight than the plants that gave rise to it. The proportion of carbon contained by a humus, however, does not seem to be increased significantly by the continued action of the causes that formed it. In a finished humus, although continuous exposure to air and rain leads to continued weight loss, there is little change in the proportions of carbon, hydrogen, and oxygen because the hydrogen and oxygen are removed, in the form of water, in proportion to the removal of carbon as carbon dioxide. After multiple extractions of humus, the humus still yields a small amount of extract. Acids do not dissolve the organic matter of humus, but potash and soda do. Humus is entirely destructible at room temperature due to the joint action of oxygen and water. As a humus loses carbon and water, a small amount is also being lost as extract to the soil drainage water. These changes explain why there is little accumulation of humus, even in places where vegetation has long been established. Incineration of humus yields a semi-vitreous ash which is not very soluble in water. The ash of the extractive juices of a humus, however, contains all the substances of plant ash. Thus, the extractive juices contribute to the fertility of the humus. Humus contains a larger proportion of nitrogen than do undecomposed plants. Humus has an antiseptic action, which retards fermentation and putrefaction.