The organic matter of the soil has engaged the attention of soil investigators ever since the latter part of the eighteenth century. The volume of literature on the subject is enormous, and perhaps in a greater degree than in any other branch of soil work we find here examples of the tendency to draw definite conclusions from inconclusive evidence. This is not surprising when it is remembered that perhaps the most important part of the soil organic matter, the humic matter, is a high molecular organic colloid, the investigation of which takes us at once into the most difficult fields of organic and of physical chemistry—fields in which the worker often needs to use special methods of experimentation which have become available only during recent years. Moreover, the study of humic matter is rendered particularly difficult, since it is a secondary product derived from the complex organic constituents of plant residues, themselves of unknown or little-known constitution, and since it is not known from which of these plant constituents it is derived.