A Key for the Classification of Forest Humus Types

»~piHE presence of organic layers at the surface is a -L characteristic feature of forest soils and serves to distinguish them from agricultural soils in which cultivation destroys the natural arrangement of organic horizons. The forester uses the term forest humus to cover the organic portions of the soil profile. From experience and observation, foresters have learned that there are great differences in the humus beneath various kinds of forest stands and that these differences influence the physical, chemical, and biological properties of forest soils. Also the character of the mineral soil may. have some influence on the -type of humus. It has been learned, further, that the type of forest humus can be altered by management with such practices as thinning, changing stand composition, controlled burning, and final cuttings. Because of the importance of forest humus to forest and watershed management, many attempts have been made to classify the different types^which can be recognized. The first such generally acceptable classification is accredited to P. E. Miiller (2). Revisions, new names, and new systems have appeared at intervals since then. This previous work has been ably reported by Romell and Heiberg (3), Wilde (5) and Heiberg and Chandler (1). None of the previous classification schemes was applicable for use over wide ranges in climate, soil and forest conditions, nor were they meant to be. Thus classification had not been standardized, and workers in different regions did not talk the same language humus-wise. There is still much to learn about the origin of the various humus types and the practical significance of their differences as they may relate to timber and watershed management. The scheme proposed by Heiberg and Chandler (1) was the product of committee work and represented the best thinking up to that time for the Northeastern United States. It was a marked improvement over earlier classifications and has found wide usage. However, questions have arisen in the field because some of the descriptions were subject to several interpretations, and some rather common humus types were not clearly identified. An effort to remedy these faults and develop a more usable classification was made by the Northeastern Forest Soils group in 1946 when a committee was appointed to consider the subject. The committee's approach was to introduce a greater degree of consistency in the type descriptions and to set up the classification in the form of a key to facilitate identification of the various humus types in the field. On the recommendation of the Northeastern group, the Forest Soils Subdivision of the Soil Science Society of America appointed a committee consisting of representatives from all forest regions of the United States to work out a forest humus classification which would be applicable, to a greater or lesser degree, to the entire country. Members of this committee tested the Northeastern key in their localities and suggested the changes needed to make it more useful. The present key is the result of this coordinated committee effort. With such a classification to serve as a frame of reference, it will be possible to determine the practical significance and genesis of the humus types as now identified. Further, with the greater understanding of forest humus types thus obtained, it will be possible to devise classifications which will be more realistic as regards both development of humus types and their application to forest management. No pretense is made that this key covers all humus types. It is believed, however, that the major categories of Mull, Duff Mull, and Mor are universally applicable and that further subdivisions of these major types can