New Nomenclature of the Higher Categories of Soil Classification as Used in the Department of Agriculture

Soils have been classified at various times for different purposes and according to many different schemes. Some schemes such as those of various Russian workers (l) are simple and easily understood, while others such as that of Sigmond (12) are much more complex and are perhaps not quite so easy to, grasp. There are many different possible ways of grouping soils according to the purposes to be served. The two principal ones of these may be called taxonomic and geographic. It is understood that the strictly taxonomic grouping is designed to place soils having similar morphology and theoretically similar genesis in the same groups. The grouping of soils on a geographic basis is designed to place soils having close geographic relationships in the same groups. In other words', soils would be grouped together if they COL:-' bine to form a landscape unit. In 1927 Marbut proposed a taxonomic classification (7) for soils which, with minor revisions, he retained until the time of his death. Those who knew him well know that he was vitally interested in constantly improving his scheme so that it would be consistent with newly discovered facts concerning soils. It is evident from the importance he assigned to "mature" or "regional" soils that Dr. Marbut was strongly influenced by his intense interest in the geographic significance of soils. Professor Shaw of the University of California has recently proposed a classification of soils (ll) in which he has combined taxonomic and geographic units. This system has its advantages in the study of a region of great soil complexity, such as is characteristic of much of the State of California. Some difficulty is experienced, however, when attempting to apply this method of classification to the soils of a continent or of the world as a whole. The most recent attempt at soil classification in the Department is that presented in the 1938 Yearbook of Agriculture (2). This classification is based on that of Marbut (Table l) as published in the Atlas of American Agriculture (6), but greater stress is laid on the groupings in Category III and in the combination of great soil groups into still higher categories. In abbreviated form the classification is. presented in Table 2. While the table is presented in the Yearbook and while there are notes on the nomenclature used, it was thought well to present the subject to the Soil Science Society of America, in order that there might be free discussion of the newly proposed classification. This classification has been discussed rather thoroughly at meetings of the inspectors and chief of the Soil Survey and represents the combined judgment of the group, even though each one might have come to somewhat different conclusions by himself. The terms finally adopted for use in the Yearbook represent a compromise of opinions of various members of the Department of Agriculture and of some of the State institutions. The soil type in the sense of the word as used in the United States is already familiar to soil scientists not only of this country but all over the world. Category I is comprised of soil types and their various phases. Soil series also is a term that is known all over the world and after a number of stages of evolution, we now have the definition as given in the Yearbook of Agriculture. The great number of soil series go to form Category II of the revised classification. Series are grouped on the basis of internal characteristics to form families of soils which go to make up Category III. The term soil family has been used by many different scientists in almost as many different ways and perhaps it is unfortunate that this name is retained to cover this particular group. The family of 'Professor Shaw (11) consists of several different soil series closely associated geographically but varying indegree of