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Threads of Genesis in the Seventh Approximation¹

ABSTRACT

A common criticism of the newly proposed system of soil classification as presented by the Seventh Approximation is its apparent departure from the genetic aspects present in the older systems. Since so much of the published text is devoted to discussions of morphology, it is easy to understand why this impression is gained at first reading. However, the impression is faulty, and the relationship between the accepted theories of soil genesis and the Seventh Approximation is more specifically drawn than was this relationship in the older systems. The major difference being that in the Seventh Approximation the relationship to genesis is drawn by specific definitions of the kind of morphology representative of a kind or a degree of genesis rather than by direct reference to a genetic process itself or its degree of activity. In nearly every case the choice of morphological character selected to be definitive for a category is based on our understanding of how these characteristics represent a specific kind or degree of genetic process.

This constitutes both strength and weakness in the Seventh Approximation. Its strong relationship to genesis imparts a desirable sense of direction to the system. This same strong adherence may lead to faulty criteria or to improper selection of definitive morphological characteristics where our knowledge of genesis is imperfect. This last is inevitable under any conditions, and the advantages strongly outweigh the disadvantages.

¹ Contribution from the Soil Conservation Service, USDA and Department of Agronomy, Colorado Agr. Exp. Sta., Fort Collins. Scientific Journal Series No. 760. Presented before Div. V, Soil Science Society of America, Nov. 27, 1961, at St. Louis, Mo.

² Soil Correlator, Soil Conservation Service, USDA and Professor of Agronomy (Soils), Colorado State University, Fort Collins, respectively.

Received for publication May 9, 1962. Accepted for publication June 2, 1962.