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The Role of Vertic Subgroups in the Comprehensive Soil Classification System¹

ABSTRACT

The application of soil surveys to nonfarm as well as to farm uses is reflected in recent modifications of the Comprehensive Soil Classification System. A case in point is the creation of vertic subgroups. Soils of vertic subgroups fail Vertisols primarily because they lack gilgai relief, intersecting slickensides, and parallelepiped structural aggregates. However, in having cracking properties peculiar to Vertisols, they are inappropriate to typic subgroups of other soil orders. Data from selected soils indicate that the cracking properties are related primarily to mineralogy and clay content. Soils dominated by montmorillonitic clay and having a clay content exceeding 40% are most common to the vertic subgroups. In addition, these soils most commonly have a Unified Soil Classification of CH or MH. They have unstable behavior with regard to engineering structures and are referred to by engineers as having a high content of "fat" clays. The Comprehensive Soil Classification System allows discussion of this unique group of soils at the subgroup level.

¹ Contribution from Soil Survey, SCS, USDA, P.O. Box 11222, Fort Worth, Texas. Presented before Div. S-5, Soil Science Society of America, Nov. 8, 1967.

² Assistant and Principal Soil Correlators, SCS, South Regional Technical Service Center, Fort Worth, Texas.

Received for publication April 4, 1968. Accepted for publication August 26, 1968.