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A Genetic Study of Two Soils Developed on Granite in Llano County, Texas¹

ABSTRACT

Studies were made of an excessively drained soil and a somewhat poorly drained soil, tentatively named Click and Bauman, respectively, to determine the effects of topography upon their genesis and resulting mineralogy. Heavy mineral data indicate that the soils have developed from relatively homogeneous parent materials. Clays were fractionated at 0.2µ for X-ray diffraction, differential thermal (DTA), and cation exchange capacity (CEC) studies. The data indicate that the clay mineralogy of both soils is mixed but that the fine clay of the Bauman soil is dominated by a montmorillonite mineral that increases with depth in the profile. The fine clay fraction of the Click soil apparently contains near equal portions of kaolinite, illite, montmorillonite, and X-ray amorphous material. Appreciable exchangeable Na is present in the subsoil of the Bauman profile but is essentially absent in the Click soil. Soluble weathering products from the surrounding slopes have moved laterally at the soil-unweathered granite interface and accumulated in the Bauman soil thereby causing its morphology and clay mineralogy to differ markedly from that in the better drained soil.

¹ Contribution from the Agronomy Department, Texas Technological College, Lubbock. Presented before Div. S-5 Soil Science Society of America, Nov. 17, 1964, at Kansas City, Mo. The assistance of H. C. Dean, J. L. Hensell and G. Dittmar, SCS, USDA, in the field studies is gratefully acknowledged.

² Formerly Graduate Assistant, Texas Technological College, now Graduate Assistant, North Carolina State University, and Professor of Agronomy, respectively.

Received for publication October 12, 1967. Accepted for publication October 19, 1967.