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Exchangeable Potassium as Affected by Mica Specific Surface in Some Soils of North Central United States¹

ABSTRACT

Mineralogical analyses of samples from 16 surface soil horizons and 9 subsoil horizons, representing 15 soil series, revealed that differences in the content of clay sized mica were closely related to the wide differences in the content of exchangeable K in ovendried samples before and after cropping. The specific surface of mica in the clay fraction was significantly correlated with the content of exchangeable K in oven-dried samples before cropping (r = 0.63), after cropping (r = 0.74), and after fertilization and cropping (r = 0.68). After being cropped and oven dried, for example, a sample of Wabash silt loam surface soil contained 466 pp2m exchangeable K, but a sample of Bates silt loam surface soil contained only 78 pp2m exchangeable K. These samples had 3.3 m²/g and 0.5 m²/g, respectively, of mica specific surface area. No significant relationship occurred between the content of exchangeable K in field moist samples and the content of mica in the soils.

The wedge-zone concept of mica weathering in crystal edges and in crystallographic (hk0) fractures normal to (001) crystal faces was used to explain the relationships between specific surface of mica and content of exchangeable K in oven-dried samples. The existence of an equilibrium between exchangeable K in the open part of the wedge with nonexchangeable K at the mica weathering side of the wedge was indicated.

¹ Contribution from the Department of Soil and Water Sciences, University of Wisconsin, Madison, supported in part by Federal grant funds through a North Central Regional Project (NC-16) and the Hatch Act, in part by the Research Committee of the Graduate School through a grant of funds from the Wisconsin Alumni Research Foundation, and in part by science facilities grant G13793-Jackson from the National Science Foundation. Presented before a joint meeting of Div. S-2 and S-5, Soil Science Society of America, Ithaca, N. Y., Aug. 20, 1962.

² Assistant Professor of Soil Science, Cornell Univ., Ithaca, N. Y. (formerly Graduate Research Assistant at Wisconsin) and Professor of Soil and Water Sciences, Univ. of Wis., respectively.

Received for publication May 31, 1966. Accepted for publication August 11, 1966.