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Published in Soil Sci Soc Am J 36:838-843 (1972)
© 1972 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Potassium Selectivity in Certain Montmorillonitic Soil Clays1

C. D. Carson and J. B. Dixon2

ABSTRACT

Potassium selectivity (kG) was relatively constant when K-saturation exceeded 15% for soil clays. As K-saturation decreased from 15% or less, depending on the soil and clay fraction, kG increased exponentially. The 2 to 0.2µm clay fractions had high kG values to about 15% K-saturation, apparently due to dioctahedral mica and vermiculite. Potassium selectivity for 0.2 to 0.08µm and <0.08µm soil clay fractions was below kG = 4.5 for about 85% of the exchange sites which corresponds to the montmorillonite contribution to cation exchange capacity (CEC) in these soil clays. Mica content correlated well with kG at a point near the steep part of the selectivity curves indicating the influence of small amounts of mica and possibly associated vermiculite on kG values. High kG values that occur over several percent K-saturation in coarse soil clay fractions indicate that this fraction is more important than the more abundant finer fractions in controlling kG values and presumably K+ availability to plants. Increases in kG values for reference montmorillonites where only a few percent K-saturation exists are attributed to mica and possibly vermiculite impurities. Dioctahedral mica and vermiculite appear to control K-selectivity in the low K-saturation range common in certain montmorillonitic soils.

NOTES

1 Contribution from Texas Agr. Exp. Sta., College Station, Texas 77843. Presented before Div. S-9 Soil Sci. Soc. Amer. Tucson, Arizona. August 26, 1970.

2 Instructor and Professor of Soil Mineralogy, respectively. Dep. of Soil and Crop Sciences, Texas AM University.

Received for publication February 16, 1972. Accepted for publication June 5, 1972.






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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome
Copyright © 1972 by the Soil Science Society of America.