Potassium Selectivity in Certain Montmorillonitic Soil Clays
1972
Carson, C. D. | Dixon, J. B.
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.
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