Release of Potassium from Soil Fractions During Cropping

Soil fractions from six soils known to vary in ability to supply K to field crops were cropped intensively in greenhouse experiments. All soils and soil fractions were Mg‐saturated prior to conducting the experiments. The K content of the clay and of the silt was linearly correlated with the logarithm of the K uptake for each fraction, except for the silt fraction of one soil. No correlation was noted between the uptake of K and the K content of the entire soil. In equilibrium experiments, K in solution in 1.0 n MgOAc after 90 days was linearly correlated to the K content of the clay. A similar correlation was obtained with the entire soil. The different pattern of K release during cropping tests as compared to the equilibrium studies is attributed to rate processes which would govern release during cropping, but which would not be a factor in the equilibrium experiment. X‐ray analyses indicated the formation of expanding lattice clays upon the removal of interlayer K by cropping. Large amounts of K were removed from the coarse clay (2.0–0.2µ), intermediate amounts from the medium clay (0.2–0.08µ), and small amounts from the fine fraction (<0.08µ). This differential release of K by the different size clay fractions can be explained by a decrease in the activity of interlayer K when the K content of the clay is decreased.