MATHEMATICAL-MODELS FOR POTASSIUM RELEASE KINETICS IN CALCAREOUS SOILS

Potassium release from the coarse (20-50 mu m), medium (5-20 mu m) and fine silt (2-5 mu m), and the coarse (2-0. 2 mu m) and medium-fine clay ( less than 0. 2 mu m) fractions of six Great Plain soils was determined by successive extraction with Ca-saturated cation exchange resins. All soils contained primarily montmorillonite-mica minerals. Results indicated that 65 to 80% of the total K released in 7000 h of extraction time occurred in the clay ( less than 2. 0 mu m) fraction. Four mathematical models (first-order rate, parabolic diffusion, power function, and Elovich) were used to describe cumulative K release. Comparisons of coefficients of determination (r**2) and standard errors of the estimate (SE) indicated that the Elovich, power function, and parabolic diffusion equations adequately described cumulative K release, whereas the first-order rate equation did not. Rate constants for the three equations were highly correlated with mica content and relative alfalfa yield and K uptake. Refs.