Mathematical Models for Potassium Release Kinetics in Calcareous Soils

Abstract

Abstract Potassium release from the coarse (20–50 µm), medium (5–20 µm) and fine silt (2–5 µm), and the coarse (2–0.2 µm) and medium‐fine clay (<0.2 µm) 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 (<2.0 µm) 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. In the past, others have used complex equations containing three simultaneous first‐order rate terms to describe K release; however, results reported herein show that simple one‐term equations can be used.