Mathematical models to study the kinetics of potassium release from swell‐shrink soils of Central India in relation to their mineralogy

Employing four mathematical models (first‐order, parabolic‐diffusion, Elovich and zero‐order), kinetics of potassium desorption from eight soils with and without cropping were studied to evaluate their ability in explaining K release from soils. The decline in the soil test K in cropped soils over original soils was drastic in easily desorbable forms compared to that of strongly held forms like 3 M H 2 SO 4 K. Results showed that parabolic diffusion as well as first‐order kinetic equation explained the K release data well for both original and K depleted (cropped) soils. Elovich and zero‐order equations were not suitable to describe the kinetic data. However, zero‐order equation explained K release data better in case of K‐depleted soils as compared to original soils. Soils with higher initial K contents registered higher release rate constants. Over the entire period of cropping the range of release rate (b) decreased from 1.26 to 1.53 × 10 −2 to values ranging from 1.12 to 1.30 × 10 −2 h −1 . In contrast, the first‐order equation, parabolic diffusion showed higher b values for cropped soils as they represent the diffusion gradient. Mica and its biotite content in both silt and clay fractions showed significant correlation (r) with b values. Similarly with the rate of K release, clay content of soils maintained significant r whereas the silt content did not.