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 s oils with and without cropping were studied to evaluate their ability in explaining K release from soils. Te decline in the soil test K in c ropped soils over original soils was drastic in easily desorbable form s compared to that of strongly held forms like 3 M H2SO4 K. Results sh owed that parabolic diffusion as well as first-order kinetic equation explained the K release data well for both original and K depleted (cr opped) soils. Elovich and zero-order equations were not suitable to de scribe the kinetic data. However, zero-order equation explained K rele ase data better in case of K-depleted soils as compared to original so ils. Soils with higher initial K contents registered higher release ra te constants. Over the entire period of cropping the range of release rate (b) decreased from 1.26 to 1.53 x 10(-2) to values ranging from 1 .12 to 1.30 x 10(-2) h(-1). In contrast, the first-order equation, par abolic diffusion showed higher b values for cropped soils as they repr esent the diffusion gradient. Mica and its biotite content in both sil t and clay fractions showed significant correlation (r) with b values. Similarly with the rate of K release, clay content of soils maintaine d significant r whereas the silt content did not.