Release kinetics of nonexchangeable potassium by different extractants from soils of varying mineralogy and depth

Nonexchangeable potassium (K) release kinetics of six major benchmark soil series of India as affected by mineralogy of clay and silt fractions, soil depth and extraction media was investigated. The cumulative release of nonexchangeable K was greater in smectitic soils (353 mg K kg(-1) at 0- to 15-cm depth and 296 mg K kg(-1) at 15- to 30-cm depth, averaged for 2 soils and 3 extractants) than in illitic (151 mg K kg(-1) at 0- to 15-cm depth and 112 mg K kg(-1) at 15- to 30-cm depth) and kaolinitic (194 mg K kg(-1) at 0- to 15-cm depth and 167 mg K kg(-1) at 15- to 30-cm depth) soils. Surface soils exhibited larger cumulative K release in smectitic and illitic soils, whereas subsurface soils had larger K release in kaolinitic soils. Among the extractants, 0.01 M citric acid extracted a larger amount of nonexchangeable K followed by 0.01 M CaCl2 and 0.01 M HCl. The efficiency of citric acid extractant was greater in illitic soils than in smectitic and kaolinitic soils. Release kinetics of nonexchangeable K conformed fairly well to parabolic and first-order kinetic models. The curve pattern of parabolic diffusion model suggested diffusion controlled kinetics in all the soils, with a characteristic initial fast rate up to 7 h followed by a slower rate. Greater nonexchangeable K release rates in smectitic soils, calculated from the first-order equation (b=91.13 x 10(-4) h(-1)), suggested that the layer edge and wedge zones and swelling nature of clay facilitated the easier exchange. In contrast to smectitic soils, higher release rate constants obtained from parabolic diffusion equation (b=39.23 x 10(-3) h(-1)) in illitic soils revealed that the low amount of exchangeable K on clay surface and larger amount of interlayer K allowed greater diffusion gradients, thus justifying the better fit of first-order kinetic equation in smectitic soils and parabolic diffusion equation in illitic soils.