Kinetics of nonexchangeable potassium release from a Tropaquept as influenced by long-term cropping, fertilisation, and manuring

We studied the effect of long-term cropping, fertilisation, and manuring on kinetics of nonexchangeable potassium (K) release from a Tropaquept in 0·01 M CaCl2 and 0·01 M citric acid media under a rice–rice cropping system in an experiment started in 1971. Under continuous cropping, the cumulative K release from the soil decreased considerably in all treatments both with and without K application. The lowest K release from the nonexchangeable fraction of the soil was observed in the 100% optimum nitrogen+phosphorus (NP) treatment (55 and 34 mg/kg in 1980 and 1994, respectively), while the highest K release was in the 100% optimum NPK+farmyard manure (FYM) treatment (99 and 66 mg/kg in 1980 and 1994, respectively). Statistical analysis showed that a parabolic diffusion equation best described the K release from soil samples collected in 1980 ( r = 0·989–0·992) and 1994 ( r = 0·994–0·998), thereby indicating that the K release was a diffusion-controlled exchange. A first-order kinetic equation also explained K release behaviour from the soil samples in both years ( r = 0·988 for 1980 and 0·971–0·988 for 1994 samples). The maximum and minimum values of rate constants were found for the 100% NPK+FYM and 100% NP treatments, respectively. A reduction in rate constants was observed with further cropping until 1994, with or without K supply, with the largest reduction being observed in the 100% NP treatment. Kinetic parameters of the first-order equation showed significant correlation with conventional measures of nonexchangeable K forms: 3 M H2SO4 and boiling 1 M HNO3 extractable K. From these results it is concluded that continuous cropping without K inputs through fertilisers or manure caused a decline in nonexchangeable K reserves and release rate while application of recommended NPK plus FYM maintained higher release rates.