RELATIONSHIP OF K FORMS AND RELEASE CHARACTERISTICS WITH CLAY MINERALOGY

The relationship between K forms (water soluble K, NH4OAc- and HNO3-extractable K) and characteristics of laboratory release of non-exchangeable K were examined as a function of clay mineralogy and taxonomy, using samples from some agriculturally important bench mark soil series of India. Potassium release characteristics were studied in samples of soils having NH4OAc- and HNO3-extractable K in the vicinity of the modal values in the various series. The release of "external" and "lattice" K was computed from the "K release curve" and computed estimates were compared within and across the soil groups. Generally, kaolinitic soils were low, mixed and illitic soils WERE medium, and smectitic soils were high in NH4OAc-K. In the estimates of HNO3-K, kaolinitic, mixed and smectitic soils were low, and illitic soils were high. The amounts and rates of K release were positively and significantly correlated with the measured K reserves. The K release was greatest in illitic and least in kaolinitic soils. The results show that while kaolinitic soils are low in both NH4OAc-K and HNO3-K and also in the release of their non-exchangeable K, the situation is different in smectitic and illitic soils. Although smectitic soils are high in NH4OAc-K, their HNO3-K is modest and the rate of K release is moderate. On the other hand, illitic soils have modest amounts of NH4OAc-K but high amounts of HNO3-K, and are associated with high K release rates. Hence, it is advisable to consider the amount of both the exchangeable and non-exchangeable K and clay mineralogy of soils in arriving at a judgement of K release and availability in soils.