Management of Potassium (K) bearing fertilisers requires to consider the following approach:
Balancing external application of potassium (K) fertilizers and utilization of soil reserve K is an important aspect of plant K-nutrition as rapid depletion of soil reserve K has negative consequences on soil quality and crop productivity. The critical limit of exchangeable K varies for soil to soil and crop to crop although K-rating limits are irrespective of crops or soils. Moreover, supply of soil K to plants is a complex phenomenon involving dynamic equilibrium among its various chemical forms. The K response in vertisols is often anomalous. These limits in Vertisols are not only higher but differ considerably from crop to crop and location to location. It was found the range of critical limit as 451-799 kg/ha, the lowest being for oilseed crops and highest for vegetable crops. The critical limits obtained from LTFE experiments is at around 315 kg K/ha for some Vertisols. The higher critical limits from soil test crop response experiments are because of the higher yield targets taken in calculating the critical limits. Such higher yield targets are seldom obtained in LTFE trials. Also, there is a difference in the way the calculations are made. The critical limit for rice is 553 kg/ha (range 250-796 kg/ha). There is ample opportunity to increase rice yield in Vertisols if K fertilization is done based on soil test. Experiments done under AICRP (STCR) have shown that the fertilizer K requirement is 34 kg K2O/ha at a soil test value of 350 kg/ha for achieving the yield target of 50-60 q/ha for rice, and this requirement changes to 26 kg, and 19 kg if the soil test values are 400, and 450 kg/ha, respectively. Wheat requires almost similar K fertilization as rice. The K fertilizer requirement for maize crop is much less though. It is only 16 kg K2O/ha at a soil test value of 400 kg/ha and 11 kg K2O/ha at a soil test value of 350 kg/ha. The recommendations for cotton is 38 kg K2O/ha. The K requirement is very high in Maharashtra, and almost nil in Karnataka. Such results are difficult to interpret only on the basis of exchangeable K. Vertisols have also been categorized on the basis of non-exchangeable K content.
With warm regards,