DEPLETION OF SOIL POTASSIUM UNDER PEARL MILLET-WHEAT AND GUAR-WHEAT CROPPING SEQUENCE IN A TROPICAL SOIL IN INDIA

A long-term field experiment was conducted to study grain yield and th e depletion of native and applied K under pearl millet-wheat and guar- wheat rotations. In the presence of P, an application of K significant ly increased the grain yield in both rotations. In general K applicati on alone, however, had no significant effect on crop yields. Total ann ual removals of K were significantly higher in plots receiving maximum amounts of P and K (P60K90) than the control plots (P0K0). The levels of exchangeable K decreased from 714 kg ha(-1) to 250 kg ha(-1) in co ntrol plots of the pearl millet-wheat rotation. Even after 13 years of cropping the values of exchangeable K were still above the critical l imit (110 kg ha(-1)). The nonexchangeable K pool was depleted from 446 0 to 1010 kg ha(-1) in 13 years of cropping of control plots. The cont ribution of nonexchangeable K to plant K was observed as high as 94% i n untreated plots in pearl millet-wheat and 84% in guar-wheat rotation s. The regression coefficients between free energy change Delta G(r) a nd exchangeable and nonexchangeable K, and between K fixation capacity and exchangeable and nonexchangeable K, were all negative.