Abstract
A long-term field experiment was conducted to study grain yield and the depletion of native and applied K under pearl millet -wheat and guar-wheat rotations. In the presence of P, an application of K significantly increased the grain yield in both rotations. In general K application alone, however, had no significant effect on crop yields. Total annual 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 control plots of the pearl millet -wheat rotation. Even after 13 years of cropping the values of exchangeable K were still above the critical limit (110 kg ha-1). The nonexchangeable K pool was depleted from 4460 to 1010 kg ha-1 in 13 years of cropping of control plots. The contribution of nonexchangeable K to plant K was observed as high as 94% in untreated plots in pearl millet -wheat and 84% in guar-wheat rotations. The regression coefficients between free energy change ?Gr and exchangeable and nonexchangeable K, and between K fixation capacity and exchangeable and nonexchangeable K, were all negative.
