Potassium Balance under Soybean-wheat Cropping System in a 44 Year Old Long Term Fertilizer Experiment on a Vertisol

Applications of fertilizers alone and in combination with organic manure significantly increased the organic carbon, available nitrogen, phosphorous, potassium, and K fractions, i.e., water soluble-K, exchangeable-K, non-exchangeable-K, lattice-K, and total-K in both surface and subsurface soils and without K input (100% NP & 100% N). Among the various K fractions, lattice K was the dominant K fraction. The highest values of these fractions were found in 100% NPK + 5 t FYM ha(-1). The relative contents of these fractions were in order, lattice-K > non-exchangeable K > exchangeable K > water soluble K, and these were higher in surface than the subsurface soils. Correlation studies revealed significant positive relationship of organic carbon and available N, P, and K with different K fractions in surface and subsurface soils. Furthermore, the minimum depletion of the K rate was recorded in 100% NPK + FYM (-2.25 kg ha(-1) yr(-1)) to that of other treatments, while control showed a maximum K depletion rate (-3.86 kg ha(-1) yr(-1)). Apparent K balance in soil was the lowest, -61 kg ha(-1) yr(-1), in control plot followed by -165 kg ha(-1) yr(-1) in the treatment of 100% NPK + FYM. Hence, much attention is required for potassium fertilizers to maintain the K status of soil and to prevent K mining.