Effect of Long-Term Application of Inorganic Fertilizers and Organic Manure on Yield, Potassium Uptake and Profile Distribution of Potassium Fractions in Vertisol under Soybean-Wheat Cropping System

The present study conducted under All India Coordinated Research Project on Long-Term Fertilizer Experiment was aimed on the effect of inorganic fertilizers with or without organic manure on yield, potassium uptake and distribution of potassium fractions after thirty six years of soybean–wheat cropping sequence during 2009–10 in a Vertisol. The investigations revealed that the maximum yield of soybean (1.84 t ha−1) and wheat (5.26 t ha−1) and K uptake by them was obtained with the treatment 100% NPK+FYM, followed by the treatment receiving 150% NPK. Available-K was found to be maximum with 100% NPK+FYM (295.2 kg ha−1), followed by 150% NPK (284.2 kg ha−1). Moreover, K fractions (water soluble-K, exchangeable-K, non-exchangeable-K, lattice-K and total-K) were significantly decreased with increasing soil depth. The contribution of different K fractions at various soil depths studied was in order of lattice-K > non-exchangeable-K > exchangeable-K > water soluble-K. All the K fractions at 0–20 cm soil depth exhibited significant and positive correlation with yield. Minimum depletion of K rate was recorded in 100% NPK+FYM (−2.08 kg ha−1 yr−1) to that of other treatments, while control showed maximum K depletion rate (−4.65 kg ha−1 yr−1). Apparent K balance in soil was lowest -58.6 kg ha−1 yr−1 in control plot followed by 100.2 kg ha−1 yr−1 in treatment 100% NPK+FYM. Hence, much attention is required for potassic fertilizers to maintain K status of soil and to prevent K mining. Thus, results of the experiment indicate that the depleting K reserves even at its optimal application rates point towards taking a fresh look at the existing recommendation for soybean and wheat crops.