Long-term fertilization effects on soil potassium and crop yields in a Vertisol

Effect of long-term fertilization and manuring on soil potassium (K) fractions in black soil (Vertisol) was investigated after 42 years of cultivation. Soils from 6 treatments, viz. control (unfertilized), N (100% of recommended N fertilizer), NP (N+ 100% of recommended P fertilizer), NPK (NP+ 100% of recommended K fertilizer), 150% NPK (150% of recommended N, P and K fertilizer), NPK+FYM (NPK+ farm yard manure), and adjacent uncultivated land were collected from 0-15 and 15-30 cm depths. Water soluble K (WSK), exchangeable K (EK) and non-exchangeable K (NEK) in soil; grain yields and K uptakes of soybean and wheat; and annual apparent K balances for different treatments were determined. Long-term variation in fertilization significantly altered the WSK, EK and NEK fractions in the soil. Cropping with K fertilizer resulted in higher WSK and EK than cropping without K fertilizer. Non-exchangeable K estimated by boiling nitric acid extraction showed higher values than that extracted by sodium tetraphenyl boron for five minutes. Wheat grain yield and total K uptake by the two crops showed positive response to K fertilization. Apparent annual K balances for 2013-14 crop cycle were negative irrespective of nutrient supply options. In general, all the K fractions and especially the exchangeable and interlayer K showed decline over uncultivated (or initial) soil. Findings of the present study, thus, highlight the need to develop a more pragmatic K fertilization approach based on exchangeable and non-exchangeable K reserves so as to minimize the decline in soil K-fertility under intensive cultivation.