Subsoil Potassium Availability in Twenty-Two Benchmark Soil Series of India

Potassium (K) is an essential plant nutrient. The K present in subsoil, either from leaching of the applied K or from genetic origin, would be available to plants in substantial amounts. A great majority of studies on K in soils have been solely concerned with samples collected from the surface soils (0–15 cm). The determination of subsoil K in benchmark soil series is thus indispensable for optimum K fertilizer recommendations. Different forms of K in subsoil (15–30 cm) were evaluated in relation to those in surface soil (0–15 cm) in 22 benchmark soil series of India. Samples were analyzed for 0.01 M calcium chloride (CaCl2), 0.01 M citic acid, 1.38 N and 6 N H2SO4, ammonium acetate (1 N NH4OAc) and sodium tetraphenyl boron (NaTPB) extractable K. Potassium extracted by the different extractants except NaTPB (non-exchangeable K) extractable K was higher in smectitic soils followed by illitic and kaolinitic soils. Results indicated substantial amounts of K in subsoils, irrespective of the dominance of a particular mineralogical suite. Potassium in the subsoil constituted up to 96% of the surface soil K. The subsoil K showed highly significant correlation with the surface soil K and the r values were 0.94, 0.97, and 0.94 for kaolinitic Alfisols and Inceptisols; illitic Inceptisols, Entisols and Aridisols and smectitic Vertisols and Vertic sub groups, respectively. All the soil groups possessed substantial amounts of subsoil K particularly for the deep-rooted crops and, therefore, the subsoil K should be considered in formulating fertilizer K requirements of crops.