Subsoil potassium availability in twenty-two benchmark soil series of India

Potassium (K) is an essential plant nutrient. The K present in subsoil, eit her from leaching of the applied K or from genetic origin, would be availab le to plants in substantial amounts. A great majority of studies on K in so ils have been solely concerned with samples collected from the surface soil s (0-15 cm). The determination of subsoil K in benchmark soil series is thu s indispensable for optimum K fertilizer recommendations. Different forms o f K in subsoil (15-30 cm) were evaluated in relation to those in surface so il (0-15 cm) in 22 benchmark soil series of India. Samples were analyzed fo r 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) extract able It. Potassium extracted by the different extractants except NaTPB (non -exchange able K) extractable K was higher in smectitic soils followed by i llitic 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 an d the r values were 0.94, 0.97, and 0.94 for kaolinitic Alfisols and Incept isols; illitic Inceptisols. Entisols and Aridisols and smectitic Vertisols and Vertic sub groups, respectively. All the soil groups possessed substant ial amounts of subsoil K particularly fur the deep-rooted crops and, theref ore, the subsoil K should be considered in formulating fertilizer K require ments of crops.