Assessment of bioavailable ammonium-nonexchangeable potassium in calcareous soils by plant growth and sodium tetraphenylboron

Laboratory and greenhouse investigations were carried out to assess bioavailability of NH4+-nonexchangeable K+ in some calcareous soils. The four soil samples were collected from 0-15 and 16-30 cm depth of an Alfisol (Gujranwala series) and three Inceptisols (Missa, Nabipur, and Shujabad series). The soils had 51-79 mg kg(-1) of ammonium acetate extractable K+. The soils were exhausted of their K+ in a greenhouse for a total of 120 days (D) by growing consecutively barley (Hordeum vulgare L.) (60D), maize (Zea mays L.) (30D), and mungbean (Phaseolus vulgaris L.) (30D). Biomass accumulated by the different crop species was significantly (P < 0.01) different on the different soils. Total uptake (283-495 mg K per pot) and concentration of K+ in the three plans species were also significantly (P < 0.01) different on the different soils. Various plant species utilized 32-76% of their total K uptake from NH4+-nonexchangeable K+ reserves of the soils. The original soil samples were simultaneously extracted for their nonexchangeable K+ by sodium tetlaphenylboron (NaTPB). The NaTPB extractions were collected at intervals of 0.5, 1.5, 2.5, 3.5, 4.5, and 5.5 hours (total 18 hours). The K+ extracted by NaTPB from various soils during 18 hours ranged from 495 to 773 mg kg(-1). The release of K+ (Y) from each soil by NaTPB as a function of time (t) was described by the power function equation (ln y = ln a + b ln t). Release rate of K+ (b value of power function) was significantly col related with cumulative plant dry matter yield (r = 0.78, P < 0.05, n = 8), cumulative total K uptake by plants (r = 0.84, P < 0.01, n = 8), and nonexchangeable K+ utilized by plants (r = 0.72, P < 0.05, n = 8).