Monitoring the changes in soil potassium by extraction procedures and electroultrafiltration (EUF) in a Tropaquept under twenty years of rice-rice cropping

Chemical extraction procedures and electroultrafiltration (EUF) fractions w ere evaluated for measuring the changes in soil K in a Tropaquept as a cons equence of continuous fertilizer use and rice-rice cropping for 20 years in a long-term fertilizer experiment. During the course of 20 years the gap i n total K uptake by crops in treatments receiving K and those without an ex ternal K supply widened, indicating a stress on soil K reserves in the latt er treatments. Readily available forms, i.e. water-soluble, 0.01 M CaCl2, c itric acid and ammonium acetate extractable K did not undergo much change. On the contrary, there was a conspicuous decrease in strongly held 3 M H2SO 4 and 1 M boiling HNO3 K (nonexchangeable K), as these forms replenished th e soil solution K removed by the crop plants. Similarly, considerable decre ase was noticed in EUF 30-35 K obtained at higher temperature and voltage, as it also represents nonexchangeable K in soils. NPK and NPK plus FYM arre sted, to a greater extent, the depletion in soil K. Huge K removals, from 1 000 to 3550 kg K ha(-1), by the rice crop in 20 years were not reflected in NH4OAc K which is commonly used as a criterion for fertilizer K recommenda tions by the soil testing laboratories in India. The total K uptake by the rice crop during 20 years corresponded closely to the changes in nonexchang eable K as measured by 3 M H2SO4 and 1 M HNO3. This suggests the need for i ncluding nonexchangeable K in soil test calibration for better K fertilizer recommendations in long-term operations.