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 were evaluated for measuring the changes in soil K in a Tropaquept as a consequence 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 in total K uptake by crops in treatments receiving K and those without an external K supply widened, indicating a stress on soil K reserves in the latter treatments. Readily available forms, i.e. water-soluble, 0.01 M CaCl2, citric acid and ammonium acetate extractable K did not undergo much change. On the contrary, there was a conspicuous decrease in strongly held 3 M H2SO4 and 1 M boiling HNO3 K (nonexchangeable K), as these forms replenished the soil solution K removed by the crop plants. Similarly, considerable decrease 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 arrested, to a greater extent, the depletion in soil K. Huge K removals, from 1000 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 recommendations by the soil testing laboratories in India. The total K uptake by the rice crop during 20 years corresponded closely to the changes in nonexchangeable K as measured by 3 M H2SO4 and 1 M HNO3. This suggests the need for including nonexchangeable K in soil test calibration for better K fertilizer recommendations in long-term operations.