Potassium balance and release kinetics of non-exchangeable K in a typic natrustalf as influenced by long term fertilizer use in rice-wheat cropping system

Abstract

Crop K requirement under negative soil K balance due to imbalanced NPK fertilization in intensive cropping system is mainly met through potassium released from non-exchangeable sources. Efforts were made to study the effects of recommended levels of N and P with or with out K fertilizers on K balance, contribution and release kinetics of non-exchangeable K in a sandy loam (Typic Natrustalf) gypsum amended alluvial alkali soil under a long-term fertilizer experiment on rice (Oryza sativa L.) wheat (Triticum aestivum L.) initiated in 1974. Soil profile samples were collected in 2003 after the harvest of rice from treatments: control (no fertilizer), nitrogen (N), nitrogen + phosphorus (NP), nitrogen + phosphorus + potassium (NPK). Use of NPK resulted in higher contents of water soluble, exchangeable, non-exchangeable and total K in soil compared to N and NP treatments but the contents were at par with control. Crop K uptake exceeded added K indicating depletion of native K in the order: NP > N > NPK > control. Addition of optimum level of N or NP fertilizers in rice-wheat cropping system for 30 years resulted in to 13 and 41% higher mining of soil K compared to NPK fertilization. Depletion of exchangeable K ranged from 85 to 157 mg kg -1 , due to initial native high status the contents were not reduced to a level where deletion of K fertilizer could influence crop yields. To meet the crop K requirement, non-exchangeable sources contributed on an average about 95% in absence of applied K and 65% with added K. The kinetics of non-exchangeable K in 0.01 M CaC12 medium were best described by first order equation as evidenced by the highest coefficient of correlation (r = -0.985 to -0.988) and the lowest value of standard error of estimate (SE = 0.153 to 0.172). Continuous cropping for 30 years with N and NP fertilizers resulted in reduction of cumulative K release compared to no fertilization. Addition of K assisted in maintenance of release of non-exchangeable K. Continuous intensive rice - wheat cropping with N and P fertilizers alone would deplete soil K more and more and may require K addition to sustain soil K fertility.