The role of potassium in sustaining yields in a long-term rice-wheat experiment in the Indo-Gangetic Plains of Nepal

A long-term soil fertility experiment (1988–1999) at the Regional Agricultural Research Station, Bhairhawa, Nepal, was analysed to determine: (1) how long the yields of rice (Oryza sativa L.) and wheat (Triticum aestivum L.) can be sustained without K but with N and N+P (NP) applied with or without farmyard manure (FYM) and green manure, and (2) the impact of K application on yields. Starting from the 1995 wheat season, the experiment was modified to accommodate K at 0, 42, and 84 kg ha–1 in plots receiving NP to study the response of rice and wheat to K. Both rice and wheat responded to K application but the response of wheat was substantially higher, indicating that the availability of native K may have been lower in wheat. Rice yields were lower in treatments without P than with P, and yields declined significantly (0.11–0.20 Mg ha–1 year–1) in all the treatments except in NP and NP+FYM. Wheat yield was more adversely affected than rice yield when P and K were not applied. In addition, wheat yields were low (average 0.5–2.1 Mg ha–1 in various treatments). Wheat yields declined (0.08–0.12 Mg ha–1 year–1) in all but FYM treatments indicating the role of FYM in sustaining yields. The interaction of K deficiency with Helminthosporium leaf blight (spot blotch and tan spot) is also suggested as one of the factors limiting wheat yields. The estimated K balance in soil was highly negative. Results suggest that farmers should apply adequate amount of K for higher and sustainable rice and wheat yields.