Yield and soil nutrient changes in a long-term rice-wheat rotation in India

Major improvements in the productivity of rice (Onza sativa L.) and wheat (Triticum aestivum L.) have occurred in South Asia since 1965-1966 when the Green Revolution began. However, after the 1980s, yield stagnated or declined. We analyzed grain yield trends, soil C, N, P, and K status, and P and K balances in a 14-yr rice-wheat experiment conducted at Punjab, India with 11 treatments comprised of various combinations of inorganic and organic sources of nutrients. Recommended levels of N, P, and K were supplemented with N through farmyard manure (FYM), wheat chopped straw (WCS), or sesbania (Sesbania cannabina Linn. & Merrill). Soil parameters were analyzed in archived soil samples collected periodically from 1988 to 1999. Rice yield declines ranged from 0.07 to 0.13 Mg ha(-1) yr(-1) dependent on treatment. Wheat yields declined by 0.04 Mg ha(-1) yr(-1) with applications of 75 and 100% N-P-K fertilizer but were maintained over the 14-yr period in the other treatments. Total soil N and available P and K declined in all the treatments except with FYM, in which total N was maintained and available P increased. Total soil C was either maintained or increased with time. Nitrogen and K depletion may have collectively contributed to the yield decline. Stable wheat yields may have been because of continuing variety improvement, resulting in higher harvest index (HI). Results show that current fertilizer recommendations are inadequate for maintaining yields. This cropping system may not be sustainable without increased K input to maintain soil K above sufficiency levels.