Long-term changes in yield and soil fertility in a twenty-year rice-wheat experiment in Nepal

A long-term rice-wheat experiment was conducted at Parwanipur, Nepal, to study the effects of organic and mineral sources of nutrients on yield and nutrient status of the soil. Twelve treatments comprising different combinations of inorganic N, P, and K; farmyard manure (FYM); and wheat chopped straw (WCS) were included. On average during a 20-year period, the control plot with an indigenous nutrient supply supported 2.06 Mg rice ha–1 and 0.69 Mg wheat ha–1. The application of 100 kg N ha–1 increased yields, but the addition of P, K, Zn, and S gave no response, indicating that the soil supply of these nutrients did not limit yield. The grain yield of rice and wheat was maintained over the years. Soil analyses of the past 6 years (1994–1999) showed that treatments receiving organic sources of nutrients increased total soil C and N from 18% to 62% and 15% to 48%, respectively, compared with the NPK treatment. There was a buildup of total P and Olsen P in plots receiving FYM. However, total and available soil K were similar in all the treatments. The apparent N and P balances for the rice-wheat system were positive in the NPK, FYM, and WCS treatments, whereas the K balance was negative in all the treatments except with the application of FYM to both rice and wheat. Rice and wheat in most years yielded <3.15 Mg ha–1 and <2.16 Mg ha–1, respectively. These yields are only 40–50% of the potential attainable yields of this region. Possible reasons for low yields are discussed.