Environmental impact and economic benefits of site-specific nutrient management (SSNM) in irrigated rice systems

Site-specific nutrient management (SSNM) provides a field-specific approach for dynamically applying nutrients to rice as and when needed. This approach advocates optimal use of indigenous nutrients originating from soil, plant residues, manures, and irrigation water. Fertilizers are then applied in a timely fashion to overcome the deficit in nutrients between the total demand by rice to achieve a yield target and the supply from indigenous sources. We estimated environmental impact of SSNM and evaluated economic benefits in farmers’ fields in southern India, the Philippines, and southern Vietnam for two cropping seasons in 2002–2003. On-farm research comparing SSNM and the farmers’ fertilizer practice showed increased yield with SSNM for the three locations, even with reduced fertilizer N rates in some cases. SSNM increased partial factor productivity (kg grain kg−1 fertilizer N) when fertilizer N use efficiency with the farmers’ fertilizer practice was relatively low such as at locations in Vietnam and the Philippines. Use of on-farm data with the DNDC model revealed lower percentage of total N losses from applied fertilizers with SSNM during an annual cycle of cropping and fallows. At the location in India, SSNM showed the potential of obtaining higher yields with increased fertilizer N use while maintaining low N2O emissions. SSNM in the Philippines and Vietnam showed greater yields with less fertilizer N through improved fertilizer use efficiency, which could reduce N2O emissions and global warming. Use of SSNM never resulted in increased emissions of N2O per unit of grain yield, and in environments where higher yield could be obtained with less fertilizer N, the use of SSNM could result in reduced N2O emissions per unit of grain yield. For the economic analysis, data were generated through focus group discussions (FGD) with farmers practicing SSNM and with other farmers not practicing SSNM. Based on FGD, the seasonal increase in yield of farmers solely due to use of SSNM averaged 0.2 Mg ha−1 in southern Vietnam, 0.3 Mg ha−1 in the Philippines, and 0.8 Mg ha−1 in southern India. Farmers practicing SSNM at the study site in India used less pesticide. The added net annual benefit due to use of SSNM was 34 US$ ha−1 year−1 in Vietnam, 106 US$ ha−1 year−1 in the Philippines, and 168 US$ ha−1 year−1 in India. The increased benefit with SSNM was attributed to increased yield rather than reduced costs of inputs.