Strategies for nutrient management in irrigated and rainfed lowland rice systems

In this paper we review key issues determining nutrient management strategies in irrigated and rainfed lowland riceland and we present two examples of site-specific nutrient management (SSNM) strategies. First, a framework for SSNM in irrigated rice is described, where attaining maximum economical yield is the priority of farmers and SSNM practices must be tailored to the large between-field differences in indigenous nutrient supply. The iterative procedure proposed includes: (1) estimation of the potential indigenous supplies of N (INS), P (IPS), and K (IKS, all in kg ha-l) and diagnosis of other nutritional disorders in year one; (2) estimation of a field- or farm-specific recommendation for fertilizer use based on nutrient interactions and economic yield target; (3) optimization of timing and amount of applied N based on actual plant growth; (4) estimation of the change in INS, IFS and IKS based on the nutrient balance after harvest; and (5) use of the adjusted INS, EPS, and IKS as model inputs for the subsequent rice crop. In a second example we discuss the SSNM strategy that is being developed for rainfed rice farmers in Cambodia. Rainfed lowland rice farmers give, priority to reducing risk and their ability to invest in cost-intensive innovative technologies is limited. Farmers' knowledge and experience become vital for the efficient management of nutrients in these environments. In our approach, technologies are generated and tested through research at selected representative sites. Extrapolation and site-specific application of new nutrient management technologies is then supported through a new agronomic soil classification and probabilistic modeling to account for farmer's knowledge and experience. The agronomic soil classification system contains three hierarchical levels and focuses on soil description in the field with relevance to nutrient management. Based on this, preliminary soil-specific fertilizer recommendations for rice have bean worked out.