This book summarizes research conducted from 1994 to 2001 to develop a new concept and the tools needed for site-specific nutrient management (SSNM) in irrigated rice systems and the tools needed for applying it in farmers’ fields. As part of IRRI's Irrigated Rice Research Consortium (IRRC), the Reaching Toward Optimal Productivity (RTOP) project has evolved into one of the largest and most important agronomic research projects in the world. More than 100 researchers and support staff in six countries collaborate in this unprecedented network of strategic on-farm and on-station research, representing disciplines such as soil science, agronomy, soil microbiology, pest management, and socioeconomics.