On-farm adaptation of knowledge-intensive nitrogen management technologies for rice systems

Efficient use of all inputs is vital to achieve and sustain high crop yields, maintain resource quality, and minimize environmental pollution. Fertilizer N is one of the major inputs in rice production. Blanket fertilizer recommendations do not take into account the high field-to-field variability and within-season dynamic changes in indigenous N supply. Since the plant growth reflects the total N supply from all sources, plant N status will be a good indicator of N availability to crops at any given time. The chlorophyll meter (SPAD) and leaf color chart (LCC) are simple, portable diagnostic tools that can measure the crop N status in situ in rice fields to determine the timing of N topdressing. Such decision aids are useful to vary N application rates to rice crops, based on crop demand and indigenous N supply. Although the chlorophyll meter cannot be owned by individual farmers due to its high cost, it is a practically useful tool for field researchers, extension specialists, and crop consultants who do not have access to well-equipped laboratories. On-farm, adaptive research is in progress in 3 countries to adapt the chlorophyll meter technique for transplanted and wet-seeded rice, local cultivar groups, and soil, crop, and environmental conditions. Initial results indicate that the SPAD threshold value of 35 is good for transplanted rice in dry season. The threshold has to be reduced to 32 for wet-seeded rice in dry season and for all rice during wet season with cloudy weather and low radiation. Thus, when calibrated with local cultivar groups and crop conditions, it can be used to accurately monitor crop N status and to advise farmers on N topdressing for rice. It can also be used effectively to verify the adequacy of existing N fertilizer recommendations to rice by the in situ monitoring of foliar N status of crops fertilized with current recommendations and to refine them to further improve N fertilization of rice. The LCC is not as accurate as the chlorophyll meter in determining the leaf N status in rice crops. However, LCC can be calibrated with the chlorophyll meter to fix the critical color shade for local rice cultivar groups and crop conditions. Farmers can, then, use the LCC to qualitatively assess foliar N status and adjust N topdressing to their rice crops. Initial feedback on the use of LCC from farmer cooperators in the Philippines is highly encouraging. Both methods are affected by factors such as varietal group, plant density, crop stress that causes leaf chlorosis, soil nutrient status, and climate; therefore, they have to be adapted to specific soil, climatic, and crop conditions. Adequate training is necessary for both extension agents and farmers to properly use the new tools for increasing the efficiency N fertilizer use on rice. Wider farmer adoption of the two diagnostic tools discussed in this paper will minimize over-fertilization of rice, increase profitability, and decrease fertilizer-related pollution of the environment.