Rice requires an adequate supply of nutrients to achieve the high yields necessary to feed growing populations. Many of these nutrients come from soil and organic inputs, such as crop residues and manures; but high yields still require supplemental nutrients from fertilizer. Existing fertilizer recommendations for rice often advise fixed rates and timings of N, P, and K for vast areas of rice production. Such recommendations assume the need of a rice crop for nutrients is constant among years and over large areas. But crop-growth and crop-need for supplemental nutrients can be strongly influenced by crop-growing conditions, crop and soil management, and climate - which can vary greatly among fields, villages, seasons, and years.

Site-specific nutrient management (SSNM) as developed in Asian riceproducing countries provides an approach for 'feeding' rice with nutrients as and when needed (IRRI, 2006). SSNM strives to enable farmers to dynamically adjust fertilizer use to optimally fill the deficit between the nutrient needs of a highyielding crop and the nutrient supply from naturally occurring indigenous sources, including soil, crop residues, manures, and irrigation water. The SSNM approach does not specifically aim to either reduce or increase fertilizer use. Instead, it aims to apply nutrients at optimal rates and times in order to achieve high rice yield and high efficiency of nutrient use by the rice, leading to high cash value of the harvest per unit of fertilizer invested.

The demand of rice for N is strongly related to growth stage. In order to achieve high yield, rice plants require sufficient N at early and mid-tillering stages (branching) to achieve an adequate number of panicles (grain bunches), at panicle initiation stage to increase spikelet (flower) number per panicle, and during the ripening phase to enhance grain filling. The supply of N from soil and organic sources is seldom adequate for high yield, and supplemental N is typically essential for higher profit from rice fields. The SSNM approach enables farmers to apply fertilizer N in several doses to ensure the supply of sufficient N is synchronized with the crop need for N.

A key ingredient for managing N to meet crop need is a method for rapidly assessing leaf N content, which is closely related to photosynthetic rate and biomass production and is a sensitive indicator of the N demand during the growing season. A chlorophyll meter can provide a quick estimate of the leaf N status, but it is relatively expensive (Peng et al., 1996). The leaf color charts (LCC), on the other hand, is an inexpensive and simple tool for monitoring the relative greenness of a rice leaf as an indicator of the leaf N status (Balasubramanian et al., 1999; Witt et al., 2005). The LCC is typically a plastic, ruler-shaped strip containing four or more panels that range in color from yellowish green to dark green.

SSNM provides two complementary and equally effective options for improved N management using the LCC. In the 'real-time' N management option, farmers monitor the rice leaf color regularly (e.g. once a week) and apply fertilizer N whenever the leaves become more yellowish-green than the critical threshold value indicated on the LCC. In the 'fixed-time/adjustable dose' option, the time for N fertilization is pre-set at critical growth stages, and farmers adjust the dose of N upward or downward based on the leaf color.

The real-time and fixed-time/adjustable-dose options for N management are typically comparable in terms of grain yield and profit when implemented according to the guidelines of SSSM. The selection of an option for using the LCC can be based on farmer preferences and location-specific factors. The fixed-time/adjustable-dose option, for example, is less time-consuming and is preferred by farmers with gainful non-rice activities and insufficient time for weekly visits to their rice fields. The real-time option is generally preferred when farmers lack sufficient understanding of the critical stages for optimal timing of fertilizer N. The effective management of N with both approaches, however, requires sufficient application of P, K, and micronutrients to overcome limitations of other nutrients.

The SSNM approach advocates sufficient use of fertilizer P and K to overcome P and K deficiencies, to avoid the mining of soil P and K and to allow best N management. Fertilizer P and K requirements, sufficient to overcome deficiencies and maintain soil fertility, are determined with a nutrient decision support system (Witt and Dobermann, 2004), which maintains the scientific principles of the underlying QUEFTS model for rice (Janssen et al., 1990, Witt et al., 1999). Outputs of the nutrient decision support system have been summarized in Table 1 and Table 2 (Witt et al., 2002), whereby fertilizer P₂O₅ and K₂O rates are obtained from an estimate of attainable yield target and either the P- or K-limited yield. The yield target must be realistically attainable by farmers. It can be estimated from the grain yield in a fully fertilized plot with no nutrient limitations and good management (for example, the NPK plot or NPK plus micronutrient plot). P- and K-limited yields are determined by the nutrient omission plot technique.

With the nutrient omission plot technique, one plot of rice is grown with abundant fertilizer supplements (NPK plot or NPK plus micronutrient plot)and the yield thus achieved is used to calculate the full demand of rice for P and K. The attained yield can also serve as a yield target. Rice is simultaneously grown in two other plots, one without added P fertilizer and the other without added K.The rice yield in the plot without fertilizer P provides an estimate of P-limited yield, and the rice yield in the plot without fertilizer K provides an estimate of Klimited yield. The yields are then used with Table 1 and Table 2 to estimate optimal rate fertilizer P₂O₅ and K₂O rates, which overcome P and K deficiencies and include sufficient P and K to prevent depletion of soil fertility arising from their long-term removal with grain and straw.

Researchers developed the SSNM approach in the mid 1990s and evaluated it from 1997 to 2000 on about 200 irrigated rice farms at eight sites in Asia. Since 2001, the on-farm evaluation and promotion of SSNM have markedly increased. In 2003 to 2005, SSNM was evaluated and promoted with farmers at about 20 locations in tropical and subtropical Asia (IRRI 2006), each representing an area of intensive rice farming.

References

• Balasubramanian, V., Morales, A.C., Cruz, R.T. and S. Abdulrachman. 1999. On-farm adaptation of knowledgeintensive nitrogen management technologies for rice systems. Nutr. Cycling Agroecosyst. 53:59–69.
• International Rice Research Institute Research findings Optimizing Crop Nutrition (IRRI). 2006. Site-specific nutrient management. http://www.irri.org/irrc/ssnm/. Accessed 23 Oct 2006.
• Janssen, B.H., Guiking, F.C.T., van der Eijk, D., Smaling, E.M.A., Wolf, J. and H. van Reuler. 1990. A system for quantitative evaluation of the fertility of tropical soils (QUEFTS). Geoderma 46: 299-318.
• Peng, S., Garcia, F.V., Laza, R.C., Sanico, A.L., Visperas, R.M. and K.G. Cassman. 1996. Increased N-use efficiency using a chlorophyll meter on high-yielding irrigated rice. Field Crops Res. 47:243– 252.
• Witt, C., Dobermann, A., Abdulrachman, S., Gines, G.C., Wang, G.H., Nagarajan, R., Satawathananont, S., Son, T.T., Tan, P.S., Tiem, L.V., Simbahan, G.C. and D.C. Olk. 1999. Internal nutrient efficiencies of irrigated lowland rice in tropical and subtropical Asia. Field Crops Res. 63:113-138.
• Witt, C., Balasubramanian, V., Dobermann, A. and R.J. Buresh. 2002. Nutrient management. pp 1-45. In Fairhurst T.H., Witt C. (eds.) Rice: a practical guide to nutrient management. Potash and Phosphate Institute (PPI), Potash and Phosphate Institute of Canada (PPIC), and International Rice Research Institute (IRRI).
• Witt C, Dobermann A. 2004. Toward a decision support system for site-specific nutrient management. pp 359-395. In Dobermann A, Witt C, Dawe D (eds). Increasing the productivity of intensive rice systems through site-specific nutrient management. Enfield, NH (USA) and Los Baños (Philippines): Science Publishers, Inc., and International Rice Research Institute (IRRI).
• Witt, C., Pasuquin, J.M.C.A., Mutters, R. and R.J. Buresh. 2005. New leaf color chart for effective nitrogen management in rice. Better Crops 89 (no. 1): 36-39.