As fertilizer prices increase, research and extension often send farmers a message of “reduce fertilizer use to save money.” But, crop yield is directly related to the amount of nutrient taken up by the crop. At some point, less fertilizer use means lower crop yield and less profit for farmers. How much fertilizer use is just right for high profit?
The answer can come from site-specific nutrient management (SSNM). This approach to farming enables farmers to optimize their use of fertilizer by matching the amount and timing of each added nutrient (nitrogen, phosphorus, and potassium) with the needs of the crop for each nutrient. Nitrogen (N) is typically the nutrient that most limits rice yields and hence the nutrient needed in largest quantity from fertilizer. Much of the N in a mature rice plant comes from the soil. On a typical rice soil in the Asian tropics, the yield of irrigated rice often reaches about 4 tons per hectare without application of N fertilizer, as long as crop management uses best practices and water is sufficient. But, markedly higher yields of irrigated rice are required to meet food needs and achieve higher profit for farmers.
How much N is needed from fertilizer to increase rice yield from a baseline—in which the crop obtains its entire N from soil—to a yield that provides the highest profit for a farmer? Based on SSNM, about 40 kg N from fertilizer must be added to increase grain yield by 1 ton per hectare in a high-yielding season (typically the dry season) and about 50 to 60 kg N is needed to increase grain yield by 1 ton in a low-yielding season (typically the wet season).
Assume, for example, that a farmer can typically achieve a rice grain yield of 5 tons per hectare in the lower yielding season during the year. Achieving this yield would then require sufficient N from fertilizer to increase yield by about 1 ton from the baseline of about 4 tons per hectare.
This requires about 50 to 60 kg fertilizer N per hectare. Assume that the farmer can typically achieve a rice yield of 7 tons per hectare in the higher-yielding season. Achieving this yield would require sufficient N from fertilizer to increase yield by about 3 tons from the baseline of about 4 tons per hectare. This corresponds to three times 40 kg or about 120 kg fertilizer N per hectare.
Through the use of such simple guidelines, extension workers and farmers can quickly evaluate current practices, thereby determining whether more or less N fertilizer is required. The required N fertilizer should be split into about three applications during the growing season based on SSNM principles for optimally “feeding” the needs of the crop for N at critical growth stages. The needs of rice for phosphorus (P) and potassium (K) are directly related to grain yield.
P in fertilizer is expressed on the basis of its oxide form—P2O5. For each ton of grain yield, a mature crop of modern high-yielding rice typically contains the equivalent of about 6 kg P2O5 within its biomass. Hence, a 6-tons-per-hectare crop contains about 36 kg P2O5 at maturity. Two-thirds of this P is in the grain.Therefore, with the harvest of grain and removal of some straw, about 5 kg P2O5 per hectare is removed from a rice field for each ton of grain yield. Hence, for a 6-tons-per-hectare crop, about 30 kg P2O5 must be replaced using P fertilizer. As a general principle, irrigated rice with a history of P fertilizer use requires about 4 to 5 kg P2O5 per hectare from fertilizer—depending on the amount of straw retained—for each ton of grain yield to maintain soil fertility and achieve high profit.
The need for K fertilizer depends upon the management of rice straw—which contains most of the K in a rice crop. It also depends on K contained in irrigation water and the K-supplying capacity of the soil, which are typically not known by farmers. SSNM provides farmers with a simple field plot technique for tailoring K fertilization to field-specific needs.
The capacity of soil to supply nutrients and promote yield can vary markedly among fields of rice farmers. The SSNM approach helps farmers determine the needs for nutrients in their specific fields based on simple observations.
Dr. Buresh is a senior soil scientist at the International Rice Research Institute.