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Nitrogen Management
USING THE SSNM APPROACH TO FORMULATE LOCALLY ADAPTED N RECOMMENDATIONS The SSNM approach can be used to formulate an N recommendation adapted to any irrigated or favorable rainfed rice environment. Table 2 lists the steps in formulating locally adapted recommendations with examples for high- and low-yielding seasons.
Table 2. Example of N management plans for two contrasting rice-growing seasons. Yield targets depend upon location and can be higher or lower than those in this example.
Step 1: Estimate an attainable yield target. This is the yield attainable by farmers with good management and average climatic conditions. Step 2: Estimate the N-limited yield or yield without fertilizer N. Step 3: Estimate yield response to N from the difference between yield target and N-limited yield. Step 4: Select an attainable agronomic efficiency. Experience in Asia indicates that an AEN of 25 is often achievable with good crop management in high-yielding seasons, and an AEN of 18 or 20 is achievable with good crop management in low-yielding seasons. Step 5: Estimate the total N required based on the yield response and AEN from Table 1. Step 6: Apply a moderate amount of fertilizer N to young rice within 14 days after transplanting (DAT) or 21 days after sowing (DAS). Use the following general guidelines to determine the early application of N before 14 DAT or 21 DAS:
Step 7: Estimate the number of fertilizer N applications by farmers from tillering onward. Step 8: Approximate the amount of N to be applied in each application. Step 9: Use the LCC with either the real-time or the fixed time/adjustable dose option to adjust fertilizer N for these applications based on the crop’s N need.
Option #1: Use of fixed time/adjustable-dose N management The distribution of the required fertilizer N as estimated for the high- and low-yielding seasons in Table 2 is illustrated for the fixed time/adjustable dose option in Figure 4. In this illustration the rice variety is an inbred with 115- to 120-day duration (from seed to seed) and is established by transplanting approximately 21-day-old seedlings.
Fig. 4. Example of fixed-time/adjustable dose N management for transplanted rice with 115- to 120-day duration. Yield targets depend upon location and can be higher or lower than those in this example. The application of fertilizer N before 14 DAT follows the guidelines in step #6 above, although flexibility is provided to apply up to 20 kg N ha−1 early in the low-yielding season when NPK fertilizers are used to supply P and K (Fig. 4). The portion of the required fertilizer N not applied early is added at preset critical growth stages. For an inbred variety with 115- to 120-day duration illustrated in Figure 4, the critical growth stages are active tillering and panicle initiation, and the remainder of the required fertilizer N is split applied at these two times. Panicle initiation is estimated to occur at about 60 days before harvest. For hybrid rice and large panicle-type rice, some fertilizer N is also often applied at heading. The baseline dose of fertilizer N at active tillering and panicle initiation is estimated from the total fertilizer N requirement illustrated in Table 2. In the example for the high-yielding season, the baseline N dose for each of two applications (one at active tillering and one at panicle initiation) is 45 kg N ha−1. This baseline N dose would be 30 kg N ha−1 for each of three applications (two during tillering and one at panicle initiation). The corresponding baseline dose for the low-yielding season is about 25 kg N ha−1 for each of two applications. This baseline N dose represents the estimated amount of fertilizer N required at active tillering and panicle initiation in fields with average crop-growing conditions and in years with average climatic conditions. More fertilizer N is required in fields with higher yield response to N and in years with favorable climate for higher yields. In such cases, the greater growth and N demand of the crop will result in more rapid yellowing of rice leaves and lower LCC readings at the critical stages of active tillering and panicle initiation. Less fertilizer N is required in fields with lower yield response to N and in years with unfavorable climate. In such cases, the rice will require less N and leaves will remain greener longer, resulting in higher LCC readings. The baseline N dose is therefore assigned to an intermediate LCC reading, approximating the leaf color at active tillering and panicle initiation in fields with average crop-growing conditions and in average years. For modern high-yielding indica rice this often corresponds to a reading of about 3.5 (intermediate between panels 3 and 4) in the standardized IRRI LCC. With this option, farmers measure leaf color before applying N at active tillering and panicle initiation. If mean leaf color is above 3.5 (for example ≥4), then less fertilizer N than the baseline is applied. If mean leaf color is below 3.5 (for example ≤3), then more fertilizer N than the baseline is applied (Figs. 4 and 5). Such adjustments in N doses at mid-tillering and panicle initiation are designed to ensure application of more N in fields and years with high plant demand for N and application of less N in fields and years with low demand for N.
Fig. 5. Using the leaf color chart for the fixed-time/adjustable dose N management for transplanted rice (from Witt et al. 2002). To tailor N recommendation to farmer conditions, the doses of fertilizer N can be rounded to the nearest half bag (1 bag = 50 kg) of urea ha−1. The N doses can also be expressed in bags of urea where 23 kg N ha−1 = 1 bag urea ha−1, 35 kg N ha−1 = 1.5 bags urea ha−1, 45 kg N ha−1 = 2 bags urea ha−1, and 60 kg N ha−1 = 2.5 bags urea ha−1.
Option #2: Use of real-time N management The distribution of the required fertilizer N estimated for the high- and low-yielding seasons in Table 2 is illustrated for the real-time N management option in Figure 6. In this illustration the rice variety is an inbred with 115- to 120-day duration (from seed to seed) and is established by transplanting approximately 21-day-old seedlings. The application of fertilizer N before 14 DAT (Fig. 6) is identical to that with the fixed-time/adjustable dose option because the LCC is not used for this early N application. The portion of the required fertilizer N not applied early is applied from tillering onward based on the crop’s need as determined by leaf color.
Fig. 6. Example of real-time N management for transplanted rice with 115- to 120-day duration. Yield targets depend upon location and can be higher or lower than those in this example. Farmers monitor the rice leaf color at 7- to 10-day intervals from tillering to about 5 to 10 days after panicle initiation for inbred rice, and up to heading for hybrid rice and large panicle-type rice. Farmers apply fertilizer N whenever the leaves become more yellowish-green than a critical threshold value indicated on the LCC (Fig. 7).
Fig. 7. Using the leaf color chart for real-time N management for rice (from Witt et al. 2002).
The dose of fertilizer N is estimated from the total fertilizer N requirement and the estimated number of N applications from tillering onward in an average field or year as illustrated in Table 2. Leaf color is typically monitored a total of four to five times for inbred rice and five to six times for hybrid rice. The effective use of real-time N management requires the selection of an N dose and a critical threshold LCC color that ensure two to three N applications in an average yielding field or year. In fields and years with above average growth and crop N demand rice leaves will turn yellow more rapidly, increasing the likelihood of more N applications and hence more fertilizer N use. In fields and years with below average yield, the rice will require less N and leaves will remain greener longer, resulting in fewer N applications and less fertilizer N use. The critical threshold value can be adjusted for cultivars and crop establishment method. Thresholds for cultivars with inherently yellowish leaves should be more yellowish green than for cultivars with inherently dark green leaves. Use of an excessively green threshold value can result in more N applications and excessive N use, whereas use of an excessively yellow threshold value can result in insufficient N use for high yield. In the example for the high-yielding season, the baseline dose is 45 kg N ha−1 when two N applications are assumed or 30 kg N ha−1 with three N applications (Table 2). Rice farmers in Asia increasingly have gainful alternative activities to rice production and increasingly prefer fewer fertilizer N applications. The higher N dose with less N applications was therefore selected for illustrating real-time N management (Fig. 6).
References Witt C, Dobermann A, Abdulrachman S, Gines GC, Wang GH, Nagarajan R, Satawathananont S, Son TT, Tan PS, Tiem LV, Simbahan GC, Olk DC. 1999. Internal nutrient efficiencies of irrigated lowland rice in tropical and subtropical Asia. Field Crops Res. 63:113-138.
Witt C, Buresh RJ, Peng S, Balasubramanian V, Dobermann A. 2007. Nutrient management. In: Fairhurst TH, Witt C, Buresh R, Dobermann A, eds. Rice: A practical guide to nutrient management. Los Baños (Philippines) and Singapore: International Rice Research Institute (IRRI), International Plant Nutrition Institute (IPNI), and International Potash Institute (IPI). p 1-45.
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