Site Specific Nutrient Management

Nitrogen Management

BACKGROUND ON MAKING AN N RECOMMENDATION

With the SSNM approach, fertilizer N recommendations for an irrigated or favorable rainfed rice-growing environment can be developed by:

1. estimating the total fertilizer N required for rice in a typical season, and then

2. formulating dynamic N management to distribute fertilizer N to best match the crop’s need for N.

Estimating the total fertilizer N required for rice in a typical season

The fertilizer N required by a rice crop is estimated through the three steps in the SSNM approach.

Step 1: Establish an attainable yield target

The yield target provides an estimate of the total amount of N needed by the rice crop because the amount of N taken up by a rice crop is directly related to crop yield (Fig. 1). The yield target depends upon the location-specific conditions of climate, rice cultivar, and crop management. It should not exceed 80 to 90% of climatic and genetic potential yield.

At maturity, modern rice varieties with harvest indices of 0.45 to 0.55 and balanced nutrition of N, P, and K typically accumulate in aboveground biomass an average of 15 kg N for each metric ton (1,000 kg) of unmilled grain yield (Witt et al. 1999). Somewhat higher amounts of N accumulate per ton of grain yield when yield targets exceed 70% of the climatic and genetic potential yield.

Step 2: Effectively use existing nutrients

Much of the N taken up by rice comes from naturally occurring (indigenous) sources, which include the soil, organic amendments, crop residue, manure, and irrigation water. This indigenous supply of N can be estimated from the total amount of N taken up by a mature rice crop that receives no fertilizer N and is not limited by other nutrients. Because the amount of N taken up by rice is directly related to yield, indigenous N supply can then be estimated from N-limited yield, which is the grain yield for a crop not fertilized with N but fertilized with other nutrients to ensure they do not limit yield (Fig. 1).

Fig. 1. Steps in determining fertilizer N required for rice.

The N-limited yield can be determined by the nutrient omission plot technique. When results from the omission plot technique are unavailable, information on use of organic amendments, soil texture, soil testing, or previous measurements of N-limited yield on similar soils can often be used to suitably estimate N-limited yield. The direct measurement of N-limited yield by the omission plot technique is not required when N-limited yield can be estimated from other information within an accuracy of ±0.5 t ha⁻¹.

Step 3: Apply fertilizer to fill the deficit between crop need and indigenous supply

Fertilizer N is required to supplement the nutrients from indigenous sources and achieve the yield target. The total fertilizer N requirement depends upon the deficit between the crop’s total N need to achieve a yield target and the N supply from indigenous sources, as determined by the N-limited yield. This deficit in N that must be filled by fertilizer N is directly related to the estimated yield response to fertilizer N, which is the difference between the yield target and the N-limited yield (Fig. 1). Modern rice varieties with harvest indices of 0.45 to 0.55 will take up about 15 to 20 kg N from fertilizer N for each metric ton increase in grain yield between the yield target and the N-limited yield.

Only a fraction of the fertilizer N applied to rice is taken up by the crop. Hence, the fertilizer N required for each ton in grain yield response will be greater than the approximately 15 to 20 kg N taken up by rice. The total amount of fertilizer N required for each ton of grain yield increase therefore depends upon the efficiency of fertilizer N use by rice.

The amount of fertilizer N required by rice (FN) is determined from the grain yield response to fertilizer N and the efficiency of fertilizer N use (AE_N) as indicated by the following equation

F_N = (Y_Target– Y_0N)/AE_N

where

F_N = fertilizer N required (kg ha⁻¹)

Y_Target= yield target (kg ha⁻¹or t ha⁻¹* 1000)

Y_0N= N-limited yield (kg ha⁻¹ or t ha⁻¹* 1000)

Y_Target– Y_0N = yield response (kg ha⁻¹or t ha⁻¹* 1000)

AE_N= increase in yield per unit of fertilizer N (kg grain yield increase kg N applied⁻¹)

The AE_N decreases with increasing rate of fertilizer N. At the fertilizer N rate corresponding to maximum profit, the optimal range for AE_N in farmers’ fields is often in the range of 18 to 25 kg kg⁻¹ (Fig. 2).

Fig. 2. Relationship between grain yield and agronomic efficiency of fertilizer N (AE_N).

Table 1 shows the simplified guidelines in estimating fertilizer N required by rice based on grain yield response to fertilizer N and the efficiency of fertilizer N use (AE_N). An AE_N of 25 is often achievable with good crop management in seasons with favorable climate (such as high solar radiation) for high yields. An AE_N of 18 or 20 is typically achievable with good crop management in seasons with less favorable climatic conditions for yield, such as rainy seasons with reduced solar radiation. An AE_N of 15 can be targeted for environments where existing fertilizer N management practices are very inefficient with AE_N in farmers’ fields of about 10 kg kg⁻¹ or less.

Table 1. Estimation of fertilizer N required for rice based on yield response to fertilizer N and efficiency of fertilizer N use.

Agronomic efficiency (kg grain increase kg applied N⁻¹) ®	15	18	20	25
Yield response (t ha⁻¹) ¯	Fertilizer N rate (kg ha⁻¹)
1	65	55	50	40
2	130	110	100	80
3	195	165	150	120
4		220	200	160
5			250	200

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Formulating dynamic N management to distribute fertilizer N to best match the crop’s need.

The required fertilizer N is apportioned in several doses during the growing season to ensure that N supply matches the crop need at critical growth stages. In the SSNM approach, fertilizers are applied using the following principles to achieve high yield and high efficiency of plant use:

1. Apply only a moderate amount of fertilizer N to young rice within 14 days after transplanting (DAT) or 21 days after sowing (DAS), when the growth and need of the plant for supplemental N is small.

2. Reduce or eliminate this early application of fertilizer N when high-quality organic materials and composts are applied or the soil N-supplying capacity is high.

3. Dynamically manage fertilizer N to ensure sufficient N supply to the crop at the critical growth stages of mid-tillering and panicle initiation. Apply fertilizer N based on the plant’s need for supplemental N, as determined by leaf N status.

4. Use the leaf color chart (LCC) (Fig. 3) to assess leaf N status and adjust fertilizer N applications to match the crop’s need for N.

5. Ensure sufficient N supply at heading for hybrid rice and large panicle-type rice.

The SSNM approach provides two complementary and equally effective options for improved N management using the LCC — the ‘real-time’ and the ‘fixed-time/adjustable dose’ options for N management. In real-time N management, farmers monitor the rice leaf color at regular intervals of about 7 to 10 days, and then apply fertilizer N whenever the leaves become more yellowish-green than the critical threshold value indicated on the LCC. In fixed-time/adjustable dose N management, the time for N fertilization is preset at critical growth stages, and farmers adjust the dose of N upward or downward based on the leaf color.

The decision on which option to use can be based on farmer preferences and location-specific factors, such as frequency of visits by farmers to their fields and their knowledge of critical growth stages for N application. The fixed-time/adjustable-dose option saves time and so is preferred by farmers who have gainful alternative activities. The real-time option is generally preferred when farmers lack sufficient understanding of the critical stages for optimal timing of fertilizer N.

Fig. 3. A standardized leaf color chart (LCC) can be used to assess leaf N status and adjust N applications to rice.

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