SITE-SPECIFIC NUTRIENT MANAGEMENT (SSNM)

Principles of P and K management

Phosphorus (P) and potassium (K) are essential elements for plant growth. Phosphorus is particularly important in the early growth stages. It promotes root development, tillering, and early flowering. Potassium strengthens plant cell walls, and contributes to greater canopy photosynthesis and crop growth. It does not have a pronounced effect on tillering, but it can increase the number of spikelets per panicle (flowers per grain bunch) and percentage of filled grain.

Rice plants obtain much of their required P and K from the soil, crop residues, organic amendments, and irrigation water; but the supply of P and K from these naturally occurring, indigenous sources are typically insufficient to sustain high rice yields. Supplemental P and K from fertilizers are thus essential for sustaining high and profitable yields of rice without depleting the fertility of soil.

Fertilizer P and K requirements for rice are determined with a nutrient decision support system (Witt and Dobermann 2004), which is derived from scientific principles within the QUEFTS model (Janssen et al. 1990, Witt et al. 1999). In this approach, recommended fertilizer P and K rates are sufficient to both overcome P and K deficiencies and maintain soil P and K fertility.

BACKGROUND ON MAKING P AND K RECOMMENDATIONS

The fertilizer P₂O₅ and K₂O 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 for the total amount of P and K needed by the rice crop because the amounts of P and K taken up by a rice crop are directly related to crop yield. 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. 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 in the nutrient omission plot technique).

Modern rice varieties with harvest indices of 0.45 to 0.55 and balanced nutrition of N, P, and K typically accumulate at maturity in aboveground biomass an average of 2.6 kg P (6 kg P₂O₅) and 15 kg K (18 kg K₂O) for each metric ton (1,000 kg) of unmilled grain yield — in the linear portion of the relationship between grain yield and nutrient accumulation in the mature crop (Witt et al. 1999). Somewhat higher amounts of P and K 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 P and K taken up by rice come from naturally occurring (indigenous) sources, which include the soil, organic amendments, crop residue, manure, and irrigation water. This indigenous supply of P can be estimated from the total amount of P taken up by a mature rice crop that receives no fertilizer P and is not limited by other nutrients. Because the amount of P taken up by rice is directly related to yield, indigenous P supply can then be estimated from P-limited yield, which is the grain yield for a crop not fertilized with P but fertilized with other nutrients to ensure they do not limit yield. Similarly, the indigenous K supply can then be estimated from K-limited yield, which is the grain yield for a crop not fertilized with K but fertilized with other nutrients to ensure they do not limit yield.

The P- and K-limited yields are determined by the nutrient omission plot technique. The K-limited yield is determined in a K omission plot receiving no fertilizer K but a sufficient supply of other nutrients to ensure they do not limit yield. The P-limited yield is determined in a plot receiving no fertilizer P but a sufficient supply of other nutrients (Fig. 1).

Fig. 1. A nutrient omission plot study conducted in a farmer’s field.

In the absence of directly determined P- and K-limited yields by the nutrient omission plot technique, P- and K-limited yields can be estimated based on soil testing, farmers’ use of organic amendments, soil properties, or previous measurements of P- and K-limited yield on similar soils. The measurement of P- and K-limited yield by the nutrient omission plot technique is not required when P- and K-limited yield can be estimated within an accuracy of ±0.5 t ha⁻¹.

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

The attainable yield target and P-limited yield are used to determine, with a nutrient decision support system, the amount of fertilizer P₂O₅ required to both overcome P deficiency and maintain soil P fertility. Similarly, the attainable yield target and K-limited yield, together with an estimate of the amount of retained crop residue, are used to determine, with a nutrient decision support system, the amount of fertilizer K₂O required to both overcome K deficiency and maintain soil K fertility. Outputs of the nutrient decision support system are summarized in Tables 1 and 2 (Witt et al. 2002).

Table 1. Guidelines for the application of fertilizer P₂O₅ according to yield target and P-limited yield in P omission plots when crop residue is retained in fields (Witt et al. 2002).

Fertilizer P is recommended even when the P-limited yield is comparable to the yield target (that is, no response to fertilizer P) to replenish the P removed with grain and straw. At maturity, modern rice varieties with harvest indices of 0.45 to 0.55 contain about 6 kg P₂O₅ in aboveground biomass (grain and crop residue) for each ton of grain yield. If most of the crop residue is retained in fields after harvest and a small amount of manure is applied to fields, then apply about 4 kg P₂O₅ ha⁻¹ per ton of grain harvested to match the export of P₂O₅ from rice fields. If most of the crop residue is removed from fields after harvest and P input from organic amendments is small, then apply about 6 kg P₂O₅ ha⁻¹ per ton of grain harvested to maintain soil P fertility. 

Table 2. Guidelines for the application of fertilizer K₂O according to yield target and K-limited yield in K omission plots (Witt et al. 2002).

The K requirement for rice is much greater than for P, and at least 80% of the K taken up by rice come from remaining straw after harvest, making straw an important source of K when calculating fertilizer K requirements. Guidelines for determining fertilizer K rates therefore consider amount of straw returned from the previous crop and enable selection from among three levels of straw input (Table 2). Fertilizer K is recommended even when the K-limited yield is comparable to the yield target (that is, no response to fertilizer K) to replenish the K removed with grain and straw.
 
With SSNM, all fertilizer P is applied before 14 days after transplanting (DAT) or 21 days after sowing (DAS). As a general principle, if the fertilizer K requirement is relatively low (≤30 kg K₂O ha⁻¹), all the K can be applied early before 14 DAT or 21 DAS. On sandy soils or when larger amounts of fertilizer K are required, K can be split applied with about 50% before 14 DAT or 21 DAS and 50% at early panicle initiation.
 

USING THE SSNM APPROACH TO FORMULATE LOCALLY ADAPTED P RECOMMENDATIONS

The SSNM approach can be used to formulate a P recommendation adapted to any irrigated or favorable rainfed rice environment. Table 3 lists the steps in formulating locally adapted recommendations for high- and low-yielding seasons.

Table 3. Example of P 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 practices and under average climatic conditions.

Step 2: Estimate the P-limited yield or yield without fertilizer P.

Step 3: Estimate the total fertilizer P₂O₅ required based on the yield target and P-limited yield from Table 1. Total fertilizer P₂O₅ requirement can also be determined with the nutrient decision support system (NuDSS).

Step 4: Apply all fertilizer P₂O₅ to young rice within 14 DAT or 21 DAS.

USING THE SSNM APPROACH TO FORMULATE LOCALLY ADAPTED K RECOMMENDATIONS

The SSNM approach can be used to formulate a K recommendation adapted to any irrigated or favorable rainfed rice environment. Table 4 shows the steps in formulating locally adapted recommendations for high- and low-yielding seasons.