SITE-SPECIFIC NUTRIENT MANAGEMENT (SSNM)

Current Version of the SSNM Approach

Through research on the development and evaluation of SSNM, much data were obtained on the relationship between yield of unmilled rice and total N, P, and K in the mature rice crop. As a result, the uptake of N, P, and K by a mature rice crop with harvest indices of 0.45 to 0.55 can be estimated from grain yield with sufficient reliability. Measurement of N, P, and K in grain and straw is no longer required for determining fertilizer N, P, and K rates by the SSNM approach. Grain yield targets and grain yield in nutrient omission plots can now be directly used in a simplified manner by estimating fertilizer N, P, and K requirements as follows:

1. Establish an attainable yield target for farmers’ fields.

   • The yield target in the SSNM approach is directly used to calculate fertilizer rates for farmers’ fields. It consequently must be reasonably attainable by farmers. A yield target higher than one attainable by farmers would lead to recommendations of more fertilizer than required for high use efficiency and profit. A yield target below that realistically attainable by farmers could result in suboptimal yield and profit. Grain yield from 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) can be used to estimate the yield target.

2. Approximate a fertilizer N rate and formulate dynamic N management.

   • The difference between the yield target and N-limited yield (i.e., yield with no N fertilizer and no limitation of other nutrients) provides an estimate of anticipated crop response to fertilizer N. The estimated yield response to fertilizer N and a targeted efficiency for fertilizer N use are used to approximate the total requirement of the crop for fertilizer N, which is dynamically apportioned among multiple times of application to best match the crop’s need for N. For more details see N management.

3. Estimate field-specific nutrient-limited yields.

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

4. Determine fertilizer P and K rate.

   • The crop’s need for fertilizer P is based on a comparison of the yield target and P-limited yield, whereas the crop’s need for fertilizer K is based on a comparison of the yield target and K-limited yield. The SSNM approach advocates sufficient use of fertilizer P and K to both overcome P and K deficiencies and maintain soil P and K fertility. For more details see P and K management.

The evaluation and promotion of this simplified SSNM approach spread through the RTOP workgroup in 2003–2004 to about 20 locations in tropical and subtropical Asia, each representing large areas of intensive rice farming under irrigated or favorable rainfed conditions. The countries involved included Bangladesh, China, India, Indonesia, Myanmar, Thailand, Philippines, and Vietnam.

The evaluation and promotion of SSNM for rice continues throughout Asia from 2005 to 2008 through the Productivity and Sustainability workgroup of the IRRC.

Site Specific Nutrient Management