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  • SSNM approach
  • SSNM in three steps
  • Initial concept of SSNM
  • Current version of the SSNM approach
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  • Principles of N management
  • Background on making an N Recommendation
  • Using the SSNM approach to formulate locally adapted N recommendation
  • Using the Leaf Color Chart (LCC) for fertilizer N management
  • FAQs on N Management
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  • Principles of P and K management
  • Background on making P and K recommendations
  • Using SSNM approach to formulate P recommendations
  • Using SSNM approach to formulate K recommendations
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  • Introduction to SSNM version 1.1
  • N management
  • P&K management
  • Implementing SSNM recommendations
  • Making an SSNM recommendation for rice
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  • SSNM: an approach for optimizing nutrient use in intensive rice production
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  • Nutrient omission plot
  • Potassium addition plot
  • Zinc addition plot

Central Vietnam

Attainable yield and nutrient deficits (2005)

Attainable yield with full fertilization and nutrient-limited yields were determined from nutrient omission plots conducted in 2005 in five farmers’ fields per soil type. The results by soil textural class are shown in Tables 1 (spring) and 2 (summer).

Rice grain yield in plots fully fertilized with N, P, and K provided an estimate of the attainable yield target, which was slightly higher in the summer than spring season. The yield of rice not fertilized with N but fertilized with other nutrients was used to determine N-limited yield. Similarly, the yield of rice not fertilized with P but receiving other nutrients indicated P-limited yield; and the yield of rice not fertilized with K but receiving other nutrients indicated K-limited yield. The difference between the yield target and a nutrient-limited yield indicated the crop response to the nutrient.

In Thua Thien-Hue, yield response to P averaged <1 t ha⁻¹ for all soils in spring (Table 1) and >1 t ha⁻¹ in summer (Table 2). Yield response to K averaged ≥1 t ha⁻¹ in clay loam and loam soils in spring (Table 1) and in all soils in summer (Table 2); yield response was higher in summer than in spring.

In Quang Nam, data were collected only in the spring season. For both clay loam and sandy loam soils, the yield response to P was about 2 t ha⁻¹, while the yield response to K was >1 t ha⁻¹ (Table 1).

Table 1. Rice yields obtained in spring 2005 with the nutrient omission plot technique in farmers’ fields in two provinces with different soil types in central Vietnam. Values shown are means of 5 replicates.

Table 2. Rice yields obtained in summer 2005 with the nutrient omission plot technique in farmers’ fields in Thua Thien-Hue Province with different soil types. Values shown are means of 5 replicates.

SSNM-based nutrient requirements for rice in Thua Thien-Hue and Quang Nam, central Vietnam

The SSNM approach was used to estimate fertilizer N, P₂O₅, and K₂O requirements based on the results in Tables 1 and 2. Total fertilizer N required for rice (Table 3) was estimated from the measured response to N (Tables 1 and 2) and an assumed target agronomic efficiency of N (kg yield increase kg fertilizer N⁻¹) of 20 for both spring and summer. See N management for more details.

Total fertilizer P₂O₅ and K₂O were estimated from attainable yield target, P-limited yield, and K-limited yield (Tables 1 and 2) using the nutrient decision support system (NuDSS). Straw input was estimated as 2 t ha⁻¹ per cropping season. 

Table 3. Fertilizer N, P₂O₅, and K₂O requirements estimated from results with the nutrient omission plot technique in Thua Thien-Hue and Quang Nam, central Vietnam, 2005. Values shown are the means and standard deviation of 5 replicates per soil type.

 * These low estimated fertilizer N rates are the result of high yields without fertilizer N in farmers’ fields selected in 2005. Fertilizer N rates would be higher in fields with lower N fertility.

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