SITE-SPECIFIC NUTRIENT MANAGEMENT (SSNM)

Mekong River Delta, Vietnam

Team Leader

Dr. Pham Sy Tan

Deputy Director

Cuu Long Delta Rice Research Institute (CLRRI)

Division of Agronomy

Omon, Cantho

Email: pstan@hcm.vnn.vn

Partner Institution

Cuu Long Delta Rice Research Institute

Site Characteristics

The Mekong River Delta (MRD) in southern Vietnam accounts for about 50% of the national rice production and almost all of the Vietnamese rice exports. The MRD is well suited for rice cultivation because of its flat topography, abundant freshwater sources, and many fertile alluvial soils.

Farmers in the Mekong River Delta typically grow two to three rice crops per year, depending on the duration of flooding in the second half of each year.

In An Giang Province, farmers grow two rice crops per year:

• Dry season rice from December to March. This is the high-yielding season.

• Wet season rice from May to August. This is the low-yielding season.

In Cantho and Tien Giang, farmers grow up to three rice crops per year:

• Dry season rice from November to February

• Early wet season rice from February to May

• Late wet season rice from June to August

Rice in the MRD is commonly established by broadcasting germinated seed, often at high seed rates (≥ 200 kg ha⁻¹).

The MRD has a monsoon climate with high temperatures throughout the year (average 27 °C) and high solar radiation. Annual rainfall ranges from 1500 to 2000 mm.

Development and Evaluation of SSNM

Research on SSNM in the MRD started in 1997 in Cantho Province. In 2002, activities expanded to An Giang and Tien Giang Provinces (Fig. 1).

Fig. 1. The sites for the development and evaluation of SSNM in the Mekong River Delta, Vietnam.

Attainable yield and nutrient deficits with inbred rice in the Mekong Delta

The nutrient omission plot technique was used to determine attainable yield with full fertilization and nutrient-limited yields. Table 1 shows the results averaged for three provinces in 2002–03. Rice grain yield in plots fully fertilized with N, P, and K provided an estimate of the attainable yield target, which was higher in the dry than wet 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. Crop response to P averaged 1.2 t ha⁻¹ in the dry season and 0.8 t ha⁻¹ in the wet season. The K-limited yield was not determined during this period.

Table 1. Attainable yield and nutrient deficits with inbred rice in alluvial soils in three provinces (An Giang, Cantho, and Tien Giang) of the Mekong Delta, 2002-03.

SD = standard deviation; n = number of replicated plots

SSNM-based nutrient requirements

The SSNM approach was used to estimate fertilizer N and P₂O₅ requirements from Table 1. The fertilizer K₂O requirement for the wet season was estimated from data obtained in 1997 and 1998 in Cantho Province. Total fertilizer N required for rice (Table 2) was estimated from the measured response to N (Table 1) and assumed target agronomic efficiency of N (kg yield increase kg⁻¹ fertilizer N) of 25 for the dry season and 18 for wet season. See N management for more details.

Total fertilizer P₂O₅ and K₂O were estimated from attainable yield target, P-limited yield (Table 1), and K-limited yield (data not shown) using the nutrient decision support system (NuDSS).

Straw input was estimated as 0.5 t ha⁻¹ for both seasons.

Table 2. Fertilizer N, P₂O₅, and K₂O requirements for rice in alluvial soil estimated from results with the nutrient omission plot technique in three provinces in the Mekong Delta, Vietnam, 2002-03.

SD = standard deviation; n = number of replicated plots

* Determined from data collected in 1997 and 1998 in Cantho Province (n = 36).

On-farm evaluation of SSNM in alluvial soil 

Fig. 2. Performance of SSNM as compared with the farmers’ fertilizer practice in the three provinces of the Mekong Delta, Vietnam. * indicates a significant difference between the two treatments at P<0.05 in a given year.
Table 3. Total amount of fertilizer N, P₂O₅, and K₂O applied to rice per season with the farmers’ fertilizer practice (FFP) and site-specific nutrient management (SSNM) in the Mekong River Delta, Vietnam in 2001-04. Values shown are the means and standard deviation of 26-40 replicates per province.

Locally Adapted Recommendation for Alluvial Soil in the Mekong Delta

Results from the nutrient omission plot technique, on-farm evaluations of SSNM, and interviews with farmers were used to formulate locally adapted recommendations for managing N, P, and K.

The rates of fertilizer P₂O₅ as determined from nutrient omission plots may have to be adjusted depending upon previous use of fertilizer P and water management. The rate of fertilizer P in both seasons is increased by 10 kg P₂O₅ ha⁻¹ if the amount of fertilizer P used in previous seasons was <30 kg P₂O₅ ha⁻¹. The fertilizer P rate in the wet season is increased for fields that are not continuously flooded and undergo alternate soil drying and wetting. Table 4 gives the recommended rates of fertilizer P₂O₅ for alluvial soil. Rates would be higher for acid sulfate soil.

Table 4. Adjustments in the SSNM recommended rates of fertilizer P₂O₅ for alluvial soil based on previous fertilizer P use and water management during the dry season.

Locally adapted SSNM recommendations for wet-seeded rice with growth durations of 95 to 100 days and 90 days in both dry and wet seasons are available at the links below.

SSNM recommendation for wet-seeded rice with growth duration of 95 to 100 days

Version 1 [PDF 159KB]

SSNM recommendation for wet-seeded rice with growth duration of 90 days

Version 1 [PDF 158KB]

Mekong River Delta SSNM version 1.0 (www.irri.org/irrc/ssnm)

Site Specific Nutrient Management