Site-Specific Nutrient Management (SSNM) for Wet-Seeded Rice in Iloilo Province, Philippines

Sta. Rita soil: 90–95 days growth duration rice (from seed to harvest)

N management: Fixed time

SSNM is a plant-based approach for ‘feeding’ a rice crop with nutrients as needed. SSNM includes the following features:

§      Applying sufficient P and K within 21 days after seeding (DAS) to meet crop needs.

§      Applying only a moderate amount of fertilizer N within 21 DAS.

§      Applying fertilizer N at tillering and later growth stages based on the need of the crop for supplemental N, as determined with a leaf color chart (LCC).

§      Applying fertilizer K at panicle initiation based on crop needs.

§      Using micronutrients based on local recommendations.

Fixed-time N management is one of two equally effective options for applying fertilizer N based on crop needs as determined with the LCC. In this option, the times for N fertilization are preset at critical growth stages, and farmers adjust the dose of N upward or downward based on leaf color. Fertilizer N is typically applied three times.

Target for recommendation

This recommendation was developed based on attainable yield targets and approximate responses of rice to fertilizer N, P, and K typically observed for irrigated rice in farmers’ fields on Sta. Rita soil, Iloilo Province. The approximations summarized in the table below represent the features of rice fields targeted by this SSNM recommendation. All yields are expressed on dry grain (about 14% moisture) basis.

SSNM recommendations for irrigated direct wet-seeded rice

Guidelines for farmers in implementing improved management practices tailored to their specific fields are given below as a series of steps at critical crop growth stages. These guidelines are based on the following premises:

• The historical application of >20 kg P₂O₅ ha⁻¹ per crop in the past four seasons (2 years) is sufficient with a yield target of 5−6 tons ha⁻¹, to build soil P status to a level where fertilizer P₂O₅ is only required to maintain soil P fertility.
• The historical retention and incorporation of rice stubble (≥30 cm) is sufficient to return 2−3 t ha⁻¹ of crop residue containing about 25−30 K₂0 ha⁻¹.

Early growth stage: Apply N, P, K, and Zn as needed

Within 14 DAS, apply fertilizer based on an identified yield target as shown in Table 1. The steps in the process of estimating fertilizer needs for a specific location are

Step 1.  Select a yield target from the two options of 5 or 6 t ha⁻¹ expressed at 14% moisture content.

Step 2.  Select a fertilizer N rate from Table 1 based on yield target.

Step 3.  Identify whether the historical level of fertilizer P₂O₅ application is above or below 20 kg ha⁻¹ per season in the past 2 years. Then, select a fertilizer P₂O₅ rate from Table 1.

Step 4.  For rice-growing areas where fertilizer Zn is recommended, select a zinc sulfate rate from Table 1 based on yield target.

Either single-element fertilizers or compound (NPK) fertilizers can be used to obtain the desired amounts of N, P₂O₅, and K₂O at the lowest cost. Fertilizer N can be supplied as urea, ammonium sulfate, or NPK fertilizer.

Table 1. Guidelines for early application of fertilizer N, P₂O₅, K₂O, and Zn within 14 days after sowing (DAS) rice in Iloilo.

Vegetative growth stage: Topdress fertilizer N at critical stages of rice growth

Apply fertilizer N several times during the growing season to ensure that the N supply matches the crop need for N, especially at the critical growth stages of tillering and panicle initiation. The crop’s need for N is indicated by rice leaf color and determined with the LCC, as shown in the figure below.

N management using the LCC at critical growth stages involves the following steps:

1.   Take LCC readings at active tillering (about 20–25 DAS). Adjust the rate of N (urea) application based on the LCC reading as shown in the figure below.

a.   If the LCC reading is <3, apply 75 kg urea ha⁻¹ (35 kg N ha⁻¹).

b.   If the LCC reading equals 3–3.5, apply 50 kg urea ha⁻¹ (23 kg N ha⁻¹).

c.   If the LCC reading is >3.5, do not apply urea.

2.   Take LCC readings at panicle initiation, about 35–40 DAS. Adjust the rate of N (urea) application based on the LCC reading as shown in the figure below.

a.   In seasons and locations with yield target = 5 t ha⁻¹ (1–2 t ha⁻¹ response to fertilizer N), apply urea based on recommendations during active tillering.

b.   In seasons and locations with yield target = 6 t ha⁻¹ (2–3 t ha⁻¹ response to fertilizer N):

§         If the LCC reading is <3, apply 100 kg urea ha⁻¹ (46 kg N ha⁻¹).

§         If the LCC reading equals 3–3.5, apply 75 kg urea ha ⁻¹ (35 kg N ha⁻¹).

§         If the LCC reading is >3.5, do not apply urea.

Panicle initiation: Apply fertilizer K₂O at targeted locations

Sufficient supply of fertilizer K₂O is essential for optimal grain filling and yield. Apply 20 kg K₂O ha⁻¹ at panicle initiation (35–40 DAS) in seasons and locations where rice stubble is removed rather than returned.

Recommendations for applying fertilizer N are illustrated in the figure below. This recommendation is specifically developed for rice cultivars with a

growth duration from seed to harvest of 90−95 days. The timing of fertilizer applications at tillering and panicle initiation should be adjusted accordingly for cultivars with longer growth duration. Panicle initiation is about 60 days before harvest, while active tillering is midway between 14 DAS and panicle initiation stage.

Fixed-time N management with LCC for direct wet-seeded rice with growth duration of 90−95 days.

Philippines Iloilo WSR FTN version 1.0 (www.irri.org/irrc/ssnm)

Site Specific Nutrient Management