Site-Specific Nutrient Management (SSNM) for Transplanted Rice in North Sumatra, Indonesia

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 14 days after transplanting (DAT) to meet crop needs.

§      Applying only a moderate amount of fertilizer N within 14 DAT.

§      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. (For details, consult your local agricultural extension representative.)

Fixed-time N management is one of two equally effective options for applying fertilizer N based on crop needs as determined with the LCC. With 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 in North Sumatra. 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 transplanted rice

Guidelines for farmers on 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 >30 kg P₂O₅ ha⁻¹ per crop in the past five seasons is sufficient to build soil P status to a level where fertilizer P₂O₅ is only required to maintain soil P fertility.
• Rice ‘straw’ refers to the portion of the rice biomass cut and removed with the panicles for threshing. Straw is considered ‘removed’ when biomass removed with the harvested panicles is not returned to the field; it is considered ‘returned’ when this harvested biomass is returned to the field. Rice biomass remaining in the field after cutting the crop at harvest is presumed to be retained in the field in all cases.

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

Within 14 DAT, 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 three options of 5, 6, or 7 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 30 kg ha⁻¹ per season in the past five seasons.
• Step 4. Select a fertilizer P₂O₅ rate from Table 1 based on yield target and past fertilizer P₂O₅ use.
• Step 5. Identify whether rice straw is historically removed or returned to the field.
• Step 6. Select a fertilizer K₂O rate from Table 1 based on yield target, straw management, and knowledge of relative soil K supplying capacity (low or high) in a geographical location.
• Step 7. For rice-growing areas where fertilizer S is recommended, select an ammonium 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 S within 14 days after transplanting (DAT) rice in North Sumatra, Indonesia.

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 23–28 DAT). Adjust the rate of N (urea) application based on the LCC reading as shown in the figure below.

a. In locations with high response to fertilizer N (yield target = 7 t ha⁻¹):

§       If the LCC reading is 3, apply 125 kg urea ha⁻¹ (58 kg N ha⁻¹).

§       If the LCC reading is between 3 and 4 (= 3.5), apply 100 kg urea ha⁻¹ (45 kg N ha⁻¹).

§       If the LCC reading is 4, apply 50 kg urea ha⁻¹ (23 kg N ha⁻¹).

b. In locations with medium response to fertilizer N (yield target = 6 t ha⁻¹):

§       If the LCC reading is 3, apply 100 kg urea ha⁻¹ (45 kg N ha⁻¹).

§       If the LCC reading is between 3 and 4 (= 3.5), apply 75 kg urea ha⁻¹ (35 kg N ha⁻¹).

§       If the LCC reading is 4, do not apply urea.

c. In locations with low response to fertilizer N (yield target = 5 t ha⁻¹):

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

§       If the LCC reading equals 3 to 4, apply 50 kg urea ha⁻¹ (23 kg N ha⁻¹).

§       If the LCC reading is above 4, do not apply urea.

2.   Take LCC readings at panicle initiation, about 38-42 DAT. Adjust the rate of urea application based on the LCC reading as done at active tillering stage, and as shown in the figure below. The rates of urea application are identical to those at active tillering, except in the following case:

a.   At locations with medium response to fertilizer N (yield target = 6 t ha⁻¹)

§       If the LCC reading is above 4, apply 50 kg urea ha⁻¹ (23 kg N ha⁻¹).

Recommendations for applying fertilizer N are illustrated in the figure below. This recommendation is specifically developed for inbred rice varieties with growth duration from seed to harvest of 115-120 days, which is common in North Sumatra. The timing of fertilizer applications at tillering and panicle initiation should be adjusted accordingly for varieties with longer or shorter growth duration. Panicle initiation is about 60 days before harvest, while active tillering is midway between 14 DAT and panicle initiation stage.

Fixed−time N management with LCC for transplanted rice with growth duration of 115–120 days.

Panicle initiation: Apply fertilizer K₂O at targeted locations

For optimal grain filling and yield at locations with low supply of plant-available K, apply fertilizer K₂O at panicle initiation based on an identified yield target as shown in Table 2. The steps in the process of targeting fertilizer K₂O to specific locations are

• Step 1. Select a yield target from the three options of 5, 6, or 7 t ha⁻¹ expressed at 14% moisture content.
• Step 2. Identify whether rice straw is historically removed or returned to the field.
• Step 3. Select a fertilizer K₂O rate from Table 2 based on yield target, straw management, and geographical location.
• Step 4. Encourage the farmer to apply the recommended fertilizer K₂O to a small plot at panicle initiation, then compare grain yield and grain filling between the plot with additional K₂O and an adjacent field without additional K₂O.

Table 2. Guidelines for application of fertilizer K₂O at panicle initiation.

Indonesia North Sumatra FTN version 2.0 (www.irri.org/irrc/ssnm)

Site Specific Nutrient Management