N-application

Two complementary approaches (real-time and fixed-time) have been used successfully in farmers’ fields to manage fertilizer N efficiently. Table 3 gives the major features of both approaches. We recommend testing both strategies side by side using participatory approaches in farmers’ fields to evaluate their performance before wider-scale dissemination. Consider socioeconomic factors when developing fertilizer N management strategies (labor availability and cost, prices of rice and fertilizer, available fertilizer sources, current application practices).

 

Basic principle of the real-time approach

The standardized LCC as developed and supplied by IRRI since 2003 contains four green panels with colors ranging from yellowish green (no. 2) to dark green (no. 5). The critical LCC value, below which a fertilizer N application is recommended, may range from 2 to 4 depending on variety and crop establishment method. Note that the critical LCC values given in Table 1 should be calibrated for local conditions.

 

Table 1. Examples of critical leaf color chart (LCC) values depending on variety and crop establishment method.

Table: Examples of critical LCC values depending on variety and crop establishment method.

 

Guidelines for using the leaf color chart

1. Take LCC readings once every 7 to 10 d, starting after 14 DAT for transplanted rice (TPR) or 21 DAS for wet-seeded rice (WSR). The last reading is taken when the crop starts flowering (first flowering). If farmers prefer to take fewer measurements, recommend the fixed-time approach (option N2) in which LCC readings are taken at critical crop growth stages such as active tillering and panicle initiation

 

2. Choose the topmost fully expanded leaf (Y leaf) for leaf color measurement because it is a good indicator of the N status of rice plants. The color of a single leaf is measured by comparing the color of the middle part of the leaf with the colors on the chart. If the leaf color falls between two values, the mean of the two values is taken as the LCC reading. For example, if the leaf color lies between values 3 and 4, it is noted as 3.5.

 

3. During measurement, always shade the leaf being measured with your body because the leaf color reading is affected by the sun’s angle and sunlight intensity. If possible, the same person should take LCC readings at the same time of day each time measurements are taken.

 

4. Take readings of 10 leaves from hills chosen randomly within a field. If six or more leaves show color values below the established critical values, immediately apply N fertilizer.

 

5. Recommended N application rates for semidwarf indica varieties are given in Table 2.

 

Table 2. Proposed amounts of fertilizer N to be applied to semidwarf indica varieties each time the leaf color falls below the critical LCC value.

a Apply about 25 kg N/ha after panicle initiation up to flowering.

 

 

What is a Fixed-Splitting Approach?

The fixed-time approach provides a recommendation for the total fertilizer N requirement (kg/ha) and a plan for the splitting and timing of applications in accordance with crop growth stage, cropping season, variety used, and crop establishment method.

 

Basic principle of the fixed-time approach

Estimate the required total amount of fertilizer N and develop a schedule for fertilizer N split applications. Use the LCC at critical growth stages to adjust predetermined fertilizer N rates.

 

1) Estimating Required Fertilizer N

Use Table 3 to derive the total fertilizer N rate based on

  • the expected yield response to fertilizer N application calculated from the difference between yield target and yield in 0 N plots (Steps 1 and 2) and

  • the attainable agronomic N efficiency (AEN,).

 

Table 3. Fertilizer N rates according to the attainable yield response (yield target – yield in 0 N plots) and the expected agronomic N efficiency (AEN, kg grain yield increase/kg fertilizer N).

 

Rule of thumb: Apply 40–60 kg fertilizer N per ha for each ton of expected grain yield response to fertilizer N application.

 

Apply less N to crops in the rainy season (less sunshine, lower yield response) and apply more N to crops in the dry season (more sunshine, higher yield response).

 

Select an expected yield response of 4 t/ha over the yield in the 0 N plot only for high-yielding seasons with very favorable climatic conditions.

 

Experience in tropical Asia indicates that an AEN of 25 is often achievable with good crop management in high-yielding seasons, and an AEN of 16.7 or 20 is achievable with good crop management in low-yielding seasons.

 

Note that the AEN is usually higher at low N rate than at high N rate. The aim of effective, environmentally sound N management in the tropics is to achieve high, economic yields while realizing an optimal AEN between 16.7 and 25 kg grain increase per kg fertilizer N. In subtropical climate, yield responses can be > 5 t/ha with optimal AEN > 25 kg/kg, in which case suggested fertilizer N rates in Table 3 would need to be adjusted.

 

  • Divide total fertilizer N recommendations into 2–4 split applications. Use more splits with long-duration varieties and in high-yielding seasons. Apply more N when the crop demand for N is large (e.g., between mid-tillering and flowering). Make a large single fertilizer N application of > 45 kg N per ha only if weather conditions are very favorable and crop response to N is large.

  • Use Tables 4–6 to develop approximate rates for N split applications. Growth stages are given, but the actual application date depends on variety (crop duration). For tropical rice, panicle initiation is about 60 days before harvest, and active tillering is approximately midway between 14 DAT or 21 DAS and panicle initiation.

  • Use the following guidelines to determine the need for early N application to young rice before 14 DAT or 21 DAS:

 

    1. Eliminate early application when yield response is 1 t/ha. At yield responses between 1 and 3 t/ha, typically apply about 20 to 30 kg N per ha. At yield response > 3 t/ha, apply about 25% to 30% of the total N.

    2. Reduce or eliminate early N applications when high-quality organic materials or composts are applied.

    3. Avoid large early fertilizer N applications (i.e., >50 kg N per ha) in transplanted rice because early growth is slow and N uptake is poor during the first 3 weeks after transplanting.

    4. Increase early N application for low tillering and large panicle type varieties when old seedlings (>24 days old) or short-duration varieties are used, where the plant spacing is wide (<20 hills/m2) to enhance tillering, or in areas with low air and water temperature at transplanting or sowing (e.g., at higher elevations).

    5. Incorporate early N into the soil before planting or apply early N within 14 days after transplanting or 21 days after sowing. Use NH4-N and not NO3-N as an early N source. There is no need to use the LCC with the early N application.

 

  • Use the LCC to assess leaf N status and the crop needs for N after 14 DAT and 21 DAS. Adjust fertilizer N rates upward when leaves are yellowish green and downward when leaves are green.

  • Apply a late N dose (e.g., at early heading) to delay leaf senescence and enhance grain filling, but only to healthy crops with good yield potential. Hybrid rice and large panicle type varieties in high-yielding seasons often require an N application at early heading. To reduce the risk of lodging and pests, do not apply excessive amounts of N fertilizer between panicle initiation and flowering, particularly in the low-yielding seasons.

  • For the standardized IRRI LCC with most rice varieties,

  • the leaf colors mentioned in Tables 4–6 correspond to LCC

  • values as follows:

    1. Yellowish green = LCC value 3,

    2. Intermediate = LCC value 3.5 (intermediate between 3 and 4), and

    3. Green = LCC value 4.

  • The fertilizer rates in Tables 4–6 are for relatively high N-use efficiencies (agronomic N efficiency or AEN) of about 16.7– 20 kg grain increase/kg fertilizer N applied in seasons with 1–2 t/ha expected response to fertilizer N and 25 kg grain increase/kg fertilizer N applied in seasons with 3–4 t/ha expected response to fertilizer N (see Table 3).

  • Use the LCC to monitor plant N status to optimize the amount of split applications in relation to crop demand and soil N supply. The N rates for specific leaf colors (LCC values) in Tables 4–6 are intended to provide sufficient flexibility to accommodate conditions when the crop response to fertilizer in a given season and location differs markedly from the expected yield increase to fertilizer N.

  • N rates in Tables 4–6 can be fine-tuned and tailored to accommodate location-specific crop-growing conditions and rice varieties.

 

  1.  

1. Transplanted rice (inbred variety) (see Table 4)

With 20–40 hills/m2, high-yielding conventional variety, continuous flooding or intermittent irrigation. Transplanted rice has slower leaf area development, dry matter accumulation, and N uptake during early growth, but high growth rates and N uptake after mid-tillering to grain filling.

 

2. Wet-seeded rice (see Table 4)

With 80–150 kg seed per ha, broadcast, high-yielding conventional variety, continuous flooding after crop emergence. Broadcast wet-seeded rice has more rapid leaf area development, dry matter accumulation, and N uptake during early growth, but a slower growth rate and N uptake after panicle initiation, particularly during grain filling. Early leaf senescence and lodging are more severe in wet-seeded rice than in transplanted rice. Wet-seeded rice needs little or no late N application.

 

Table 4. An approximate fertilizer N splitting for transplanted and wet-seeded inbred rice with high N-use efficiency.

 

 

3. Transplanted rice (hybrid) (see Table 5)

With 20–30 hills/m2, hybrid rice with high yield potential, continuous flooding or intermittent irrigation. Transplanted hybrid rice often responds to late N application in high-yielding seasons.

 

Table 5. An approximate fertilizer N splitting for transplanted hybrid rice with high N-use efficiency.

 

4. Transplanted rice (large panicle type) (see Table 6)

High-yielding rice with very large panicles (panicle weight type rice), relatively low tillering and good resistance to lodging. Includes some new plant type rice and some hybrid rice such as the Chinese “super” hybrid rice.

 

Table 6. An approximate fertilizer N splitting for large panicle type (panicle weight type) rice with high N-use efficiency.

 

 

 

Notes:

  • Do not topdress N when heavy rainfall is expected