How much fertilizer N is needed by a rice crop?

• The fertilizer N needed by a rice crop can be estimated as shown in the table below from the expected increase or response in grain yield due to fertilizer N application and the expected agronomic efficiency of fertilizer N use by the crop.
   

  Agronomic efficiency (kg grain increase kg−1 applied N) ®	15	18	20	25
  Expected yield response  (t ha−1) ¯	Fertilizer N  rate (kg ha−1)
  1	65	55	50	40
  2	130	110	100	80
  3	195	165	150	120
  4		220	200	160
  5			250	200

   

• How is yield response determined?

  • The grain yield response with fertilizer N application is the difference between the yield target and yield without fertilizer N (N-limited yield). The yield target is the yield attainable by farmers with good crop and nutrient management and average climatic conditions. The yield without fertilizer N indicates the ability of the soil to supply N to the crop. It can be determined with an N omission plot receiving no fertilizer N but sufficient quantities of other nutrients to ensure that they do not limit yield.
     

• What agronomic efficiency for fertilizer N is achievable?

  • The agronomic efficiency of fertilizer N (AE_N) is the increase in yield per unit of fertilizer N applied. Experiences in Asia indicate that an AE_N of 25 is often achievable in the tropics with good crop management in high-yielding seasons, and an AE_N of 18 to 20 is achievable in the tropics with good management in low-yielding seasons. An AE_N of 15 is a realistic target for environments where existing fertilizer N management practices are very inefficient with AE_N in farmers’ fields of about 10 or less.
     

• How is the need for fertilizer N affected by cropping season?

  • The estimated need for fertilizer N based on yield target and yield without fertilizer N can be adjusted for contrasting cropping seasons with different achievable AE_N as shown in the table below.

     

    Season	Yield target  ®	5 t ha−1	6 t ha−1	7 t ha−1	8 t ha−1	9 t ha−1
    	Approximate yield without fertilizer N (t ha−1) ↓	Fertilizer N rate (kg N ha−1)
    Low-yielding (AEN = 18)	3	110	-	-	-	-
    	4	55	110	-	-	-
    	5	0	55	-	-	-
    High-yielding (AEN = 25)	3	80	120	160	200	-
    	4	-	80	120	160	200
    	5	-	-	80	120	160

     

    Yield targets are lower in the low-yielding season (i.e., rainy season with relatively low solar radiation) than the high-yielding season (i.e., dry season with higher solar radiation). An AE_N of 25 kg kg⁻¹ can often be achieved with high yield and good fertilizer N management in high-yielding seasons in the tropics. This is equivalent to 40 kg fertilizer N for every one ton increase in grain yield. An AE_N of about 18 kg kg⁻¹ can often be attained with high yield and good fertilizer N management in low-yielding seasons in the tropics. This is equivalent to 55 kg fertilizer N for every one ton increase in grain yield.

     

• How can yield without fertilizer N be approximated?

  • The yield without fertilizer N indicates the ability of the soil to supply N to the crop. It can be determined with an N omission plot receiving no N fertilizer but sufficient quantities of other nutrients to ensure that they do not limit yield. As a rule of thumb, experiences with N omission plots across irrigated rice-growing areas in Asia indicate the grain yield of rice without fertilizer N is about 3 t ha^-1 at low soil fertility or in very intensively cultivated fields, 4 t ha^-1 at medium soil fertility, and 5 t ha^-1 at high soil fertility.

   

How is fertilizer N best distributed during the season?

• For best effect, farmers should apply fertilizer N several times during the growing season to ensure that the N supply matches the crop need for N at the critical growth stages of tillering, panicle initiation, and grain filling.

  ·         Apply only a moderate amount of fertilizer N to young rice within 14 days after transplanting (DAT) or 21 days after sowing (DAS), when the growth and need of the plant for supplemental N is small.

  ·         Reduce or eliminate early application of fertilizer N when high-quality organic materials and composts are applied or the soil N-supplying capacity is high.

  ·         Apply sufficient fertilizer N at the critical growth stages of active tillering and panicle initiation to match the needs of the rice crop for supplemental N

  ·         Use the leaf color chart (LCC) to assess leaf N status and adjust fertilizer N applications to match the needs of the rice crop for N.

  ·         Ensure sufficient N supply at early heading for hybrid rice and large panicle-type rice in seasons with large response to fertilizer N.
   

• How much fertilizer N should be applied early?

  • Use the following general guidelines to determine the early application of N before 14 DAT or 21 DAS:

    • Typically apply about 20 to 30 kg N ha⁻¹ in seasons with yield response to N between 1 and 3 t ha⁻¹.
    • Apply about 25 to 30% of the total N in seasons with yield response to N >3 t ha⁻¹.
    • Eliminate early application when yield response to N is ≤1 t ha⁻¹.
    • Do not use the LCC with the early N application.

    ·         Reduce or eliminate early N application when high-quality organic materials and composts are applied or the soil N-supplying capacity is high.

    • Increase the N application up to 30 to 50% of the total N when old seedlings (>24 days old) and short-duration varieties are used.
    • Increase early N application in areas with low air and water and temperature at transplanting and for low tillering and large panicle type varieties.
       

• How much fertilizer N should be applied at active tillering and panicle initiation?

  • Rice requires N during the tillering stage to ensure a sufficient number of panicles, and rice requires N at panicle initiation to ensure a sufficient number of filled spikelets per panicle. The critical time for N application at panicle initiation is about 60 days before harvest of tropical rice. The critical time for N application at active tillering is about midway between 14 DAT or 21 DAS and panicle initiation.

     

    The need of the rice crop for fertilizer N can be determined by leaf N status, which is related to leaf color. Dark green leaves have ample N, whereas yellowish green leaves are deficient in N. The leaf color chart (LCC) is a simple and inexpensive tool to rapidly assess leaf N status based on leaf color.

     

    The LCC can be used to guide the application of fertilizer N to maintain an optimal leaf N content for achieving high rice yield with effective N management. As a rule of thumb, the more yellowish green the leaf color, the greater the need of the crop for fertilizer N. The need for fertilizer N at active tillering and panicle initiation also increases in proportion to the response in grain yield to fertilizer N.
     

• When is fertilizer N needed at heading?

  • Inbred rice normally does not require fertilizer N at heading or flowering if the N application at the critical growth stage of panicle initiation was adequate. Hybrid rice and large panicle type (panicle weight type) rice in high-yielding seasons often require a fertilizer N application at early heading.

     

    As a general guideline do not apply fertilizer N at early heading or flowering except in the following cases:

    ·         For hybrid rice, apply about 20 kg N ha⁻¹ at early heading when the expected response to fertilizer N is ≥3 t ha⁻¹ and leaf color is yellowish green (LCC value 3).

    ·         For large panicle type rice, apply about 25 kg N ha⁻¹ at early heading when the expected response to fertilizer N is ≥3 t ha⁻¹ regardless of leaf color.
     

How is fertilizer N adjusted with the LCC?

• Rice farmers should measure leaf color immediately before applying fertilizer N at active tillering and panicle initiation. The amount of fertilizer N to be applied can then be adjusted upward or downward based on leaf color, which reflects the relative ‘hunger’ of the crop for fertilizer N.
   

• What is an LCC?

  • The LCC is usually a plastic, ruler-shaped strip containing four or more panels that range in color from yellowish green to dark green. Several types of LCCs with varying shades of color have been developed and distributed to rice farmers. This created uncertainties regarding which LCC to use and led to requests for a standardized LCC.

     

    In October 2003, the External Review Panel of the Irrigated Rice Research Consortium recommended “standardization of the LCC with harmonized color formula accompanied by calibration with local rice leaf characteristics and training of extension workers and farmers to use the LCC … as soon as possible”. IRRI in collaboration with the University of California Cooperative Extension soon afterwards released a standardized LCC with improved quality assurance for the reproducibility of colors and enhanced matching of the colors to the reflectance spectra of rice leaves.

     

    The standardized LCC is 14 cm long, made of high-quality plastic, consisting of four color shades from yellowish green (No. 2) to dark green (No. 5). The color strips are fabricated with veins resembling rice leaves.

     

    The LCC is usually a plastic, ruler-shaped strip containing four or more panels that range in color from yellowish green to dark green. Several types of LCCs with varying shades of color have been developed and distributed to rice farmers. This created uncertainties regarding which LCC to use and led to requests for a standardized LCC.

     

• How is fertilizer adjusted with the LCC at active tillering and panicle initiation?

  • As a rule of thumb:

    • Apply little or no fertilizer N when the leaf color is green (LCC value 4).
    • Apply a relatively high rate of fertilizer N when the leaf color is yellowish green (LCC value 3).
    • Apply an intermediate rate of fertilizer N when the leaf color is intermediate between 3 and 4.

     

    The rate of fertilizer N depends upon the ability of the soil to supply N to the crop as determined from the yield in a plot without fertilizer N (N-limited yield). If information is not available for yield without fertilizer N, it can be approximated from the expected ability of the soil to supply N to the crop.

     

    Use the following procedure to select a fertilizer N rate for any given field and season:

    • Select one of the three options for approximate yield without fertilizer N. If yield without fertilizer N (N-limited yield) is known, select the nearest of the three N-limited yields in the table below.  If yield without fertilizer N is not known:
      • Assume medium ‘soil N fertility’ for clayey soils with 1 or 2 crops per year, and little or no input of manures and organic materials.
      • Assume high ‘soil N fertility’ for clayey soils with 1 or 2 crops per year, and relatively high soil organic matter or input of manures and organic materials.
      • Assume low ‘soil N fertility’ for sandy and loamy soils with little or no input of manures and organic materials, and for fields with intensive cultivation of 3 high-yielding crops per year with little or no input of manures and organic materials.
    • Select a yield target, which is the yield attainable by farmers with good crop and nutrient management and average climatic conditions.
    • For a given soil N-supplying status and yield target, select an N rate based on the LCC reading.

     

    Approximate yield without fertilizer N (t ha−1) ↓		Yield target ®		5 t ha−1	6 t ha−1	7 t ha−1	8 t ha−1	9 t ha−1
    N-limited yield	Soil N fertility	LCC reading	Leaf color	Fertilizer N rate (kg ha−1)
    4 t ha−1	Medium	LCC ≤ 3	Yellowish green	35	45	45-60*	60	◄
    		LCC = 3.5	Intermediate	25	35	35-45*	45	◄
    		LCC ≥ 4	Green	0	0	25	25-35*	◄
    5 t ha−1	High	LCC ≤ 3	Yellowish green	--	35	45	45-60*	60
    		LCC = 3.5	Intermediate	--	25	35	35-45*	45
    		LCC ≥ 4	Green	--	0	0	25	25-35*
    3 t ha−1	Low	LCC ≤ 3	Yellowish green	45	45-60*	60	60	◄
    		LCC = 3.5	Intermediate	35	35-45*	45	45	◄
    		LCC ≥ 4	Green	0	25	25-35*	25-35*	◄

     

    * Use the lower rate at active tillering and the higher rate at panicle initiation.

     

    The fertilizer N rates in the above table can be fine-tuned and tailored to accommodate location-specific crop-growing conditions and rice varieties.
     

• Farmers can use the LCC to frequently monitor the plant N status of rice. In such case, farmers can take LCC readings at 7 to 10 day intervals beginning at early tillering. Fertilizer N can be applied whenever the leaf color becomes more yellowish green than a critical threshold LCC value.

   

  The critical threshold value for most modern high-yielding rice varieties is intermediate between LCC value 3 and 4 (LCC value 3.5) for transplanted rice and LCC value 3 for wet seeded rice with relatively dense plant population. The critical threshold value can be adjusted for rice varieties.

   

  As a rule of thumb:

  • During the period between 14 DAT or 21 DAS and panicle initiation, apply the rates of fertilizer N indicated in the table below whenever the leaf color becomes more yellowish green than the critical LCC value.
  • After panicle initiation up to early heading, apply about 25 kg N ha⁻¹ whenever the leaf color becomes more yellowish green than the critical LCC value.

   

  Expected yield response to fertilizer N(t ha−1)	Fertilizer N rate during the period after 14 DAT or 21 DAS up to panicle initiation (kg N ha−1)
  1–2	25
  2–3	35
  3–4	45

   

• How does yield response to fertilizer N compare with the yield target?

  • The rates of fertilizer N in the above table are based on the expected yield response to fertilizer N. In many cases the yield response to fertilizer N is not known, whereas the yield target is known.

     

    The expected yield response to fertilizer N can be converted to yield target based on knowledge of the approximate yield without fertilizer N. Use the table below to convert expected yield response to fertilizer N into a yield target.

     

    Expected yield response to fertilizer N ®		1 t ha−1	2 t ha−1	3 t ha−1	4 t ha−1
    Approximate yield without fertilizer N (t ha−1) ↓		Yield target (t ha−1)
    N-limited yield	Soil N fertility
    4 t ha−1	Medium	5	6	7	8
    5 t ha−1	High	6	7	8	9
    3 t ha−1	Low	4	5	6	7