Farmers in many parts of the world tend to apply excess amounts of N in quest of achieving higher yield of rice. Urea is one of the cheapest among all fertilizers and is required in large quantity. This is another reason why farmers usually apply urea in amounts more than required, sometimes to compensate for deficiency of other more expensive fertilizers e.g. TSP, MP etc. Excess use of urea is detrimental to crops and environment. Because of over use of urea, more vegetative growth often takes place at the cost of reproductive growth resulting in lower yield. Plants get lodged causing sterility, more insect and disease infestation lead to lower yield and higher pest management costs. It is, therefore, necessary to apply urea in a judicious way based on plant's demand.

Need-based N application
N deficiency is the most commonly detected nutrient deficiency symptom in rice. Old leaves and sometimes all leaves become light green to chlorotic at the tip. Leaf colour and canopy appearance are the visual indications of nitrogen deficiency in rice plants and are, therefore, the indicators for determining the time of urea application. Because leaf N content is closely related to photosynthetic rate and biomass production, it is a sensitive indicator of dynamic changes in crop N demand within a growing season. Farmers, therefore, generally use leaf colour as a visual and subjective indicator of the crop's N status and need for N fertilizer application. Soil and plant analysis for N availability is another means of determining the need for applying N fertilizer. However, soil analysis is not an easy way of doing so and such analytical facility is also not that available to farmers. Thus, it is not practical as part of routine soil analysis. Soil tests for N fertilizer recommendation in flooded rice field have not been successful.

The chlorophyll meter, also known as soil plant analysis development (SPAD) meter can quickly and reliably assess the leaf area based N status of a crop. It has been successfully used for rice and other cereal crops. The principle underlying SPAD as a diagnostic tool to determine rice plant's need for additional fertilizer N is based on the fact that rice leaf N concentrations are positively correlated with rice yield and that leaf N correlates with leaf greenness. Leaf Colour Chart (LCC) is another simple and inexpensive instrument to assess the need of the rice plants for nitrogen and to apply the right amount of nitrogen fertilizer (urea).

What is a Leaf Colour Chart (LCC)?
LCC is basically a four inch (previously it was 6 inch) plastic colour device having four separate strips of colour in it. The colour gradients are from light yellowish green to dark green. The first leaf colour chart was developed in Japan. Chinese researchers developed a much improved LCC and calibrated it for indica, japonica and hybrid rice. This chart later became a model for the LCC currently distributed by IRRI's Crop Resources and Management Network (CREMNET). The colour chart is an ideal tool to optimise nitrogen use in rice cropping irrespective of nitrogen source applied -- organic or inorganic.

The instrument is being increasingly used in many Asian countries like Japan, Vietnam, China, Philippines and India to determine the real time based N application.

The strip 2 (of the modified version) is yellow, located on the left most corner of the LCC and the strip 5 is deep green on the right most corner. The strips in between (numbering 3-4) are green in varying proportion, with the gradients being increasingly deeper rightwards. There is a critical value for LCC by which the need for urea application is determined, varying over transplanted and direct wet-seeded rice. The suggested LCC critical value is 3.5 for transplanted aman and boro rice. The equivalent value for high-density direct wet-seeded rice is 3.0. If the "greenness" of paddy canopy is found at or below the critical value, it means the paddy needs urea and if the "greenness" is found above the critical value, it will indicate that there is already adequate N supply for the rice crop and there is no need to apply more.

How to use the colour chart?
* Take LCC readings from 14 days after transplanting for transplanted rice or 21 days after seeding for direct wet-seeded rice in Aman season. But in Boro season, it should be done one week later. The last reading is taken when the crop is at the stage of first flowering.

*Take readings at the same time of the day (9 am to 11 am or 2 pm to 4 pm) with the sun at your back to shade the leaf being measured. The same person should take the leaf colour measurements throughout the crop period who has the experience or judgment in colour variation isolation.

*Select at random at least 10 disease or insect pest damage free rice plants in a field with uniform plant population.

*Compare the colour of the uppermost fully expanded leaf of the 10 selected plants by placing its middle part on top of the colour strips in the chart. If the leaf colour falls between two grades, the mean of the two values is taken as the LCC reading. For example, if the leaf colour lies between chart values 3 and 4, it is noted as 3.5. Do not detach or destroy the leaf. During taking reading, you should move in the fields very carefully.

*If six or more leaves read bellow a set critical value (3.5 for transplanted and 3.0 for direct wet-seeded rice) apply 7.5 kg urea per bigha (33 decimal) in T. aman and 9.0 kg urea per bigha land in boro season.

*Repeat the process every 7-10 days or at critical growth stages (early tillering, active tillering, panicle initiation and first-flowering).

*If the LCC value on the day of measurement is found above the critical value, take the LCC reading again after 5 days and apply urea, if needed.

Remember that LCC is used to asses only N requirement of rice plants, other fertilizers must be applied as recommended. The visual symptoms of N deficiency can be confused with those of sulfur deficiency which tends to first affect the younger leaves or all leaves on the plant. Slight N deficiency can also be confused with iron deficiency but the latter affects the emerging leaf first.

Potential benefits of LCC
LCC as a tool for guiding N management in rice has been tested at on-farm and on-station. The use of LCC-based N management has been found to save about 20 kg N ha-1 (equivalent to about 45 kg urea, which is worth taka 270/-) in West Bengal, India. In Bangladesh, LCC has been validated by BRRI scientists under farmers' conditions since 2000 in collaboration with the International Rice Research Institute (IRRI). It has been observed that LCC-based N management increased yields of both aman and boro rice to some extent. However, there is significant amount of urea saving due to LCC-based N management. This urea saving was 50-60 kg per hectare, which is equivalent to Taka 300-400. In Bangladesh, the potential area for LCC-based N management has been estimated to be about four million hectares of which about 1.5 million hectares are for T. aman and about 2.5 million hectares for boro season. LCC-based N management may result in a potential national benefit of Taka 1306 million which will come from savings of urea. The figure will be multiplied several times if the value of additional rice yield is considered.

LCC has been successfully tested and validated with the high yielding rice varieties. The technology is currently being disseminated by BRRI and the Department of Agricultural Extension (DAE). Some NGOs are also disseminating the technology to the farmers. Efforts should be made by all concerned to strengthen the upscale of this useful technology by the farmers so as to improve national resource conservation, improve land productivity and protect environment.

Limitation of LCC use
*LCC can assess only the leaf colour but sometimes the assessment through LCC may not be 100 percent accurate.

*Genetic variation of different rice varieties in respect of natural leaf colour must be considered. The critical value 3.0 or 3.5 is not always true for all the cultivars.

*Perception of the LCC reading takers regarding leaf colour variation should be justified.

*Taking of LCC reading should be avoided just after rain, especially in monsoon even the day after rain since it may produce misleading result.

Dr. Musherraf Husain is Chief Scientific Officer, ARD, BRRI, Gazipur.