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Use of Ďleaf colour chartí in nitrogen management  

By Dr Muhammad Farooq & Dr Shahzad M.A. Basra

THE growth of a plant depends on the availability of sunlight, water and various chemical elements. About 16 elements are recognized as essential in rice nutrition: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, iron, manganese, copper, boron, zinc, molybdenum and chloride.

Among these, carbon, hydrogen, and oxygen are absorbed directly from the air and water; the rest must be present in the soil.

Rice requires large amounts of nitrogen. Nitrogenís fundamental importance as a primary nutrient element is augmented by the fact that many improved rice varieties cultivated around the world have been bred to show a marked response to the application of nitrogenous fertilisers.

To know how much N fertiliser should be applied is very important. One way is to calculate the amount of N and other nutrients required for a target yield of a crop based on nutrient analysis; then determine the amount of nutrients that indigenous sources can supply; finally, the difference between the crop need and the indigenous supply is the amount of nutrients that must be added through fertilizers.

For example, a rice crop yielding 6 t ha-1 in requires 90 kg N ha-1 of which 40 to 70 kg N ha-1 are likely to come from the indigenous sources; the remaining 20 to 50 kg N ha-1 must be supplied through fertilisers.

Generally, crop response to applied N fertilizer is used as a basis for developing blanket N recommendations for large areas. The blanket recommendation, however does not consider variability soil N supply and changes in crop demand.

Farmers generally apply too much N (and little P and K and other nutrients) that results in high pest and disease incidence and serious lodging. The consequence of high N application is high pesticide use to control pests, more expenditure on pesticides, and reduced yield and poor grain quality due to lodging.

In addition, excess N is leached into water sources that get polluted over time. Farmers suffer from more pesticide-related health risks. Overall, reduced yield, reduced profit, and higher health risks and environmental pollution result from over application of N fertilizers.

Rice fields generally show high variability in soil N supply and cropís need for N fertiliser. This means that rice crops in different fields require different amounts of N input. Farmers, therefore, require an effective strategy to handle such field-to-field variation in N fertiliser requirement of rice crops.

Research has developed methods to vary N inputs to rice crops based on crop demand and soil N supply. This is called crop-need-based N management. It involves the matching of external fertilizer N supply to actual crop demand and crop growing condition in individual fields. It can treat N deficiency on a timely basis. It prevents over-application of N fertilizer and requires periodic monitoring of crop N status and application of N fertilizer as and when required.

Precise application of N fertilizer based on plant need and location in the field greatly improves fertiliser use efficiency in rice. The optimum use of N can be achieved by matching N supply with crop demand.

Farmers generally use leaf colour as a visual and subjective indicator of the rice cropís nitrogen status and need for N fertilizer application. Leaf colour intensity is directly related to leaf chlorophyll content which, in turn, is related to leaf N status.

A potential solution has been tried to regulate the timing of N application in rice using a chlorophyll meter (or SPAD meter) or a LCC to determine the plant N. The concept is based on results that show a close link between leaf chlorophyll content and leaf N content.

Moreover, leaf areaĖbased N concentration (Na) varies within a narrow range at different growth stages. The close relationship between Na and SPAD or LCC readings facilitates the use of a single critical value for SPAD or LCC to monitor leaf N status at all growth stages. Thus, the chlorophyll meter or LCC can be used to quickly and reliably assess the leaf N status of crops at different growth stages. The LCC, because of its low cost for farmers, has shown much promise in real-time N management.

The leaf colour chart (LCC) reinforces the farmerís knowledge and provides a simple, easy-to-use, and inexpensive tool for efficient N management in rice. It is made of plastic and consists of four different colour strips (previously with six strips of 1-6). The critical values of LCC indicate the need for N application. For transplanted and direct seeded rice these critical values are 3.5 and 3, respectively.

The colour chart is an ideal tool to optimise N use in rice cropping, irrespective of the source of nitrogen applied - inorganic, organic, and/or bio-fertilisers. The successful adaptation and use of the colour chart will promote timely and efficient use of N fertiliser in rice and minimise the fertilizer-related pollution of surface and ground water.

Moreover, with the need-based N fertiliser application, rice plants will remain healthy, thereby reducing the need for application of pesticides. It is thus an eco-friendly tool in the hands of small farmers.

How to use the LCC: Sampling should be started at the panicle initiation growth stage of rice. Because the sun angle and light intensity affects leaf colour. Experts say sampling should be done between 10am and 2pm. While sampling, always the sun should be on the back

. Regardless of the method, experts recommend repeating the process several times within each field to obtain representative results.

* Randomly select at least 10 disease-free rice plants or hills in a field with uniform plant population.

* Select the topmost fully expanded leaf from each hill or plant. Place the middle part of the leaf on a chart and compare the leaf colour with the colour panels of the LCC. Do not detach or destroy the leaf.

* When the leaf colour falls between two shades, the mean value is taken as the reading, e.g., 2.5 for colour between two and three. Do not detach or destroy the leaf.

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

* Measure the leaf colour under the shade of your body, because direct sunlight affects leaf colour readings. If possible, the same person should take LCC readings at the same time of the day every time.

* If more than five out of 10 leaves read below a set critical value, apply nitrogen fertilizers.

To mobilise the on-farm adaptation of LCC, extension wing of Agriculture Department may arrange demonstration plots at the farmersí fields. Fields can be divided into two equal parts: one for N fertilization with farmerís practice and the other for LCC-based N application. P, K, Zn are applied at locally recommended rates and time for both plots.

Farmers can compare the total amount of N fertilizer used and grain yields obtained for the two methods to determine their relative efficiency and profitability. They can also evaluate the level of incidence of pests and diseases in both plots.

Several factors influence LCC readings e.g., varieties, plant density, sunlight, soil and plant nutrient status (other than N), and biotic and abiotic stresses that induce discoloration of leaves. One should understand these limitations and know how to tackle them while using the LCC.

The use of LCC in not being used in Pakistan is at farmersí field. It is the need of the hour to adapt and popularise this cost effective method for judicious N use. Extension wing of the Agriculture Department may be assigned the duty to introduce LCC to Pakistani farmers and train the farmers.

Courtesy:  The Dawn

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