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Use of ‘leaf colour chart’ 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. Nitrogen’s
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 crop’s 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 crop’s 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 area–based 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 farmer’s
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 farmers’ fields. Fields can be divided into two equal
parts: one for N fertilization with farmer’s 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 farmers’
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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