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Real Time N Management |
Nitrogen is a large nutrient input for cassava production and nitrate is
susceptible to movement and loss from soils, hence increasing N use efficiency
assumes greater significance. Since soil tests do not account for the dynamics
of soil N supply and for inputs from other sources such as irrigation water,
they are of limited value for predicting the variability in soil N supply. If we
can synchronize the N needs of the crop with N supply, it will greatly increase
N use efficiency. Since plant growth reflects total N supply from all sources,
plant N presumably is the best indicator of N availability to crops at any given
time (Singh et al., 2007). Samples of whole plant tissue or leaves are used to
assess the nutrient status of growing plants (Balasubramanian et al., 1999).
Nitrogen concentration of youngest fully expanded leaf (YFEL) blade without
petiole has been found to reflect the recent history of N availability to
cassava (Howeler, 2001). The method of leaf sampling and analysis is
destructive, time consuming and expensive; it is also limited by lack of
analytical laboratories in developing countries (Turner and Jund, 1994; Peng et
al., 1995). Hence measurement of leaf N concentration by conventional laboratory
procedures has limited use as a diagnostic tool for optimizing N top dressing.
In order to synchronise N supply with crop demand, tools which can
instantaneously assess nitrogen status of cassava have been developed and
standardized. These are simple, quick and non-destructive diagnostic techniques
to decide when to apply N fertilizers to cassava. Two decision aids viz. leaf
colour chart (LCC) and hand held chlorophyll meter which have already outpaced
agronomic nutrient management research and proven to be very effective to
monitor leaf N status and to improve the timing of N top dressing in many other
crops have been standardized for cassava.
The leaf colour chart (LCC) developed from a Japanese prototype (Furuya, 1987)
by International Rice Research Institute (IRRI), The Philippines and has been
successfully used for N management in many crops has been calibrated for use by
farmers to qualitatively assess foliar N status and adjust N top dressing
accordingly.
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Leaf colour chart has been calibrated for N top dressing of cassava cultivars,
Sree Vijaya and M-4 |
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The chlorophyll meter has proven to be another appropriate analytical tool to
quickly identify nitrogen deficiencies at the beginning of the growth of the
crop. It is a simple, portable hand held device that clamps on a leaf and
measures light transmittance at 650 and 940 nm. The 650 nm wavelength coincides
with the spectral region associated with maximum chlorophyll activity, while the
940 nm wavelength provides internal calibration to the instrument; compensating
for leaf thickness, water status and other plant factors. The primary
application for the chlorophyll meter has been to determine the potential
efficacy of additional nitrogen treatments to crop plants.
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Studies by Byju and Haripriya Anand (2009 a and b); Haripriya Anand and Byju
(2008 a and b); Haripriya Anand and Byju, 2009; Byju et al. (2008) and Byju et
al. (2010) have resulted in adapting the leaf color chart and chlorophyll meter
to cassava to use these simple tools for N top dressing for enhancing N use
efficiency and thereby yield. Standardization of the critical leaf for LCC and
chlorophyll meter as well as determination of the threshold LCC and chlorophyll
meter values for selected cassava varieties have been attempted in the studies
mentioned above.
Studies in CTCRI, India have conclusively indicated that leaf color chart (LCC)
and chlorophyll meter (SPAD-502) can be used to determine the leaf nitrogen
concentration in the youngest fully expanded leaf (YFEL) blade of cassava. Since
plant growth reflects total N supply from all sources, plant N presumably is
found to be the best indicator of N availability to crops at any given time. Our
results indicate that the LCC score, SPAD value and leaf N concentration of
cassava are influenced by leaf position, growth stage, cultivar and N fertilizer
rate. Significant (p < 0.01) positive correlation between tuberous root yield
and LCC score (r2 = 0.85) as well as between tuberous root yield and SPAD value
(r2 = 0.83) existed at 30 days after planting (DAP) which was better than the
relationship of tuberous root yield with leaf N concentration (r2 = 0.79). Same
trend was noticed at 60 DAP also. The study indicated that same threshold value
for LCC score or SPAD value can be used at 30 and 60 DAP for both cultivars,
Sree Vijaya and M-4. The results also indicate that we can use a single
regression equation across the cultivars and across the growth stages of 30 and
60 DAP for the relationship between LCC score and leaf N concentration and
between SPAD value and leaf N concentration. A LCC threshold value of 2.65 is
suitable to determine the optimal timing of N top dressing of the cultivars
which corresponded to a SPAD value of 25.
The field experiments were also conducted in the farm of Central Tuber Crops
Research Institute, India to validate the SPAD and LCC based nitrogen top
dressing to cassava and the results showed that farmers can apply nitrogen
according to the need and indigenous nitrogen supply in farmers fields. Future
plans include validation of the technology in diverse soil and climatic
conditions and for other important varieties cultivated in India. |
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References
Balasubramanian, V., Morales, A.C., Cruz, R.T. and Abdulrachman, S.
1999. On-farm adaptation of knowledge intensive nitrogen management technologies
for rice systems. Nutrient Cycling in Agroecosystems, 53: 59-69.
Byju, G. and Haripriya Anand M. 2009 a. Leaf color chart and
chlorophyll meter based leaf nitrogen estimation and their critical values for
real time nitrogen management in cassava. Communications in Soil Science and
Plant Analysis. 40 :2816-2832.
Byju, G. and Haripriya Anand M. 2009 b. Differential response of
short- and long- duration cassava cultivars to applied mineral nitrogen. Journal
of Plant Nutrition and Soil Science. 172(4): 572-576.
Byju, G., M. Nedunchezhiyan and G. Ramanandam. 2008. Soil
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Byju G., C.S.Ravindran, M. Haripriya Anand and V.S. Santhosh Mithra.
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Furuya, S. 1987. Growth diagnosis of rice plants by means of leaf colour.
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Haripriya Anand M. and G. Byju. 2008 b. Leaf colour chart-a farmer
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Haripriya Anand M. and G. Byju. 2009. Leaf colour chart and
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Technology & Environment.
Howeler, R.H. 2001. Cassava mineral nutrition and fertilization. In
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Bellotti, A.C. (eds); CABI Publishing, Oxon, UK and New York, USA, 115-147.
Peng, S., Laza, R.C., Garcia, F.V. and Cassman, K.G. 1995.
Chlorophyll meter estimates leaf area based nitrogen concentration in rice.
Communications in Soil Science and Plant Analysis, 26(5&6): 927 – 935.
Singh, Y., Singh, B., Ladha, J.K., Bains, J.S., Gupta, R.K., Singh, J.
and Balasubramanian, V. 2007. On farm evaluation of leaf colour chart for
need- based nitrogen management in irrigated transplanted rice in north western
India. Nutrient Cycling in Agroecosystems, 78: 167 – 176.
Turner, F. T. and Jund, M.F. 1991. Chlorophyll meter to predict
nitrogen top dress requirement for semi dwarf rice. Agronomy Journal, 83:
926-928.
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