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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.
 
Leaf colour chart has been calibrated for N top dressing of cassava cultivars, Sree Vijaya and M-4
 
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.

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.
 
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 fertility research for cassava in India. 8th Asian Cassava Research Workshop, 20-24 October 2008, Vientiane, Lao PDR.

Byju G., C.S.Ravindran, M. Haripriya Anand and V.S. Santhosh Mithra. 2010. Balanced fertilizer application to cassava by site specific nutrient management approach for enhanced yield, income and nutrient use efficiency. International Conference on Balanced Nutrient Management for Tropical Agriculture, April 12-16, 2010 at Swiss Garden Resort and Spa, Kuantan, Pahang. Organised by The Malaysian Society of Soil Science. Abstracts pp. 61-65.

Furuya, S. 1987. Growth diagnosis of rice plants by means of leaf colour. Japan Agriculture Research Quarterly, 20: 147-153.

Haripriya Anand M. and G. Byju. 2008 a. Chlorophyll meter and leaf colour chart to estimate chlorophyll content, leaf color and yield of cassava. Photosynthetica. 46(4):511-516.

Haripriya Anand M. and G. Byju. 2008 b. Leaf colour chart-a farmer friendly decision tool for nitrogen management for cassava in laterite soils of Kerala. 18th Swadeshi Science Congress, 5-7 November 2008, Thiruvananthapuram, Kerala, India. Abstracts pp. 69-70.

Haripriya Anand M. and G. Byju. 2009. Leaf colour chart and chlorophyll meter based nitrogen management for increased yield and N use efficiency of cassava. Proceedings of the 21st Kerala Science Congress, 28-31 January 2009, Kollam, Kerala, India. pp. 5-7. Kerala State Council for Science, Technology & Environment.

Howeler, R.H. 2001. Cassava mineral nutrition and fertilization. In Cassava: Biology, Production and Utilization; Hillocks, R.J., Tresh, J.M. and 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.
 
Central Tuber Crops Research Institute
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