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Concept of SSNM
It is very clear that cassava nutrient management by blanket fertilizer recommendations over wide areas and soil types over the past 40 years or so in India have resulted in significant yield increase. But when we extrapolate the results from experimental stations to farmers’ fields, the yield cannot be increased beyond a certain level due to the high temporal and spatial variability of soil and plant properties. Studies in other crops clearly showed that further increase in yield and nutrient use efficiency can be possible only by managing this large spatial and temporal variability existing in soil nutrient supply, nutrient use efficiency and crop response to nutrients among different farms (Doberman and White 1999, Pathak et al., 2003).
 
The concept of SSNM developed for rice in Asian countries (Doberman and White, 1999) has been tried for cassava by conducting on farm and on station experiments for the past 8 years in different cassava production domains in India. Studies conducted by CTCRI indicated that the indigenous nutrient supply is variable among cassava farms in India and is not related to the common soil test procedures adopted. This indicates that blanket fertilizer recommendations over large areas (states) are not enough to meet the future demand of cassava production and a plant based integrated management of all nutrients is needed for further yield increase. The concept of SSNM is based on the hypothesis that in high yielding situations, the ability to predict soil nutrient supply and plant uptake in absolute terms rather than relative yield response is the basis for plant nutrient management (Dobermann and White, 1999). The QUEFTS (Quantitative Evaluation of Fertility of Tropical Soils) model (Janssen et., 1990; Smaling and Janssen 1993) takes into account the interactions of N, P and K. The methodology followed by Witt et al. (1999) to validate the model for rice by conducting experiments in six Asian countries and Pathak et al. (2003) for wheat in India has been used to develop SSNM technology for cassava in India. The basic relationships between tuberous root yield and nutrient uptake of cassava have been developed and the QUEFTS model has been modified for prescribing site specific NPK recommendations for cassava.
 
The relation between yield and nutrient supply is divided into several steps in the QUEFTS model by considering the interactions among nutrient supply, uptake and use. Four steps are involved in QUEFTS: (1) determination of indigenous nutrient supply. Relationship between soil test data and nutrient uptake in fertilizer omission plots is used for it; (2) determination of the uptake of N (UN), P (UP) and K (UK) as functions of potential supply of N (SN), P (SP) and K (SK) ie, supply from soil plus fertilizer, taking fertilizer nutrient recovery efficiency into account; (3) estimation of yield ranges as functions of actual uptakes of N, P and K when they are maximally accumulated and maximally diluted. The internal efficiencies when they are maximally accumulated and diluted are designated as ‘a’ and ‘d’ respectively; and (4) estimating the final yield by combining three yield ranges (one each for N, P and K) considering NPK interactions.
 
Using ‘a’ and ‘d’ values for N, P and K, fertilizer recovery efficiencies and some minimum values for INS (5.0), IPS (1.0) and IKS (5.0), the balanced NPK uptake requirements were derived under a set of constraints ie, yield target and yield potential (Byju et al., 2006, 2008, 2009, 2010 a,b). Since it is impossible to maximize the uptake efficiencies of N, P and K simultaneously, Smaling and Janssen (1993) proposed to maximize the mean of the uptake efficiencies. The ratios of UN/SN, UP/SP and UK/SK are yield-producing uptake efficiencies and the mean of these three ratios is the total yield-producing uptake efficiency. In this version of QUEFTS, yield-producing uptake efficiencies were set to above 95 per cent (UN/SN=UP/SP=UK/SK ≥ 0.95.
 
The maximum yield of cassava obtained under experimental conditions in each of the cassava production regions was fixed as the climatic yield potentials and the values ranged from 17.5 – 24.5 t ha-1 tuberous roots dry matter. Yield goals were fixed at 70 to 80 per cent of the climatic yield potential because beyond that level, internal nutrient efficiencies in the plant showed decline. Moreover, about 80 per cent of the climatic yield potential seems to be the maximum possible yield that can be obtained by most farmers under field conditions.
 
References:
Byju, G., C.S. Ravindran and M. Nedunchezhiyan. 2006. Site specific nutrient management as a precision farming tool for cassava cultivation in Tamil Nadu, India. Abstract of papers: 14th Triennial symposium of the International Society for Tropical Root Crops, 20-26 November 2006, Mascot Hotel, Thiruvananthapuram, Kerala, India. pp. 185-186.
 
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 and V.S. Santhosh Mithra. 2009. Precision farming techniques in cassava. In: (Eds) Information technology applications in horticultural crops. Central Potato Research Institute, Shimla, Himachal Pradesh, India.
 
Byju G., A.C. Hridya, and C.S.Ravindran. 2010a. A modified QUEFTS model for site specific nutrient management of cassava (Manihot esculenta Crantz). Proceedings of 22nd Kerala Science Congress, 28-31 January 2010, KFRI, Peechi, Kerala, India, pp. 3-5.
 
Byju G., C.S.Ravindran, M. Haripriya Anand and V.S. Santhosh Mithra. 2010b. 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.
 
Dobermann, A., P.F. White. 1999. Strategies for nutrient management in irrigated and rain fed lowland rice systems. Nutrient Cycling in Agroecosystems 46: 1-10.
 
Janssen, B.H., F.C.T. Guiking, D. Van der Ejik, E.M.A. Smaling, J. Wolf, H. Reuler. 1990. A system for quantitative evaluation of fertility of tropical soils (QUEFTS). Geoderma 46: 299-318.
 
Smaling, E.M.A., B.H. Janssen. 1993. Calibration of QUEFTS, a model predicting nutrient uptake and yield from chemical soil fertility induces. Geoderma 49: 21-44.
Central Tuber Crops Research Institute
Sreekariyam, Thiruvananthapuram, Kerala - 17