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 Two linear functions describing the maximum accumulation and dilution of phosphorus in total plant dry matter (DM) of cassava were developed using the data set with a HI of 0.40 and above (see table below). Two straight lines were drawn by including all the data points and these lines represent the maximum accumulation (a) and dilution (d) of a nutrient in the plant as done earlier for rice by Witt et al (1999).

Table 1: Descriptive statistics of the data set used for developing empirical models describing the relationship between tuberous root yield and phosphorus content in cassava
Parameter  Unit n Mean SD Minimum Median Maximum
Tuberous root yield t ha-1 480 24.50 7.95 5.60 23.37 36.52
Total dry matter t ha-1 480 16.37 6.90 6.45 16.57 41.20
Harvest Index   480 0.40 0.03 0.35 0.40 0.48
P in total DM1 kg ha-1 180 20.00 8.07 3.40 19.24 29.91
IE2, Phosphorus kg root kg-1 P removed 180 426.20 72.38 232.60 451.17 764.70
RIE3, Phosphorus kg P removed t-1 roots 180 2.30 0.09 1.70 2.27 2.40

Three sets of constants representing the slopes of the boundary lines for maximum accumulation (a) and maximum dilution (d) were used to simulate the optimal relationships between tuberous root yield and uptake of P. The constants were calculated by excluding the upper and lower 2.5, 5 and 10 percentiles (2.5th/97.5th, 5th / 95th or 10th / 90th) of all internal nutrient efficiency data presented in table above. When a greater percentage of the upper and lower internal efficiency values was eliminated, the envelopes were found to be narrowed and there was a skewed distribution of the IEs for P and K which had little effect on the model output. So, it is proposed to use the model parameters by eliminating the upper and lower 2.5 percentiles (2.5th / 97.5th) for a standard version of QUEFTS for a practical decision making on fertilizer requirement of cassava. Proposed standard slopes describing the envelope of tuberous root yield vs phosphorus uptake in cassava are aP=250, dP=750.


Witt, C., A. Dobermann, S. Abdulrachman, H.C. Gines, Wang Guanghuo, R. Nagarajan, S. Satawatananount, Tran Huc Son, Pham Sy Tan, Le Van Tiem, G.C. Simbahan, D.C. Olk. 1999. Internal nutrient efficiencies of irrigated lowland rice in tropical and subtropical Asia. Field Crops Research 63: 113-138.
Central Tuber Crops Research Institute
Sreekariyam, Thiruvananthapuram, Kerala - 17