Requirement 
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 DM^{1} 
kg ha^{1} 
180

20.00 
8.07 
3.40 
19.24

29.91 
IE^{2}, Phosphorus 
kg root kg^{1} P removed 
180 
426.20 
72.38 
232.60 
451.17 
764.70 
RIE^{3}, 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.


Reference
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: 113138. 
