Requirement

Two linear functions describing the maximum accumulation and dilution of
potassium 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 potassium 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 potassium 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 
K in total DM^{1} 
kg ha^{1} 
180

125.43 
61.55 
22.90 
123.28 
277.83 
IE^{2}, Potassium 
kg root kg^{1} P removed 
180 
61.10 
19.05 
26.60 
65.38 
111.91 
RIE^{3}, Potassium 
kg P removed t^{1} roots 
180

14.50 
0.86 
11.60 
15.21 
21.80 

^{1} DM – dry matter, ^{2} IE – internal efficiency, ^{3} RIE – reciprocal internal efficiency
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 (2.5^{th}/97.5^{th}, 5^{th} / 95^{th} or 10^{th} / 90^{th}) 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 (2.5^{th} / 97.5^{th}) 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 potassium uptake in cassava
are aP=32, dP=102.


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. 
