Nitrogen
Requirement 
Two linear functions describing the maximum accumulation and dilution of
nitrogen in total plant dry matter (DM) of cassava were developed using the data
set with a HI of 0.40 (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
nutrient content in cassava. 
Parameter 
Unit 
n 
Mean 
SD 
Minimum 
Median 
Maximum 
Tuberous root yield 
t/ ha 
480 
24.50 
7.95 
5.60 
23.37 
36.52 
Total dry matter 
t/ ha 
480 
16.37 
6.90 
6.45 
16.57 
41.20 
Harvest Index 

480 
0.40 
0.03 
0.35 
0.40 
0.48 
N in total DM1 
kg/ ha 
480 
115.38 
59.69 
25.80 
118.64 
220.14 
IE2, Nitrogen 
kg root/ kg N removed 
480 
250.39 
102.08 
138.43 
206.23 
489.44 
RIE3, Nitrogen 
kg N removed/ troots 
480 
4.56 
1.49 
2.04 
4.85 
7.22 


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 N, P and K. 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 nitrogen uptake in cassava are aN=35, dN=80. 
1 DM – dry matter, 2 IE – internal efficiency, 3 RIE – reciprocal internal
efficiency 


