|
Field Specific N Management |
The internal efficiencies of nitrogen at maximum
accumulation (a) and dilution (d) recommended for use in QUEFTS for cassava are
35 and 80 kg tuberous root yield (dry weight) / kg N removed. When the internal
efficiency values of nitrogen were analysed from unfertilized and fertilized
plots separately, it could be observed that the values were higher in
unfertilized plots and this is a clear indication that nitrogen is a limiting
nutrient in those major cassava production regions.
The N uptake requirements at different yield potentials of cassava showed that
the relation between yield and N uptake is linear at lower yield targets,
indicating that plant growth is limited by N uptake. At higher yield targets
that are closer to yield potential, there was great reduction in the internal
efficiency values (Table 1). When the yield target approaches the yield
potential, the IE values decreased drastically from the linear level and reached
minimum values. The results indicate that maximizing the nutrient efficiencies
by balanced N application will give more profit to farmers than aiming for
higher yield targets closer to potential yield.
The N uptake requirement in total plant dry matter for 1000 kg tuberous root in
the linear part of the relation was 17.6 kg irrespective of the yield potential.
The corresponding IE value for N was 56.2 kg kg-1. It can be observed that both
the N uptake and IE values of cassava were similar to the values from the data
set used for developing the model. The IE of nitrogen was found to be lower in
the data set which could be due to nutrient imbalances or differences in
potential yields at various experimental locations. It can also be seen that the
linear part of the relationship is always 75 to 80 per cent of the whole yield
range.
The indigenous N supply (INS) was calculated from the unfertilized plots. This
is based on the assumption that nitrogen is the most important nutrient element
that restricts cassava growth. The soil indigenous nutrient supply and different
soil test values (pH, organic carbon, available N, P and K) were plotted in all
possible combinations to develop the relationships between them. The regression
equations and their correlation coefficients developed for the four major
cassava production regions in India are given in Table 2. Due to very similar
soil conditions, and lack of suitable data set, the relationships developed for
Tamil Nadu region have been adopted for Maharashtra too. When we added other
soil test values, there was no improvement in the relationships.
There were wide variations in the fertilizer nutrient recovery efficiencies of N
by cassava. The average value of recovery efficiency of N was 50 per cent.
Another major observation was the considerable variation in REn with the amount
of fertilizer applied. We developed the relationships between recovery
efficiency and amount of N fertilizers applied as
REn = -0.0032N2 + 0.7589N + 3.1533
|
|
Tuberous root yield (t/ha dry matter) |
N uptake
(kg/ha) |
Internal efficiency
(kg/kg) |
Reciprocal internal efficiency
(kg/1000 kg) |
|
2.45
|
44
|
56
|
17.9
|
|
4.90
|
87
|
56
|
17.7
|
|
9.80
|
173
|
56
|
17.7
|
|
14.70
|
260
|
57
|
17.7
|
|
19.60
|
368
|
53
|
18.8
|
|
21.35
|
433
|
49
|
20.3
|
|
21.70
|
447 |
49 |
20.6
|
|
22.05 |
481
|
46
|
21.8 |
|
22.40 |
485 |
47
|
21.5
|
|
22.75 |
507 |
45 |
22.3 |
|
23.10 |
527 |
44 |
22.8 |
|
23.45 |
548 |
43 |
23.4 |
|
23.80 |
575 |
41 |
24.1 |
|
24.15 |
621 |
39 |
25.7 |
|
24.49 |
640
|
34 |
29.8 |
Table 1: Nitrogen uptake
requirements, internal efficiencies (kg tuberous root per kg nutrient) and
reciprocal internal efficiency (kg nutrient per 1000 kg tuberous root) for
cassava as calculated by QUEFTS for certain yield targets. |
|
|
|
Location
|
Regression equations |
n |
R2 |
|
Kerala
|
INS = 188.84 OC - 6.2265 |
35 |
0.78
|
|
Tamil Nadu |
INS = 221.94 OC + 4.8519 |
23 |
0.86 |
|
Andhra Pradesh |
INS = 129.11 OC + 54.055 |
24 |
0.41 |
|
Maharashtra
|
INS = 221.94 OC + 4.8519 |
10 |
0.90 |
Table 2: Indigenous nitrogen supply (kg/ha) from five cassava
production regions expressed in soil chemical properties. |
|
|
|
Yield target |
10 |
20 |
30 |
40 |
|
Yield in N omission plot |
N rate (kg/ha) |
|
10 |
25 |
100 |
§ |
§ |
|
20 |
0 |
25 |
100 |
§ |
|
30 |
0 |
0 |
25 |
150 |
|
40 |
0 |
0 |
0 |
25 |
Table 3: Rate of application of fertilizer N for specific yield
targets based on yield in N omission plots for Kerala and Andhra Pradesh states
|
|
§ - not able to achieve the yield target |
|
|
|
Yield target |
10 |
20 |
30 |
40 |
50 |
|
Yield in N omission plot |
N rate (kg/ha) |
|
10 |
35 |
100 |
240 |
§ |
§ |
|
20 |
0 |
35 |
100 |
230 |
§ |
|
30 |
0 |
0 |
35 |
90 |
230 |
|
40 |
0 |
0 |
0 |
0 |
35 |
Table 4: Rate of application of fertilizer N for specific yield
targets based on yield in N omission plots for Tamil Nadu and Maharashtra states
§ - not able to achieve the yield target |
|
|
|
Steps involved in determining N fertilizer rate for a particular site |
|
1. |
Fix the yield target of the particular site. |
|
2. |
Determine the yield in nitrogen omission plot by conducting the very simple
nutrient omission plot trials
. |
|
3. |
Estimate the total fertilizer requirement based on yield target and yield in
nutrient omission
plots and using the QUEFTS model or simplified charts as given
above. |
|
4. |
The above fertilizer rate is for
high yielding cassava cultivars
for other
varieties, 50 per cent of the recommendation can be given based on previous
results. |
|
|