Field Specific P Management
The internal efficiencies of phosphorus at maximum accumulation (a) and dilution
(d) recommended for use in QUEFTS for cassava are 250 and 750 kg tuberous root
yield (dry weight) / kg P removed. When the internal efficiency values of
phosphorus were analysed from unfertilized and fertilized plots separately, it
could be observed that the values were higher in fertilized plots and this is a
clear indication that phosphorus is not a limiting nutrient in those major
cassava production regions in India.
The P uptake requirements at different yield potentials of cassava showed that
the relation between yield and P uptake is linear at lower yield targets,
indicating that plant growth is limited by P 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 P application will give more profit to farmers than aiming for
higher yield targets closer to potential yield.
Table 1: Phosphorus 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. |
|
|
Tuberous root yield (t/ha dry matter) |
P uptake
(kg/ha) |
Internal efficiency
(kg/kg) |
Reciprocal internal efficiency
(kg/1000 kg) |
|
2.45 |
5 |
459
|
2.2 |
|
4.90 |
11
|
450
|
2.2 |
|
9.80 |
22
|
451
|
2.2 |
|
14.70 |
33
|
451
|
2.2 |
|
19.60 |
46 |
428
|
2.3 |
|
21.35 |
55
|
389
|
2.6 |
|
21.70 |
58 |
377 |
2.7 |
|
22.05 |
57
|
389
|
2.6 |
|
22.40 |
61 |
369
|
2.7 |
|
22.75 |
62 |
365 |
2.7 |
|
23.10 |
66 |
348 |
2.9 |
|
23.45 |
70 |
337 |
3.0 |
|
23.80 |
73 |
324 |
3.1 |
|
24.15 |
78 |
309 |
3.2 |
|
24.49 |
145 |
260 |
3.9 |
|
The P uptake requirement in total plant dry matter for 1000 kg tuberous root in
the linear part of the relation was 2.2 kg irrespective of the yield potential.
The corresponding IE value for P was 451.7 kg kg-1. It can be observed that both the P uptake and IE values of cassava
were similar to the values from the data set used for developing the model. The
IE of phosphorus 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 P supply (IPS) was calculated from the plots that did not receive
any P fertilizer. 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.
Table 2: Indigenous phosphorus supply (kg/ha) from five cassava
production regions expressed in soil chemical properties.
|
Location
|
Regression equations |
n |
R2 |
|
Kerala
|
IPS = 0.3302 Bray1 P + 8.3511 |
21 |
0.73 |
|
Tamil Nadu |
IPS = 0.6067 Olsen P + 1.084 |
15 |
0.82 |
|
Andhra Pradesh |
IPS = 0.3586 Bray 1 P + 3.3456 |
11 |
0.77 |
|
Maharashtra
|
IPS = 0.6067 Olsen P + 1.084 |
10 |
0.83 |
|
There were wide variations in the fertilizer nutrient recovery efficiencies of P
by cassava. The average value of recovery efficiency of P was 30 per cent.
Another major observation was the considerable variation in REp with the amount
of fertilizer applied. We developed the relationships between recovery
efficiency and amount of P fertilizers applied as REp = 39.498e-0.0061P.
With the help of QUEFTS model and with the relations developed, field specific
fertilizer recommendations can be made for the major cassava growing regions in
India. Table 3-4 gives an idea about the differences in N fertilizer rate at
different regions.
Table 3: Rate of application of fertilizer P2O5 for specific yield
targets based on yield in P omission plots for Kerala and Andhra Pradesh states. |
|
Yield target |
< 10 |
10-20 |
20-30 |
> 30 |
|
Yield in P omission plot |
P2O5 rate (kg/ha) |
|
<10 |
10 |
50 |
140 |
§ |
|
10-20 |
0 |
10 |
50 |
§ |
|
> 20 |
0 |
0 |
15 |
80 |
§ - not able to achieve the yield target |
Table 4: Rate of application of fertilizer P for specific yield
targets based on yield in P omission plots for Tamil Nadu and Maharashtra
states. |
|
Yield target |
< 10 |
10-20 |
20-30 |
30-40 |
> 40 |
|
Yield in P omission plot |
P2O5 rate (kg/ha) |
|
<
10 |
15 |
50 |
100 |
§ |
§ |
|
10-20 |
0 |
15 |
50 |
90 |
§ |
|
20-30 |
0 |
0 |
15 |
60 |
120 |
|
> 30 |
0 |
0 |
0 |
20 |
80 |
§ - not able to achieve the yield target |
Steps involved in determining P2O5
fertilizer rate for a particular site
|
|
1. |
Fix the yield target of the particular site. |
|
2. |
Determine the yield in phosphorus 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
and for other varieties, 50 per cent of the recommendation can be given based on previous results. |
|
|
|