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Calculate P Fertilizer Requirement |
The requirement of P fertilizer of cassava is estimated by taking into account
the indigenous nutrient supply, target yield and agronomic efficiency of P
fertilizer use by cassava.
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P fertilizer (kg/ha) = |
Target yield (t/ha) – Yield in P omission plot (t/ha) |
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Agronomic efficiency of P (AEP) |
ie. tuberous root yield increase (ton) per kg P fertilizer applied
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Indigenous Phosphorus Supply
The average value of indigenous phosphorus supply (IPS) was found to be similar
in all crops sampled, but more than two fold ranges were measured among the
cassava fields in each year (table 1). The IPS ranged from about 7 to 19 kg P/
ha and coefficient of variation (CV) of IPS ranged from 26 to 32 per cent in
each year. Among the four crops sampled in the same cassava field, average
standard deviation of nitrogen supply was 2 kg/ha. This average estimation error
(or crop to crop variability) of a field-specific measurement is due to the
spatial variability arising because of rotation of nutrient omission plots and
changes in climatic and crop management factors, different varieties grown in
the same field in different years, and errors associated with plant sampling and
chemical analysis. Over the short term, the present average levels of indigenous
supply would be sufficient to get a tuberous root yield of 25 t/ ha assuming
nitrogen requirement of 0.73 kg/ ton tuberous root yield.
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Table 1: Variation of the indigenous phosphorus supply (kg/ha)
among cassava farms in India |
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Year
|
Mean
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Min
|
Max
|
CV
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2003
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13.35 |
7.89 |
18.95 |
28.82 |
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2004 |
13.61 |
8.45 |
16.76 |
32.12 |
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2005 |
12.91 |
7.41 |
17.63 |
26.65 |
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2006 |
15.41 |
9.63 |
18.47 |
30.45 |
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Tuberous root yield and P uptake
There was a significant increase in tuberous root yield and phosphorus uptake in
SSNM in all the four crops grown during 2003-2006 compared to FFP (Table 2). The
average yield difference between SSNM and FFP for the four crops grown was 8.72
t ha-1 (23%, P=0.005) and the differences in tuberous
root yield over the years were not statistically significant (P=0.247). In nine
farms, the average yield exceeded 40 t ha with a maximum of 47.69 t/ha and in
five farms, yields in the SSNM exceeded 10 t/ha compared to FFP, clearly
indicating the superiority of SSNM approach.
Significant differences were noticed in plant P uptake in SSNM compared with FFP
treatments. On an average, plant P uptake increased by 5.17 kg/ha (21.73%,
P=0.002). Similar trends in phosphorus uptake were observed over the years
(P>0.05).
Table 2. Effect of site specific nutrient management (SSNM) on root yield, plant
phosphorus accumulation and P fertilizer use in cassava farms in India. |
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|
Treatment |
D |
P > ITI |
|
SSNM |
FFP |
|
Tuberous root yield, t/ha |
37.35 |
28.63 |
8.72 |
0.005 |
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Plant P uptake, kg/ha |
23.79 |
18.62 |
5.17 |
0.002 |
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P Fertilizer, kg/ha |
91 |
51 |
40 |
0.004 |
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D-SSNM – FFP
P > ITI - probability of a significant mean difference between SSNM and
FFP
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Fertilizer Use
The average rate of p in FFP plots was 51 kg P2O5/ha. Detailed survey indicated
that most of the farmers applied fertilizers without taking into account the
actual soil fertility status. The P status was not found to be significantly
correlated with IPS (r = 0.31). On an average, 40 kg/ha more fertilizer P was
used in SSNM treatment compared to FFP (+78 %, P=0.004). Higher rate of
application of fertilizer P in SSNM treatments was fixed based on the prediction
of the calibrated QUEFTS model that accurately accounted for the low native soil
P status measured as plant P uptake in nutrient omission plots.
Phosphorus Uuse Efficiency
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The phosphorus use efficiency in the SSNM treatment was increased significantly
in SSNM treatment where the field- and season- specific P management was
practiced (Table 3). More P fertilizer was applied in SSNM plots compared with
the FFP and there was significant increase in agronomic efficiency (AEP),
recovery efficiency (REP) and physiological efficiency (PEP) of phosphorus.
Across the four crops grown, the AEP increased by 17 kg kg-1 (24%, P=0.022), REP
by 0.01 kg kg-1 (10%, P=0.003) and PEP by 49 kg kg-1 (74%, P=0.003). The results
of the study provide on-farm evidence that the present phosphorus management
practice in India for cassava is inconsistent with the physiological nutrient
requirements of the crop and that is one of the major reasons that prevents
further increase in productivity of cassava (Byju et al., 2006, 2008, 2009,
2010a, b). In addition to this, it also results in nutrient imbalance and
losses. The importance of entire P supply before 10 days after planting is
highlighted in many studies (Mohankumar 2000., Nayar et al. 1985) as against
what is observed in FFP.
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Table 3: Effect on site specific nutrient management (SSNM) on
fertilizer P use efficiency of cassava farms in India. |
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Treatment |
D |
P>ITI |
P>IFI |
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SSNM |
FFP |
|
AEP, kg tuber/kgP |
87 |
70 |
17 |
0.022 |
0.003 |
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REP, kg P/kg P |
0.11 |
0.10 |
0.01 |
0.003 |
0.008 |
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PEP, kg tuber/kg P |
279 |
187 |
92 |
0.003 |
0.017 |
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References
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