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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.

P fertilizer (kg/ha) = Target yield (t/ha) – Yield in P omission plot (t/ha)
Agronomic efficiency of P (AEP)
ie. tuberous root yield increase (ton) per kg P fertilizer applied
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.

Table 1: Variation of the indigenous phosphorus supply (kg/ha) among cassava farms in India
Year Mean Min Max CV
2003 13.35 7.89 18.95 28.82
2004 13.61 8.45 16.76 32.12
2005 12.91 7.41 17.63 26.65
2006 15.41 9.63 18.47 30.45
 
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.
Treatment D P > ITI
SSNM FFP
Tuberous root yield, t/ha 37.35 28.63 8.72 0.005
Plant P uptake, kg/ha 23.79 18.62 5.17 0.002
P Fertilizer, kg/ha 91 51 40 0.004
     D-SSNM – FFP
    P > ITI  - probability of a significant mean difference between SSNM and FFP
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
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.

Table 3: Effect on site specific nutrient management (SSNM) on fertilizer P use efficiency of cassava farms in India.
Treatment D P>ITI P>IFI
SSNM FFP
AEP, kg tuber/kgP 87 70 17 0.022 0.003
REP, kg P/kg P 0.11 0.10 0.01 0.003 0.008
PEP, kg tuber/kg P 279 187 92 0.003 0.017
References
Central Tuber Crops Research Institute
Sreekariyam, Thiruvananthapuram, Kerala - 17