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Calculate K Fertilizer Requirement
The requirement of K fertilizer of cassava is estimated by taking into account the indigenous nutrient supply, target yield and agronomic efficiency of K fertilizer use by cassava.

K fertilizer (kg/ha)   = Target yield (t/ha) – Yield in K omission plot (t/ha)
Agronomic efficiency of K (AEK)
ie. tuberous root yield increase (ton) per kg K fertilizer applied
Indigenous Potassium Supply

The average value of indigenous potassium supply (IKS) 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 IKS ranged from about 98 to 207 kg/ha of K and coefficient of variation (CV) of IKS ranged from 18 to 29 per cent in each year. Among the four crops sampled in the same cassava field, average standard deviation of potassium supply was 15 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 potassium requirement of 5.33 kg/ton tuberous root yield.

Table 1: Variation of the indigenous potassium supply (kg/ha) among cassava farms in India
Year Mean Min Max CV
2003 164.25 115.65 206.93 19.32
2004 169.41 104.63 186.91 24.31
2005 174.37 98.67 188.43 18.41
2006 157.61 111.41 198.43 28.65
Tuberous root yield and K uptake

There was a significant increase in tuberous root yield and potassium uptake in SSNM plots in all the four crops grown during 2003-2006 compared to FFP plots (Table 2). The average yield difference between SSNM and FFP  plots for the four crops grown was 8.72 t/ha (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 K uptake in SSNM  plot compared with FFP treatments. On an average, plant K uptake decreased by 7.87 kg/ha (3.91%, P=0.011). Similar trends in potassium uptake were observed over the years (P>0.05).

Table 2: Effect of site specific nutrient management (SSNM) on root yield, plant potassium accumulation and K fertilizer use in cassava farms in India
Treatment D P > ITI
Tuberous root yield, t/ha 37.35 28.63 8.72 0.005
Plant K uptake, kg/ha 201.01 208.89 -7.88 0.011
K Fertilizer, kg/ha 105 123  -18 0.006
     D-SSNM – FFP
    P > ITI  - probability of a significant mean difference between SSNM and FFP
Fertilizer Use

The average rate of K in FFP plots was 123 kg K2O/ha. Detailed survey indicated that most of the farmers applied fertilizers without taking into account the actual soil fertility status. The K rates were not found to be significantly correlated with IKS (r = 0.34). On an average, 18 kg/ha less fertilizer K was used in SSNM treatments than in FFP (-15%, P=0.006). Lower fertilizer K rate in the SSNM treatment resulted from model based predictions that accurately accounted for the high native soil K status measured as plant K uptake in K omission plots.

Potassium  Use Efficiency
The potassium use efficiency in the SSNM treatment was increased significantly in SSNM treatment where the field and season specific P management was practiced (Table 3). Less fertilizer K was applied in SSNM plots compared with the FFP and there was significant increase in agronomic efficiency (AEK), recovery efficiency (REK) and physiological efficiency (PEK) of potassium. Across the four crops grown, the AEK increased by 41 kg/kg (64%, P=0.000), REK by 0.13 kg/kg (65%, P=0.002) and PEK by 22 kg/kg (68%, P=0.040). The results of the study provide on-farm evidence that the present potassium 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 application of 50 per cent of K fertilizer before 10 days after planting and the rest 45-60 days later is highlighted in many studies (Mohankumar 2000., Nayar et al. 1985) as against what is observed in FFP.

Table 3: Effect of site specific nutrient management (SSNM) on fertilizer K use efficiency of cassava farms in India
Treatment D P>ITI P>IFI
AEP, kg tuber/kgK 115 74 41 0.000 0.030
REP, kg K/kg K 0.37 0.24 0.13 0.002 0.040
PEP, kg tuber/kg K 69 47 22 0.040 0.025
Central Tuber Crops Research Institute
Sreekariyam, Thiruvananthapuram, Kerala - 17