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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
Internal efficiency
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.
Central Tuber Crops Research Institute
Sreekariyam, Thiruvananthapuram, Kerala - 17