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Field Specific N Management
The internal efficiencies of nitrogen at maximum accumulation (a) and dilution (d) recommended for use in QUEFTS for cassava are 35 and 80 kg tuberous root yield (dry weight) / kg N removed. When the internal efficiency values of nitrogen were analysed from unfertilized and fertilized plots separately, it could be observed that the values were higher in unfertilized plots and this is a clear indication that nitrogen is a limiting nutrient in those major cassava production regions.

The N uptake requirements at different yield potentials of cassava showed that the relation between yield and N uptake is linear at lower yield targets, indicating that plant growth is limited by N 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 N application will give more profit to farmers than aiming for higher yield targets closer to potential yield.

The N uptake requirement in total plant dry matter for 1000 kg tuberous root in the linear part of the relation was 17.6 kg irrespective of the yield potential. The corresponding IE value for N was 56.2 kg kg-1. It can be observed that both the N uptake and IE values of cassava were similar to the values from the data set used for developing the model. The IE of nitrogen 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 N supply (INS) was calculated from the unfertilized plots. This is based on the assumption that nitrogen is the most important nutrient element that restricts cassava growth. 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.

There were wide variations in the fertilizer nutrient recovery efficiencies of N by cassava. The average value of recovery efficiency of N was 50 per cent. Another major observation was the considerable variation in REn with the amount of fertilizer applied. We developed the relationships between recovery efficiency and amount of N fertilizers applied as
REn = -0.0032N2 + 0.7589N + 3.1533

Tuberous root yield (t/ha dry matter) N uptake
Internal efficiency
Reciprocal internal efficiency
(kg/1000 kg)
2.45 44  56  17.9
4.90 87 56 17.7
9.80 173 56 17.7
14.70 260 57 17.7
19.60 368 53 18.8
21.35 433 49 20.3
21.70 447 49 20.6
22.05 481 46 21.8
22.40 485 47 21.5
22.75 507 45 22.3
23.10 527 44 22.8
23.45 548 43 23.4
23.80 575 41 24.1
24.15 621 39 25.7
24.49  640 34 29.8
Table 1: Nitrogen 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.
Location Regression equations n R2
Kerala INS = 188.84 OC - 6.2265 35 0.78
Tamil Nadu INS = 221.94 OC + 4.8519 23 0.86
Andhra Pradesh INS = 129.11 OC + 54.055 24 0.41
Maharashtra INS = 221.94 OC + 4.8519 10 0.90
Table 2: Indigenous nitrogen supply (kg/ha) from five cassava production regions expressed in soil chemical properties.
Yield target 10 20 30 40
Yield in N omission plot N rate (kg/ha)
10 25 100 § §
20 0 25 100 §
30 0 0 25 150
40 0 0 0 25
Table 3: Rate of application of fertilizer N for specific yield targets based on yield in N omission plots for Kerala and Andhra Pradesh states
     § - not able to achieve the yield target
Yield target 10 20 30 40 50
Yield in N omission plot N rate (kg/ha)
10 35 100 240 § §
20 0 35 100 230 §
30 0 0 35 90 230
40 0 0 0 0 35
Table 4: Rate of application of fertilizer N for specific yield targets based on yield in N omission plots for Tamil Nadu and Maharashtra states
  § - not able to achieve the yield target
Steps involved in determining N fertilizer rate for a particular site
1. Fix the yield target of the particular site.
2. Determine the yield in nitrogen 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  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