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Field Specific K Management
The internal efficiencies of potassium at maximum accumulation (a) and dilution (d) recommended for use in QUEFTS for cassava are 32 and 102 kg tuberous root yield (dry weight) / kg K removed. When the internal efficiency values of potassium 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 potassium is a limiting nutrient in those major cassava production regions in India.

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

Table 1: Potassium 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) K uptake
Internal efficiency
Reciprocal internal efficiency
(kg/1000 kg)
2.45 38 64 15.6
4.90 76 65 15.5
9.80 153 64 15.6
14.70 229 64 15.6
19.60 354 55 18.1
21.35 396 5 18.5
21.70 403 54 18.6
22.05 418 53 18.9
22.40 440 51 19.6
22.75 452 50 19.9
23.10 462 50 20.0
23.45 489 48 20.8
23.80 520 46 21.8
24.15 555 43 23
24.49 470 37 27.2

The K uptake requirement in total plant dry matter for 1000 kg tuberous root in the linear part of the relation was 15.6 kg irrespective of the yield potential. The corresponding IE value for K was 64.6 kg kg-1. It can be observed that both the K uptake and IE values of cassava were similar to the values from the data set used for developing the model. The IE of potassium 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 K supply (IKS) was calculated from the plots that did not receive any K 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 IKS = 0.7398 Exch K - 9.9405 22 0.83
Tamil Nadu IKS = 0.2499 Exch K + 29.051 15 0.81
Andhra Pradesh IKS = 1.0296 Exch K - 41.702 11 0.81
Maharashtra IKS = 0.2499 Exch K + 29.051 10 0.95

There were wide variations in the fertilizer nutrient recovery efficiencies of K by cassava. The average value of recovery efficiency of K was 50 per cent. Another major observation was the considerable variation in REK with the amount of fertilizer applied. We developed the relationships between recovery efficiency and amount of K fertilizers applied as REK = -0.0021K2 + 0.6608K + 0.8699.

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 K2O for specific yield targets based on yield in K omission plots for Kerala and Andhra Pradesh states
Yield target < 10 10-20 20-30 > 30
Yield in K omission plot K2O rate (kg/ha)
<10 40 100 150 §
10-20 0 35 100 150
> 20 0 0 35 100
 § - not able to achieve the yield target
Yield target < 10 10-20 20-30 30-40 > 40
Yield in K omission plot K2O rate (kg/ha)
< 10 35 100 250 § §
10-20 0 35 100 250 §
20-30 0 0 35 100 250
> 30 0 0 0 35 100
 § - not able to achieve the yield target
Steps involved in determining K2O fertilizer rate for a particular site
1. Fix the yield target of the particular site.
2. Determine the yield in potassium 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