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How to conduct nutrient omission plot trial?

The indigenous nutrient supplying capacity of a particular field/region can be estimated by conducting very simple nutrient omission plot trials that can be conducted by farmers themselves in their fields or at a community level by extension agencies. Use good quality planting materials and follow recommended crop management practices as per the recommendations of CTCRI and conduct a simple trial as shown in figure below.
                                                                                Farmers Field
NPK Plot

6 x 6 plants
N omission plot

6 x 6 plants
P omission plot

6 x 6 plants
K omission plot

6 x 6 plants

Figure. Design of a nutrient omission plot trial in a farmer’s field.

It is better to select site specific recommendation domain based on soil homogeneity and in each domain select 10 cassava fields and conduct the trials in an area occupying 144 plants as shown above. Divide the experimental area into four equal parts each with 36 plants and give a bund of 30 cm width and height around each plot. This is done to avoid movement of nutrient between the plots.

In N omission plot, apply P and K fertilizers and do not apply any N fertilizer. Similarly, in P omission plot, apply N and K fertilizers and do not apply any P fertilizers. In K omission plot, apply N and P fertilizers and do not apply any K fertilizer. The NPK plot is far the estimation of target yield and here apply N, P and K fertilizers in sufficiently large amount to get the target yield of the region. Apply micronutrient fertilizers also in all four plots, in areas where there are any micronutrient problems.

Estimate the yield of 16 plants from each plot after avoiding the 20 border plants as indicated in figure below. Cut all the tuberous roots from 16 plants and take the weight in kg. Multiply the weight in kg by 0.7716 and record the value as tuberous root yield in tons/ha. Based on the average yield in nutrient omission plots and depending on target yield, the NPK fertilizer rates for the field/location can be estimated using modified QUEFTS model.

Figure: An individual plot of nutrient omission trial.
Yield is measured from the inner plants shown in green colour
Central Tuber Crops Research Institute
Sreekariyam, Thiruvananthapuram, Kerala - 17