The amount of fertilizer to be applied per hectare on a given field is determined by the following: The amount of nutrients needed by plant for optimum growth and productivity, the availability of nutrient in the soil (level of soil fertility), the moisture status of the soil, the type of crops to be grown and the types and grades of fertilizers available.
Usually mineral fertilizers are delivered in 50-kg bags while the nutrient content (active ingredients) is given in percentages e.g. N 15 P15 K15. Meaning that each 50kg bag contained 15% N 15% P 15% K.
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Steps to Follow in Calculating the Rate of Fertilizer to Be Applied
Determine the quantity of active ingredients contained in each bag, this could be done by dividing the percentage of each nutrient by two. Example, N15 P15 K15 would contain active ingredients N 15/2= 7.5kg P15/2= 7.5kg K 15/2=7.5kg.
If the recommendation is to apply N60-P60-K60 per hecter, the easiest option for the farmer is to buy a multi nutrient (compound) fertiliser grade N15-P15-K15.
One 50-kg bag contains N7.5kg -P7.5kg-K7.5kg of active ingredients.
To find the number of bags required to supply the recommended nutrients:
1 bag = 7.5kg x = 60kg find X
X = 60 divided by 7.5 = 8 bags of NPK.
Example: how many bags of ammonium sulphate (AS) (with 21% N and 24% S) are needed to supply 60 kg/ha of N?
21 divided by 2 gives 10.5. Thus, approximately six bags of AS would supply 60.5 kg of active ingredients of N.
In addition, six bags of AS will supply 72 kg/ha of sulphur.
Thus eight bags of 50 kg of N15-P15-K15 are needed to apply the recommended rate of 60 kg/ha N, 60 kg/ha P2O5 and 60 kg/ha K2O.
When the recommendation per hectare isN60-P30-K30, with 50- kg bags of a N15-P15-K15 grade the farmer would divide 30 by 7.5 = 4. Since each 50kg bag contains 7.5 kg of active ingredients of NPK.
In this case he should apply only four 50-kg bags of NPK fertilizer per hectare, giving half of the recommended rate of nitrogen and the full rate of phosphate and potassium as basal dressing.
The remaining 30 kg/ha N should be applied in the form of a straight nitrogen fertilizer as one or two top-dressings in line with good agricultural practices.
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