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Home / Crop

Fertilizers: How to Calculate fertilizer amounts to use in maize

Maize is an integral component in the diet of a majority of Kenyans. This crop is usually under various threats like pests, currently fall armyworm and desert locusts, diseases and many diseases. To be able to withstand such attacks, a healthy crop alongside other control methods are important.

Too often farmers apply inorganic fertilizers based on guesswork and estimates. Let’s explore the mathematics behind fertilizer quantities based on C.A.N and urea fertilizers, the most common topdressing fertilizers for maize. It is important that a farmer understands the right amount as it saves on operational costs.


Scientists recommend 160kg of N (Nitrogen) for a good harvest, usually 30 bags from a single acre. To be able to harvest 30 bags from an acre with Urea as the topdressing fertilizer, a farmer therefore needs to apply around 350 kgs of urea fertilizer. The reason for 350 kgs of urea is, for every 100 kilos of urea you apply, the plant benefits with 46 kgs of Nitrogen therefore to meet the 160 kg demand for Nitrogen by maize a farmer needs to apply 350 kgs of Urea.

The math behind this is:

100 kg of urea = 46 kg of Nitrogen, therefore, how many kgs of Urea will supply 160 kgs of Nitrogen? 350 kgs of urea is required to supply an acre of maize with the needed 160 kgs of Nitrogen.


C.A.N stands for Calcium Ammonium Nitrate, a compound fertilizer. Compound fertilizers are fertilizers that have more than one type of element i.e. nutrient. As indicated by the name C.A.N contains calcium and 24 - 28% nitrogen (Nitrate) depending on the manufacturer.

If a farmer plans to use 26% C.A.N for topdressing in two splits assuming that the soil has zero nutrients, for one acre 615 kgs of C.A.N is required.

The 26% means that for every 100 kilos of C.A.N 26% a farmer applies to the maize crop; 26 kilos of nitrogen are made available in the soil.

How much does a farmer spend for either fertilizer?


To supply the crop with the much needed nitrogen using Urea, a farmer needs 350 kilos of the fertilizer. A 50 kg bag of urea retails at a recommended retail price (rrp) of Ksh 3200. To get 350 kgs a farmer needs to buy seven 50 kgs bags and spend Ksh 22,400 to purchase the required nitrogen amounts for a 30 bag harvest from an acre.


For C.A.N to supply 160 kilos of nitrogen to the maize crop, a farmer needs 615 kgs of the fertilizer which retails at Ksh 2200 per 50 kg bag. Therefore, a farmer needs around 12.3 bags of C.A.N. To total amount a farmer spends would then be Ksh 27,060 to be able to supply the right amounts of nitrogen for a harvest of 30 bags an acre.

With the above, farmers can make the right decision on which fertilizer to use as a top-dress.

It is important that farmers remember that timing of the application is very important. It is therefore recommended that farmers split the application at least twice: at knee height (The top of the maize reaching the knees of an adult) and when the maize tassles (male flower at the top forms).

Keep visiting farmers.co.ke for the next story on how characteristics of urea and C.A.N fertilizers affect the nutrition of a maize crop.

Author; Njoki Thuo – Mwakughu (BSc Agriculture, Egerton, MSc AICM, UoN)

Email; [email protected]

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