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Understanding Fertiliser

10 July 2024

There are many different terms used in the measuring of fertiliser applications, such as hundredweight/catchweight/bag (cwt) per acre, units per acre or kilograms per hectare. Here we explain the difference and how to calculate what you are putting on.

Handy conversions

1 cwt = 50kg

1 cwt x 2.47 = kg product/ha

Units/acre x 1.25 = kg/ha

Kg/ha x 0.8 = units/acre

The rate of nutrient being applied, and the rate of product are different.

  • The nutrient content is the amount of elemental nutrient (N, P, K,S) required by the crop. When you are given a recommendation for fertiliser this will be the amount of nutrient required.
  • The amount of product is the physical weight of fertiliser applied to deliver the required amount of nutrient.

The numbers on a 600kg fertiliser bag refer primarily to the amount of Nitrogen (N), Phosphate (P), Potassium (K) and Sulphur (S) contained per tonne on a percentage basis.

bag of fertiliser
  • Under the imperial system 1 unit = 1%. A 600kg bag contains 21 % Nitrogen and 60% Sulphur per tonne.
  • If working on a cwt basis each 50kg would also contain 21% Nitrogen and 60% Sulphur.
  • Under the metric system the unit of measurement is kg.
  • To calculate the nutrient in a tonne of fertiliser:
    1000 / 100   x    % nutrient in tonne of fertiliser = kg of nutrient per tonne of product.

For example, the N content in a tonne of fertiliser with analysis 21-0-0+60So3 is:

1000 / 100     x      21 = 210kg of Nitrogen per tonne of product

Or    1000 x 21% = 210kg

Calculating how much product to apply

The following calculations can be used to calculate how much product needs to be applied to deliver the required amount of nutrient required by the crop and soil.

The crop requires 100kg of Nitrogen/ha.  The fertiliser analysis is 21-4-15+7.5so3.

Nutrient required / Nutrient content in 1t fertiliser        x 1000 = kg of product/ha required to deliver 100kg N.

For example:

100 /  210  (21% of 1t)       x 1000 = 476kg/ha product

Calculating how much nutrient has been applied

The following calculations can be used to calculate how much nutrient has been applied from a fertiliser application:

Units per acre

1cwt/acre of 21-0-0+60so3 was applied to grassland for grazing

  • 1cwt is 50kg/acre. Each cwt contains 21 units of N = 21 units/acre


To convert units/acre to kg/ha, multiply by 1.25.

  • 21 x 1.25 = 26kg N applied per ha

Imperial ----> Metric

Kilograms per Ha

123kg/ha of 21-0-0+60so3 was applied to grassland for grazing

  • Application rate (kg/ha) x (% nutrient/100) = kg/nutrient/ha

    123kg/ha x (21/100) = 26 kg N/ha


To convert kg/ha to units/acre, multiply by 0.8.

  •  26kg N/ha x 0.8 = 21 units N/acre

Metric ----> Imperial

You can find some handy calculators on the FAS Companion App, downloadable from the relevant app store. Visit for more info.

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