Putting the right amount of fertiliser in the ground is good stewardship of money and the environment. Using too much will cost you more than you need to spend, and may wash away to pollute areas that don't need any fertiliser. Fertilisers are made up of a variety of substances, and calculating just how much of that is the actual fertiliser versus other material is important. For example, if you want to use 2.27kg. of nitrogen on an acre field and you are using ammonium sulphate at 21 per cent nitrogen you would use this calculation.
Divide the total number of pounds of nitrogen you need by the percentage of nitrogen in your fertiliser. In this example, this is 5 divided by .21.
Calculate the answer at 10.8kg. of ammonium sulphate to reach 2.27kg. of nitrogen.
Change the total number of pounds of fertiliser and the percentage found in the product according to your needs.