A fertilizer grade consists of three numbers, for example “10-3-10,” sometimes called its N-P-K.
A 20-14-18 fertilizer and a 10-7-9 fertilizer have the same fertilizer ratios; other things being equal, they would have the same effect, provided twice as much of the 10-7-9 were applied. One advantage of the higher analysis 20-14-18 fertilizer is the saving in transportation costs.
Sometimes additional terms are added to the N-P-K string, especially for fertilizers made for soils with specific deficiencies. These additional terms contain the symbol for one of the chemical elements and a number that is the percentage by weight of that element. For example, a fertilizer marked “20-20-20-10S” would be 10% elemental sulfur by weight.
Fertilizer grades don't capture all the properties desirable in a soil amendment. Compost, for example, has a very low N-P-K grade.
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Last revised: 28 December 2007.