What the numbers on a fertiliser bag mean

One corrected abbreviation changed the account of our October banana feeding: DAP became APS. The difference was more than a name. Each product carries a different nutrient analysis, just as the numbers 12:61:0 and 19:19:19 describe different contents.

Reading the bag makes those differences visible before they are lost in a list of fertilisers.

What N, P and K actually mean

The three headline numbers commonly describe percentages by weight of nitrogen (N), phosphate expressed as P₂O₅, and potash expressed as K₂O. The last two are label conventions, rather than percentages of elemental phosphorus and potassium. [1]

A 10 kg bag marked 10:10:10 therefore supplies 1 kg of nitrogen and the equivalent of 1 kg each of P₂O₅ and K₂O. The weight of the product and the weight of its nutrients are different things.

The names that recur in our logs

• Urea is a nitrogen fertiliser. Its nitrogen contribution belongs in the total even when another product in the same programme also supplies nitrogen.
• MOP means muriate of potash, a potassium source. “Potash” in a working note still needs a product label to establish the actual grade.
• SSP means single superphosphate. Alongside phosphate, grades of SSP can supply sulphur; check the declared analysis. [6]
• DAP means diammonium phosphate. The familiar 18:46:0 grade supplies nitrogen and phosphate, with no potash declared in those three numbers.
• APS means ammonium phosphate sulphate. The 20:20:0:13 product supplies nitrogen, phosphate and sulphur. The fourth number is sulphur; it does not turn the zero into potassium.
• 12:61:0 and 19:19:19 are grades, rather than instructions. The former has no potash declared; the latter includes all three headline nutrients. Solubility, intended use and the manufacturer's directions still matter. [1, 3, 4, 5]

A correction worth keeping

Our October banana record first called one component DAP. The corrected entry identifies APS 20:20:0:13. That is a meaningful change: the products have different nutrient analyses. Repeating the earlier abbreviation would change our understanding of the feeding round.

The same caution applies to zinc and sulphur. They appear in farm discussions alongside NPK, but their inclusion should answer a crop or soil need. They are not automatic additions to every feeding.

Reading a blend without mistaking it for a plan

For any nutrient, multiply the product weight by the percentage on its label, then add the contributions from each product. One kilogram of 20:20:0:13 contains 200 g N, 200 g P₂O₅ equivalent and 130 g sulphur. This is label arithmetic, not a suggested application. [5]

A ratio of products is not the same as an NPK ratio. Two mixtures with the same total weight can supply very different nutrients.

The field completes the picture

A suitable soil test, interpreted for the crop and region, helps establish what is already present. Previous fertiliser, manure and other inputs must be counted too. Soil condition and growth stage affect the decision; a pale leaf alone cannot prescribe a bag. [2, 3]

Product, grade and amount establish the nutrient contribution. Area or plant count, date and method show where and how it was supplied. Together they explain far more than the number of bags bought.