The three numbers on a bag of fertilizer indicate the respective percentages of nitrogen (N), phosphorous (P) and potassium (K) it contains. Bags sometimes also show the percentages of calcium (Ca), magnesium (Mg), sulphur (S) and some micronutrients.
For example: Fertilizer marked 15-15-30 contains 15% nitrogen, 15% phosphorous and 30% potassium.
To convert these percentages into kg or lb, just multiply them by the weight of the bag.
For example: A 30-kg bag of 15-15-30 contains:
|Nitrogen||15% of 30 kg||= 4.5 kg|
|Phosphorous||15% of 30 kg||= 4.5 kg|
|Potassium||30% of 30 kg||= 9 kg|
|Total||60% of 30 kg||= 18 kg|
The rest of the bag (12 kg or 40%) may contain secondary elements, micronutrients, impurities and fill.