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Plant Structure, Function, and Use

Plants and Soils

What do the numbers on a bag of fertilizer indicate?

The three numbers on a fertilizer bag, such as 15-20-15, refer to the percentages by weight of macronutrients. The first number stands for nitrogen, the second for phosphorus, and the third for potassium. In order to determine the actual amount of each element in the fertilizer, multiply the percentage by the fertilizer’s total weight in pounds. For example, in a 50-pound bag of 15-20-15, 7.5 pounds is nitrogen, 10 pounds is phosphorus, and 7.5 pounds is potassium. The remaining pounds are filler.



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