In the described fertilizer, phosphorus compounds make up what percentage?

• A. 15%
• B. 20%
• C. 30%
• D. 25%

Answer: (C)

Phosphorus content in fertilizers is given as a percentage by weight, and phosphorus is usually labeled as P2O5 (phosphorus pentoxide) on the N-P-K rating. When the description states the phosphorus compounds make up a certain percentage, that number is the amount of P2O5-equivalent in the fertilizer. In this case, phosphorus compounds account for 30% of the mixture, so the phosphorus portion is 30% by weight.

If you ever need to think in terms of elemental phosphorus instead of P2O5, you can convert using molecular weights: P2O5 contains 62 g of elemental P per 142 g of P2O5, about 43.7% P by mass. So 30% P2O5 corresponds to roughly 13% elemental P.