Final answer:
The equation for the percentage of P2O5 in fertilizer involves interpreting the middle number of the NPK rating on the fertilizer package, which represents the percent of phosphorus pentoxide equivalent in the product.
Step-by-step explanation:
The equation for the percentage of P2O5 in fertilizer involves understanding the NPK rating of a fertilizer. The NPK rating stands for nitrogen (N), phosphorus (P), and potassium (K), with the percentages of each represented by three numbers in the order of N-P-K on the fertilizer package. The middle number represents the percentage of phosphorus, specifically the equivalent amount in phosphorus pentoxide (P2O5).
To calculate the percentage of P2O5 in a fertilizer, look at the second number in the NPK rating. For example, for a 16-4-8 fertilizer, the 4 represents 4% P2O5. For a 50-pound bag of this fertilizer, this indicates that there are 2 pounds (or 4% of 50 pounds) of P2O5. If you wish to find out the actual amount of elemental phosphorus, you would need to convert from P2O5 to phosphorus (P) using the appropriate molar masses and ratios from their molecular formulas. Phosphate fertilizers often include a mixture of different forms of phosphorus, including polyphosphate and orthophosphate.