P2O5equivalent

P2O5equivalent is a conventional way of expressing the phosphorus content of fertilizers and phosphate-containing materials as the amount of phosphorus pentoxide (P2O5) that would supply the same quantity of phosphorus. The term is widely used in fertilizer trade and labeling to provide a uniform basis for comparing different phosphate sources, even when the materials have different chemical forms.

The relationship between elemental phosphorus (P) and P2O5 is fixed by their chemical composition. P2O5 has a molecular weight of about 141.94 g/mol, with two phosphorus atoms contributing roughly 61.95 g of P. Therefore, the mass fraction of P in P2O5 is about 0.4364. Consequently, P content (as a percentage) equals 0.4364 times the P2O5 percentage, and the P2O5equivalent of a given phosphorus percentage is about 2.291 times the P percentage. For example, a fertilizer with 16% P2O5 contains about 6.98% elemental P, and a product with 3% elemental P has a P2O5equivalent of about 6.87%.

Usage and caveats: P2O5equivalent expresses total phosphorus content, not necessarily plant-available phosphorus. Some phosphate sources, such as rock phosphate or poorly soluble phosphates, may have lower nutritional availability than their labeled P2O5equivalent suggests. In some regions, labeling trends are shifting toward expressing phosphorus as elemental P rather than P2O5, but P2O5equivalent remains common for comparison across diverse phosphate materials. When interpreting labels, it is important to distinguish between total phosphorus content and the portion that is readily available to plants, which can depend on solubility and soil conditions.