Phosphate fertilizer grading convention:\nCommercial phosphatic fertilizers are graded on the basis of their ________ content.

Introduction / Context:
Fertilizer labeling uses standard oxide conventions—N-P2O5-K2O—to report guaranteed analyses. Even though plants absorb elemental nutrients (e.g., P as orthophosphate), industry practice expresses phosphorus in terms of P2O5 equivalent for historical and regulatory reasons. Understanding this convention is fundamental in interpreting product specs and comparing fertilizers.

Given Data / Assumptions:

• The question concerns grading/labeling, not plant uptake chemistry.
• We follow standard commercial practice and legal definitions.
• We must choose the correct oxide equivalent used globally.

Concept / Approach:
Phosphatic fertilizers (e.g., single superphosphate, triple superphosphate, DAP, MAP) declare phosphorus as percent P2O5 equivalent on the bag. This does not mean the product literally contains P2O5; it is a conventional expression allowing uniform comparison across different chemical forms. Thus, “P2O5” is the correct basis for grading.

Step-by-Step Solution:
Identify the labeling system: N-P2O5-K2O convention.Match phosphorus term to P2O5 equivalent.Select “P2O5” as the correct answer.

Verification / Alternative check:
Regulatory documents and fertilizer handbooks consistently define phosphorus labeling in P2O5 terms for guaranteed analyses.

Why Other Options Are Wrong:
P2O3 and PCl5 are not used for fertilizer grade; H3PO4 is an acid feedstock, not the labeling basis; “PO4^3- expressed as elemental P only” does not follow the oxide convention.

Common Pitfalls:
Confusing agronomic phosphorus (elemental P basis) with labeling conventions; conversion factors exist to translate between P and P2O5 when needed.

Final Answer:
P2O5