Fertiliser analysis convention: Potassic fertilisers are graded and labeled on the basis of their equivalent potassium oxide content. Which expression fills the blank correctly?

Introduction / Context:
Fertiliser labels worldwide commonly use oxide-equivalent notation: P as P2O5 and K as K2O. This historical convention persists for consistency across grades and origins, even though plants take up elemental forms (e.g., K+). The question asks which basis is used to grade potassic fertilisers.

Given Data / Assumptions:

• Standard commercial labeling practices are assumed.
• Products may be KCl, K2SO4, or other potassium salts.
• Label lists K content as % K2O equivalent.

Concept / Approach:
Regardless of the chemical form (chloride, sulphate, nitrate), potassium content is converted to the oxide-equivalent K2O for labeling. Thus, “MOP 60% K2O” denotes muriate of potash with about 60% K2O equivalent. The convention allows comparison across products with different anions (Cl−, SO4^2−) by referencing a common basis.

Step-by-Step Solution:

Verification / Alternative check:
Fertiliser standards and trade literature list K grades as % K2O; similarly P grades are listed as % P2O5.

Why Other Options Are Wrong:

• KCl/K2SO4/KNO3: real products but not the unit of grading.
• Elemental K only: not the standard label basis.

Common Pitfalls:
Confusing product identity with label basis; the oxide-equivalent system is a convention, not a chemical requirement.

Final Answer:
K2O