Why is potassium content on fertilizer labels expressed as potassium oxide equivalent?

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A periodic table element box for potassium shows atomic number 19, symbol K, atomic weight 39.0983, electron configuration [Ar]4s1, and name potassium. Icons indicate it is an alkali metal, solid at room temperature, body-centered cubic crystal structure, and has strongly basic higher-valence oxides. A key explains the symbols for alkali metals, solid state, body-centered cubic structure, and strong basicity. — potassium Properties of potassium.

In the context of fertilizer labeling, potassium content is expressed as an equivalent amount of potassium oxide ( $K_{2} O$ ) primarily to provide a standardized metric for comparison. Because potash deposits vary significantly in their actual potassium concentration, using $K_{2} O$ as a common denominator makes it easier to evaluate and compare the nutrient value of different sources and products.1

It is important to note that while this term appears on commercial packaging, the fertilizer itself does not actually contain potassium in the form of $K_{2} O$ . In reality, the potassium in fertilizers is most commonly present as potassium chloride ( $K C l$ ).12