What is the chemical form of potassium used in fertilizers?

The correct answer is potassium oxide, represented as K2O. This form is commonly used in fertilizers to denote the potassium content.

In agronomy, potassium is an essential nutrient for plant growth and development, and it is often expressed in terms of its oxide form because K2O is a standardized way to represent the potassium content in fertilizers. When analyzing fertilizers, potassium content is typically converted to K2O to standardize the reporting, making it easier for farmers and agronomists to understand and compare nutrient levels across different products.

While other options like KCl (potassium chloride) and KNO3 (potassium nitrate) are also sources of potassium, they refer to different chemical compounds with additional elements and properties. K2SO4 (potassium sulfate) is another potassium source, but again while it contains potassium, it doesn't specifically represent the elemental form of potassium in fertilizers as K2O does.