Which of the following plant nutrients is a cation in soils?

In soil chemistry, cations are positively charged ions that play crucial roles in plant nutrition and soil fertility. Potassium is a well-known macronutrient that exists as a cation in soil environments, specifically as K+. This form allows it to be readily available for absorption by plant roots. Potassium is vital for various plant functions, including photosynthesis, enzyme activation, and regulation of stomatal opening, which impacts water usage efficiency.

Sodium also exists as a cation and can be found in soil. However, in the context of primary plant nutrients, potassium is more significant as it is essential for plant growth while sodium is only beneficial in small amounts and often associated with soil salinity issues. Nitrogen typically exists in forms such as nitrate (NO3-) and ammonium (NH4+), with the latter being a cation, but it is not generally highlighted as a primary nutrient cation compared to potassium. Phosphate (PO4^3−) is an anion, not a cation.

Therefore, when identifying cations in soils that are essential for nutrient availability, potassium stands out as the primary answer.