Which of the following is a basic cation in soil?

A basic cation in soil refers to positively charged ions that contribute to soil fertility and plant nutrition. These cations include essential nutrients such as calcium, magnesium, and potassium. Among the options provided, calcium ions (Ca²⁺) fit this category, as they play a crucial role in soil chemistry and plant health.

Ca²⁺ is important for cell wall structure in plants and helps maintain soil structure by promoting the aggregation of soil particles, which enhances water retention and aeration. This cation also participates in various physiological processes within plants. Additionally, there is a direct relationship between the availability of calcium and the overall health of the soil ecosystem.

In contrast, the other options listed represent ions that may have different roles in soil chemistry. For example, H⁺ ions contribute to soil acidity, while Al³⁺ and Fe³⁺ can influence soil properties but are not classified as basic cations. Al³⁺, for instance, often contributes to soil acidity and can be toxic to plants in high concentrations, whereas Fe³⁺ is involved in processes such as oxidation-reduction reactions but does not represent a basic cation.