Which of the following ions contributes to soil acidity?

The presence of hydrogen ions (H+) is the primary factor contributing to soil acidity. Soil pH is a measure of the concentration of hydrogen ions; as the concentration of H+ increases, the pH decreases, leading to more acidic conditions. Acidity in soil can significantly impact nutrient availability, microbial activity, and plant growth.

In contrast, the other cations listed—calcium (Ca2+), potassium (K+), and sodium (Na+)—do not contribute to soil acidity. Instead, calcium is often associated with alkalinity and can help mitigate acidity when applied as lime, while potassium and sodium also do not add to the acidic character of the soil. Thus, hydrogen ions play a crucial role in defining the acidic nature of soils, making this the correct choice.