Which cations are considered exchangeable bases on soil cation exchange sites?

• A. Hydrogen, magnesium, potassium, sodium.
• B. Calcium, magnesium, potassium, sodium.
• C. Calcium, manganese, copper, zinc.
• D. Nitrogen, phosphorus, sulfur, boron.

Answer: (B)

Exchangeable bases are the basic cations that occupy soil cation exchange sites and help neutralize acidity, primarily coming from minerals in the soil. The usual four are calcium, magnesium, potassium, and sodium. These ions readily adsorb to negative sites on clay and organic matter and can be swapped with other cations in the soil solution, influencing base saturation and overall soil fertility. Hydrogen and aluminum also occupy exchange sites, but they are associated with acidity (exchangeable acidity) rather than bases, so they aren’t counted among exchangeable bases. The other options include manganese, copper, and zinc, which are micronutrients but not the typical exchangeable bases used to describe base saturation, and nitrogen, phosphorus, sulfur, and boron, which are not exchangeable cations.