Differentiate exchangeable bases and base saturation and their role in interpreting soil fertility.

• A. Exchangeable bases are soil organic acids; base saturation is the pH of the soil.
• B. Exchangeable bases are trace minerals; base saturation is the total nutrient content.
• C. Exchangeable bases are water molecules; base saturation is the soil's capillary rise.
• D. Exchangeable bases are Ca, Mg, K, Na on cation exchange sites; base saturation is the proportion of all exchange sites occupied by these bases, indicating fertility status.

Answer: (D)

Exchangeable bases are the Ca, Mg, K, and Na ions held on cation exchange sites on clay minerals and organic matter, the ones that can be swapped with other cations in the soil solution. Base saturation is the percentage of all exchange sites that these bases occupy, calculated as the sum of Ca, Mg, K, and Na on the exchange complex divided by the soil’s total cation exchange capacity, times 100. This concept matters for interpreting soil fertility because a higher base saturation generally indicates more plant-available base cations and less acidity on the exchange complex, implying better nutrient supply and fertility. In acidic soils, H and Al occupy many exchange sites, lowering base saturation and often limiting nutrient availability, whereas liming or management that increases Ca and Mg raises base saturation and improves fertility. The other descriptions mix up what bases are or what base saturation measures, such as equating bases with organic acids or pH, or treating base saturation as total nutrient content or a physical property, which don’t reflect the exchange-site balance that governs nutrient availability.