Explanation

Soil anion exchange capacity generally increases as pH decreases because the concentration of hydrogen ions (H+) increases, which can displace other cations but also affects the surface charge and availability of binding sites for anions in many soil contexts, particularly in variable charge soils where lower pH reduces negative charge. However, for fixed charge soils, it might not change much. Wait, let's re-evaluate. Anion retention is often associated with positive charges on soil particles (like Fe/Al oxides). As pH decreases, these surfaces become more positively charged, increasing anion adsorption/retention. Therefore, a decrease in soil pH increases the capacity to retain anions.