Aluminum, when added to soil, lowers the base saturation by replacing basic cations, which leads to soil acidification and a reduced pH.
The student asked which substance, if added to soil, would lower the base saturation. Out of the options given, the correct answer is E) Aluminum. Adding elements such as sodium, potassium, magnesium, or calcium typically increases the base saturation because they are considered basic cations. Base saturation refers to the proportion of soil base cations that are occupied by these basic cations. Conversely, aluminum (Al³+) can lower the base saturation because it's a trivalent cation that is acidic when it replaces basic cations on the cation exchange sites in the soil. This process can lead to soil acidification and a decrease in pH, thus reducing the base saturation.