Soil metals acids and bases

Question

I have read in environmental science textbooks that aluminum can acidify soil, whereas metals such as sodium, calcium, magnesium, and potassium can alkalinize soils. I understand that Al³⁺ can hydrolyze water to form 3H⁺ and lower pH.

Why are alkali and alkaline earth metals described as alkalinizing soils? I understand that the neutral elemental forms of these metals (alkali, alkaline earth) react with water to form OH^-, but the prevailing cation-form is much less reactive, except that it might precipitate with OH^- from water self-ionization, and shift the equilibrium towards more H⁺.

I also know that some of these cations are commonly found in minerals with carbonate and silicate that can lead to OH^-, but this seems to driven by the counter-ion, not the metal cation.

Answer 1

Most alkali and alkaline earth metals, especially the most abundant ones calcium, magnesium, potassium and sodium (list is alphabetical) exist in silicates. Silicic acid/hydrated silica is very weakly acidic whereas the alkali and alkaline earth hydroxides are moderately strong or fully strong bases; so the silicates should be expected to give basic products when released into water solution. If carbon dioxide is serving as the chemical erosion agent, the metals described here will turn it into soluble bicarbonate ions, which are basic in water. The reaction below with orthoclase feldspar is given as an example (based on combining chemical reactions given in https://www2.tulane.edu/~sanelson/eens211/weathering&clayminerals.htm):

$\ce{\underset{orthoclase}{4KAlSi3O8} +4CO2 +6H2O-> 4 K^+ +4\underset{basic}{HCO3^-} +\underset{kaolinite clay}{Al4Si4O_{10}(OH)8} +8\underset{quartz}{SiO2}}$