Base-forming cations
Hi, I have a question about base-forming cations: In the SoilWeb200 (http://soilweb200.landfood.ubc.ca/interactions-among-soil-components/3-adsorption-of-ions/), it says "Ca2+, Mg2+, K+, NH4+, and Na+ are called base cations or base-forming cations", and "in soil, their salts undergo hydrolysis, which increases alkalinity by the release of OH- ions into soil solution." On the other hand, in the textbook (P273, Brady & Weil) it says "...these cations are non-hydrolyzing", "...they generally do not produce OH- ions either" and "rather, their effect in water is neutral, and soils dominated by them have a pH no higher than 7 unless certain anions are present in the soil solution." It seems these statements are contradicting each other, or am I missing something? Could you please clarify this? Thank you!
You missed couple of important parts of the text explanation. The text reads as follows:
"These cations [Ca2+, Mg2+, K+, and Na+] are nonhydrolizing and so do not produce acid (H+) upon reacting with water as Al3+ or Fe3+ do." This means that so called base-forming cations will not produce H+ and as a result of hydrolysis that in turn leads to acidification of the soil.
The textbook explanation then continues: "They [base-forming cations], generally do not produce OH- ions either . Rather, their effect in water is neutral and soils dominated by them have a pH about 7 unless certain anions are present in soil solution. The basic hydroxyl (OH-)-generating anions are particularly carbonate (CO32-) and bicarbonate (HCO3-). These anions originate from the dissociation of such minerals as calcite or from dissociation of carbonic acid."
This second part of textbook explanation is along the lines we provided in the lectures, that salts of base-forming cations (not these cations on their own) are contributing to the increasing concentration of OH- ion and consequent increase of soil pH.