Delta_G:
Yeah there's some measure that gardeners use and call pH that is more of a measure of how much hydroxide salts are present in the soil. I don't understand it very well as I'm not a gardener either. But I am a chemist who has been asked about it a few times and figured out that they have something other than solution concentration of hydronium that they all pH.
pH is a measure of H+ ion, not the hydroxide ion (which is just one of many bases that will
affect the pH by binding H+) Soil chemistry is very complex with a myriad of acid-base reactions,
pH only measures H+, not the other species, so its a part of the story only.
H+ ion is measured because its easiest to measure, the proton is the only ion that can diffuse
easily through a large variety of solids, including the thin glass of a pH electrode, so its concentration
can be measured unaffected by other charged species in the sample.
In water protons hitch a ride on water molecules, the H3O+ ion, but water is a weak base so they
can jump off easily.