But this isnt basic water, this is the soil solution. Soil Chemistry is a whole sub-branch of its own, much like biochemistry. You do not deal with simple, basic solutions of water and one or two chems. A soil chemist I know once told me a story about a conversation he had with a physical chemist. The guy said "we spend all this time coming up with these rules for solutions and you guys go and throw dirt into them..." Soil Chemistry is messy. With soil chem one must deal with something called reserve acidity. H+ ions in soluton do not necessarily come strait from the disassociation of water. Instead they arrive in the soil solution indirectly, with the majority being absorbed to the Cation Exchange surfaces of clay particles. Ca2+ has this little tendency to become absorbed to these surfaces and be taken out of the solution.
That is why Ca(OH)2 makes such an excellent liming material, it will not stay wrapped up with the hydroxide ions, but instead find its way to negatively charged clay surfaces.