We have plenty of proof (insofar as anything is ever proved in science) of the age of the Earth.
If we don't know all the significant processes of salt addition and removal and their rates, now and in the past, then there's no way that calculating an age that way can be meaningful. It's a non-issue. The whole argument is unsupportable. If Humphreys' had actually come up with something supportable, it would be publishable. As it is he's just repeating senseless bafflegab.
Also, I find it amusing how creationists become the ultimate uniformitarians when it suits them. The list of things that Humphreys assumes never change would be a long one.
(Radiometric dating is OT here, but just to forestall; there are no similarly unsupported assumptions underlying radiometric dating, just premises based on multiple iindependent lines of evidence and incredibly well-established and tested theories.)