Ray,

I admit to using limestone as base when I was broke and didn't know any better. The main disadvantage, IMO, is that you get little biological activity from limestone, at least compared to LR. This is primarily due to the lack of porosity in limestone compared to LR. Limestone may have started off as coral, geographically speaking, but over a few million years it has been compacted and lost much of its porosity. Biological activity is pretty much directionally proportional to surface area and porosity is one of the main factor in determining surface area.

Gary