But gypsum is mainly Calcium Sulfate; that would tend to lower the pH, but apparently not enough to be a risk for Marts, in your case Moby because of all that crushed gravel base to your driveway. I guess I'm assuming you don't have crushed granite as a base.
One question I have (for Rick) is: If there is so little soluable Iron at pH levels above 7, then how is it that Martagons can be so green?
