I don't think there's that much salt in the oyster shell since I've used it to raise and buffer the pH in some aquariums. And I put a LOT in those aquariums!
I also have mixed some of my chicken's oyster shell in the tomato garden for added calcium since I don't need it anymore for my ancient hens who are way past egg laying. My soil is already pH of over 8 without anything added and I don't think the oyster shell raises it any higher than that.
Around here I can get all kinds of crushed granite at the landscaping places. They have stuff called breeze or crusher fines which is 1/4 inch down to dust. But I'm near giant granite boulders called the Rocky Mountains so that might make it much more available.
