Granite chips or granite grit would be a good option to fill space and increase drainage. But it is very inert and will not have much influence on the chemical composition of the mix, if that is what you were wondering. In nature, over very very long periods, granite can weathered and break down into small particles, resulting in acidic soil. But this occurs too slowly to have an effect on your potting mix, and would not be much different than quartz or another inert stone (but limestone is a different story because it can dissolve more rapidly).

The fact that H. morelianum grows in granitic soils, however, is a good clue that it probably prefers a rather acidic substrate.

Bill

Home Depot sell decomposed granite. I use it in my soil mix.

Maids

Bill and Ray,

That was very interesting and useful information. Thank you!

Thanks Bill and Ray, I don't know in what kind of rock ledges that H. morelianum grows, just that it grows in rocky areas and on ledges. It was recommended to use horticultural pumice in the mix but it is hard to come by locally. I ordered a 5lb bag online but that didn't go very far. I was wondering if I could use granite chips to replace some of the horticultural pumice in a soil that needs greater aeration and drainage as granite is porous which is what I was looking for. From Bill I learned that the granite breaks down slowly which might be equivalent to a neutral ph which is what the pumice has and from Ray I learned where to buy decomposed granite - so thank you both!. - J