Isn't Cactus soil high in sand Mike? If you can't buy compost for the organic materials you need here are a few alternatives.
1/4 native clay soil + 1/4 sand + 1/2 potting soil (without moisture crystals)
or
1/4 native clay soil + 1/4 sand + 1/4 potting soil (without moisture crystals) + 1/4 composted manure.
I'd add bark mulch to improve soil aeration, 25% by volume.
