Posted: 08/03/2013 at 11:42
My father grows cacti on the roof in Malta, and very successful he is too. The soil must be completely free-draining, and when I mean completely, I mean more like a sieve. I would not think it bad to have 10% compost or soil, mixed with 90% fine grit and sharp sand. In fact the best way he found to propagate cacti was to stand the cuttings in sand that is kept damp, but stood on a saucer full of pebbles, so the water drains out the bottom. Contrary to popular myth, cacti love being watered and fed, in the summer a general feed every week will see them grow quickly, they are greedy sods!! The trick is the substrate they are planted in, if it holds moisture they will rot. The mix I have detailed sounds madness, and when you water them, it is alarming to see how much seems to flow straight through! But remember, the cactus itself holds all the water it needs, provided you have the correct soil mix, overwatering will never be a problem!
Managed to fish out some pictures, you have to love cacti flowers, they are VERY short lived typically less than 24hrs, and most cacti flower at night to be polinated by moths!!