We mix our own cactus soil, designed for everyone who lives other than in the desert, so there’s no sand in it at all, which is a good ingredient if you live in the desert, but we don’t. One of our regulars asked a question about the ingredients in our mix.
I was looking at your pre-mixed soil for sale (the one that’s recommended for a very fast draining soil for cacti); I noticed that there is a percentage of COIR in the mix. Doesn’t that retain moisture, thus, creating a less desirable draining vehicle. Maybe it has some other attributes that justifies it’s addition to the mix…
Now, I would answer this differently than Hap, who does a good job of just getting down to basics. I would have said something along the lines of Yes, it does hold some moisture – all plants need some moisture… But that would have been rude of me, so it’s a good thing Hap answered instead.
Our soil mix is mostly lava and pumice, the organic materials are coir and composted rice-hulls. Both of them are nice and rot resistant, both being the seed-husks of water transported seeds, means they are filled with natural anti-fungal properties, which leads to long term soil stability and healthy plants. Coir and rice-hulls can last eight to ten years in potting soil. Commonly used peat only lasts about two to three years, and has a host of other drawbacks as well…. The coir and rice-hulls are both “long fiber”, so they do hold moisture, but not too much. They also “bond” nutrients well, so the slow release complex organic fertilizer we add has a good life span in the soil mix.