The Best Cactus Soil

     There are a lot of misconceptions about cactus soil..Some people have the idea that cactus will thrive in pure sand with no moisture; this is NOT the case. First lets go over the condition of the soil (moisture), because even with all the best ingredients, if you don't get your watering right you could end up with a dead or dying plant.
     Most cactus have very sensitive roots, which are highly developed to take advantage of any and all moisture they can get. These fine feeder roots can, and often do, die back when the soil is allowed to stay dry too long.
     The problem here is, once you do finally water your cactus again, the plants ability to uptake moisture and nutrients is diminished (because you killed its fine root hairs) and now the plant will be sitting in wet soil for an unusually long period of time while it develops more feeder roots, and this can open the door for rot.
     The key is to keep the soil from drying out completely. I like to dig my finger down in the soil a bit and first gauge its appearance (is it still somewhat dark?(you should take note of the appearance of your soil mix when totally dry and when fully moistened)), and then determine what the soil feels like (does it still feel slightly damp?). I may also hold my finger at an angle to a light source and see the level of moisture present on my finger tip as a further verification. It is important to note this test is performed on the sub-surface soil; the surface may appear dry, but you want to know what is going on in the root zone.
     I know when it is time to water when:

1. The sub-surface soil is somewhat dark (on a 1-10 scale, 1 being totally dry in appearance, and 10 being fully moistened in appearance, from 3-4 is a good range to start with).
2. The sub-surface soil feels SLIGHTLY damp ( if you were to construct a scale similar to the one above, but swap out appearance with feeling, the same range would probably serve you well).

     Now that we have that out of the way, lets get into the components of the soil. Keep in mind that this recipe probably isn't optimal for all species, but it is a good start in my opinion. This mix works very well for my Pereskiopsis, Zygocactus, and my columnar cacti such as Trichocereus and Myrtillocactus. There are some species of cacti accustomed to growing in limestone or gypsum veins, so the native habitat of the species should be mimicked to a degree.. Something to keep in mind though, is that although a species may be designed to tolerate such austere conditions, doesn't mean it cant utilize a bit more in the way of nutrients and moisture.
     Anyway, the mix I use is somewhat simple by most standards I suppose, but it works for me. It is composed of (by volume) 3/4 coarse orchid bark mix, and roughly 1/4 all purpose potting soil. There are some specifications however. The potting soil, for one should have a fair amount of perlite, some compost content, and preferably a mild time release fertilizer. You can experiment with different brands, but I prefer Black Gold. Some peat is alright, but there are some brands of potting soil that seem to use peat as their base, and you should avoid these types of mixes; it is good for storing nutrients, but it holds too much water if it is used in excess, and if you let it dry out on accident it is difficult to re-moisten. It is very important that you get the orchid bark right as well. There are many brands that market their orchid bark and "coarse", but few that actually are. Here are two examples of orchid bark, both advertised as coarse, just look at the difference:

 

 

     How is that for a size difference. The one on the bottom is the one you want. I unfortunately bought a lot of the one on the top because it was a good price, and I couldn't see into the bag, so I wasn't aware of what I was getting. The big chunks you see in the bottom example are ideal, as they allow very easy air flow through the soil, and cactus roots love fresh air. The orchid mix on top surprising compacts quite a bit, which in addition to the lack of fresh air to the roots, also causes the soil to retain more water (further impeding airflow)...You get what you pay for I guess...

 

     There you have it, a short primer on cactus soil and optimal moisture content..Far from the definitive work on the subject but this should help some of you guys who are just getting started, and maybe some veterans, who knows? ;)

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