Avoid the bark and the pre-packaged soil mixes. The soil mixes are best for containers and even then you need to alter it with some other material. I suggest trying to find a landscape materials yard that sells bulk quantities of decomposed granite (and other stuff). Here in AZ they are all over the place. Ive had great success mixing in some 1/4" minus granite with my native soil for raised succulent beds. 1/4 minus has enough gritty and finer particles to provide good drainage. You can have a few tons of this delivered to you much cheaper than anything you can get at HD or Lowes. You can also get a nice top-dressing material fron them. If you cant find one in the phone book, maybe call up some landscaping contractors and ask them where they get their non-plant materials. Good luck!

I dont know a great deal about terrariums but I know a little about cacti. Cacti are arid plants, where they live there is very little humidity. Although most varieties you find at your big box stores are fairly adaptable. With that being said, dont get discouraged if a few die from rot. Again I dont know much about the needs of terrariums soils but cacti need loose soil. Perlite and sand are great for drainage as long as they are the right size grainules. You want to avoid anything smaller then an 1/8 inch and no larger then 1/4 inch. Silica sand and crushed perlite will compact very tightly and hender drainage and root growth. I use a mixture of perlite, peat and turkey grit. Turkey grit can be found anywhere animal feed is available.I generally make a mix of equal parts. Remember, a cactus is more likely to be killed by the abundance of water then the lack of it. In the picture, it appears that they used white aquarium gravel and a fine white sand, probably silica. These are tops dressings to make the terrarium look snazzier not actual components of the soil mix. I hope this is useful to you in some way or another. Keep us posted on your progress with pics! plant_junkie

Hm. It's probably the wave of new growers, too, Albert. I can't imagine anyone growing succulents in gritty mix with pumice and/or turface going back to using perlite unless they have no choice. Even as an amendment to potting soil, perlite is usually something I'd tolerate but not prefer. When newbies come by asking about plants, for example, we generally just say mix perlite with potting soil because it's the easiest thing to get and to do especially if they only have one pot of random echeveria. Although, I'd say that for moisture control, I'd rather use a dried-up, hydrophobic block of solidified peat than potting soil with tons of perlite. Once peat has become hydrophobic, it will not suck up water unless soaked for hours. So, just don't soak it. I've kept a pot of mammillaria alive this way for over a year (looking, buying and moving house at the time). But, kids, this is risky and repotting is tricky so don't do it at home!

@meanom most people here mix gritty mix as equal parts of: * granite stone that is 1/4" to 3/8" (no fines or sand) * turface MVP that is filtered, and many people here "cheat" and filter out only particles smaller than 1/16" * bark filtered 1/4" to 3/8". Fortunately, I found a local supplier that prefilters to 1/4" I would recommend using some root hormone to get the roots growing into that medium. Then fertilize maybe starting two weeks later.