The question is about the size, ratio and composition of the drainage and soil layers in some improvised terrarium. It's made out of a 8'' (20cm) truncated/shaved off glass sphere (basically a cookie/candy jar); accessories include some colorful fish tank gravel, activated coal, a nylon bag, some potting and dark soil, as well as a couple of small random plants...

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I read that:

  • Maintaining the soil moisture is difficult even with an adequate drainage layer.
  • It's theoretically possible to restrict root growth; a smaller area to grow roots in means less soil to grow in I would imagine.
  • How deep the root zone is varies from plant to plant.
  • Coal is only a must in you have a closed up terrarium.

I've seen different types of layering, proportions, even mixing the coal with the gravel or even with the soil in some instances... is it just about aesthetics?

  • Is there an ideal drainage to soil layer ratio in a terrarium; is it 1:1?
    • How would you setup the layers here, in particular the soil layer, so as to restrict root growth and ultimately constrain growth to the jar; 1 inch per?
    • In terms of composition, how much coal does one use and is it layered above or below the gravel, or would you mix it in any other way; does it matter where it is and can it be detrimental in some cases?
    • Do you see any issues with having the soil layer made out of different types of mix (such as potting and this dark mix) side by side?
  • Do you intend to build a closed or an open terrarium? I ask this because I have noticed that open terrariums go well even in the absence of coal, the drainage being made only from gravel, about an inch of it under the soil. I don't have experience with closed terrariums, so I don't know how much gravel and coal you need.
    – Alina
    Commented Aug 18, 2017 at 14:46
  • @Alina I made two experiments with a 8'' and 6'' glass jar. I wanted to do closed ones because I thought a lot of humidity would accelerate growth and I thought that would help an aloes for instance (total newb with all green stuff). In the smaller one I put some pieces of rock and moss only. In the bigger one I put a small plant with green leaves that are wine red underneath and the aloe and a clover. I closed both but it would fog. One of the moss element went rotten and the moss never seemed really happy with either open/close. The plant with red leaves grows too much so that one is open no
    – user17747
    Commented Aug 18, 2017 at 20:39
  • @Alina Thanks for your comment. I might write all that in some tentative answer based on my very little experience with that. It's a trial run. I ended up having a low space to soil ratio inside the spheres and I don't have the most delicate of hands. A plant that seemed small in the store looked big once I had to put it in, so I had to remove roots or try to split it and I realized that's no easy task and I ruined most of the plants trying to do that. But I did learn quite a few things... Thanks again!
    – user17747
    Commented Aug 18, 2017 at 20:42


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