Substrate for a closed terrarium - what's important?

Erde im Garten
Normal garden soil. Source: piqsels.com

The substrate in a closed terrarium is one of the most important building blocks. It's literally the base for all life inside. Without a substrate your plants would not only be unable to anker themselves. They also wouldn't get any nutrients and would soon wilt. The substrate is also important for insects, worms and snails and makes up a basic part of their habitat. They use it to hide or even to eat.

You'll notice that this is an important topic. But what exactly makes a good terrarium substrate? What is a bioactive substrate and what do you need to mix it yourself? All of this will be explained in this article.

What does the substrate do?

Like I said, the substrate has many functionalities. Just like:

  • Letting your plants anchor themselves
  • Harbouring nutrients
  • Quick drainage and distribution of water
  • Decomposing waste

In order for your plants to sprout and thrive your substrate needs to be full of nutrients. You'll probably close your terrarium up forever. The plants can really only access the nutrients that were in the substrate from the beginning. Those are being recycled by the nutrient cycle over and over again.

Waldboden voller Blätter
Dead leaves have a lot of stored nutrients. Source: piqsels.com

In order to be able to absorb nutrients your plants need water. Under normal circumstances your plants would be rained on equally. But in a closed terrarium the water will mostly stay on the sides as condensation will appear mostly on the walls. And since it's important for a quickly draining substrate that easily distributes water everywhere.

Lastly the waste has to be decomposed and absorbed so the ground doesn't get cluttered with biomass. Over time, biomass will block the water front draining and keep important nutrients with the living plants still need. To prevent that, you'll need a bioactive substrate for your closed terrarium. But how can you achieve all of that?

Letting your plants anchor themselves

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Eine Pflanze in einem Erdhaufen
Plants like loose soil to anchor themselves in. Source: piqsels.com

This is the easiest on the list. You'll probably not even have to worry about it. Plants can anchor themselves in various kinds of substrates. Plants can do with anything similar to garden soil texture-wise. You can use potting soil, forest soil, peat, sand or even cotton as a substrate for your closed terrarium.

In harder materials your plants will have it a lot... well harder. If you see any dandelions growing out of the concrete, you can be sure that there's soft soil below that. In materials such as gravel, porcelain shards or active carbon your plants won't be able to anchor well.

Fun Fact! Some plants grow lithophytically, meaning they really grow on stones. Some orchids fall into that category and can actually make a nice addition in your closed terrarium.

Harbouring nutrients

As you just read cotton you were probably wondering whether or not I'm actually qualified to talk about plants. And you're completely right. Cottons alone doesn't work (and frankly speaking, I'm not qualified). Cotton or sand have little to no nutrients in contrast to normal soil. But your plants obviously need those as well. The best way is to take soil from a forest which is extremely rich in nutrients. Forest soil contains many, many dead and living plants and insects you might not even see at first.

Schubkarre mit einem Sack Blumenerde
Potting soil isn't bioactive but full of nutrients. Source: piqsels.com

You can also use potting soil for terrariums that is full of nutrients and is specifically designed for plant growth. Potting soil on it's own hoever isn't bioactive. Thus, it will not decompose rotting plant matter well. We'll look at that in a bit.

Another good possibility for a nutritious substrate is fertilizer. Fertilizer is available as a natural variant (I'm sure you know, what I'm talking about) or as an artifical variant that smells a bit nicer. You'll have to be careful with fertilizer though, as you'll need to find a good balance. Too little fertilizer and your plants will stagnate or will because of a lack of nutrients. Too much fertilizer comes with it's own risks as well.

With too much fertilizers your plants will rapid grow in height but will focus too little on their roots. They already get enough nutrients, so why should they? But that'll make them unstable and stagnate as soon as the nutrients are used up. Furthermore, fertilizer can drastically change your soils pH value. Most plants need a pH value between 5,5 and 7,5 to grow well. You really need to be careful. Forest soil will be a much safer option to get nutrients. Only use fertilizer if you know exactly what you're doing. Don't forget that a closed terrarium is an independent ecosystem and any excess cannot escape.

Quick drainage of water

A closed terrarium has a daily water cycle but it'll never rain in there. Due to condensation most of the water will collect on the walls rather than drip from the top of the container. This leads to your substrate being drier in the middle than on the sides.

Coconut fibre helps to make your substrate loose. Source: piqsels.com

In order for the water in your closed terrarium to be evenly distributed you need a substrate resistant to compaction. If your substrate compacts too much you'll get standing water sooner or later. You can see this on unplanted field after long rainfalls. This is a great breeding ground for bacteria and mold, which you do not want. Your substrate needs to be a little springy to resist compaction. You can achieve this by adding any kind of fibre. Coconut fibre, reed or dried sphagnum moss work quite well. Mix it in a ratio of about 2:1 soil to fibre. You really don't need to be exact.

Mixing soil isn't rocket science after all. :)

Additionally you should reduce the overall surface area of your substrate. Because of water's surface tension it likes to stay in materials with higher surface area. If your substrate is made of very fine material the water won't drain into the false bottom as nicely. You can reduce the overall surface area by adding larger objects like gravel or pieces of charcoal. This will also improve the overall ventilation and gives your plants roots better access to oxygen.

Another great possibility for distributing water is a false bottom. The big gaps between the buffer material leads to a very even distribution of water and prevents standing water at the bottom of your soil. You should always have a false bottom. I explain how to make one in this article.

Decomposing waste

Sooner or later some of the plants inside your closed terrarium will die. It doesn't matter how well your biotope works. Just like humans and animals plants have a maximum lifespan. Since you'll definitely get some biomass this way it'll have to be removed somehow. This is where bioactivity is important.

A bioactive substrate has many bacteria, insects, worms and snails eating dead plant matter. Their excrements can then be absorbed again by plants. They are an important part of the nutrient cycle of a closed terrarium. Also helpful are mosses, algae and mushrooms. The more diverse the micro fauna and flora are the better the stability of your ecosystem will be. It'll only be probably as soon as one of these components gets out of control and overtakes the others by taking too many nutrients. A closed terrarium full of mushrooms or algae isn't only not that pretty, it'll also collapse after a while.

Kellerassel auf Baumrinde
Isopods are detrivores and love to eat dead materials. Source: piqsels.com

You won't have to worry about making your substrate bioactive if you're using soil from your garden or a forest which already has many small insects inside that will multiply by themselves. If it's likely that you'll get larger waste products (like leaves) in your closed terrarium you could think about manually adding isopods. They come in many more forms than you'd think. Rubber ducky isopods are especially cute but accordingly expensive. If you don't care about their looks you can just look for isopods under stones, dead wood or in your basement.

Other small helpers are springtails. You can read about them in this article.

I talked about potting soil above which isn't bioactive. You can however easily change that by adding soil from outside. It really doesn't have to be much since the animals and bacteria inside will make themselves feel at home and multiply by themselves.

Different layers of substrate in a closed terrarium

If you want to give your closed terrarium a more aesthetic look you can stack multiple different layers of substrate. For example, you can fill the lowest layer with puffed clay balls to absorb water. Then a layer of gravel ontop, then sand and lastly normal soil. You can do that as much as you like. It won't affect your plants in a negative way as long as they can anchor their roots in a nutrient-rich substrate. Theoretically you can also put a thin layer of gravel on the very top as long as you've planted your plants in soil.

Summary

A closed terrarium needs a bioactive substrate filled with bacteria, worms, insects and snails to eat and decompose waste. To prevent standing water, the substrate needs to be loose and a little springy. If you're using a false bottom, you'll need to reduce the overall surface area of your substrate by adding larger objects like stones.

You're best suited with forest soiled which you can enrich by adding fibers and gravel or charcoal. If you've got some potting soil at home you can also add that.

Important note for carnivorous plants

Carnivorous plants need a different substrate than usual plants. Many key aspects from this article like quick drainage of water and the ability to achor also apply to them. What's important in terms of nutrients for carnivorous plants is explained in this article.

Are you done?

Falls du bereits ein schönes, ewiges Terrarium gebaut hast, schicke mir doch ein Bild an jan@terrarium.blog! 🙂 Alternativ kannst du es mir gerne auch bei Instagram an @terrarium.blog senden. Schöne Bilder teile ich gerne mit deinem Profilnamen, um sie der Welt zu zeigen.

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