Over the past ten years I have experimented with a handful of clays in the terrarium, both as substrates and backgrounds. Below are the types I’ve used, with some details on the qualities and uses for each type.
Pelleted type -this clay really needs to be kneaded, or else certain parts will remain dry while others are oversaturated. Letting it sit in water does not suffice, because whatever clay isn’t touching water will not even get wet. Bentonite has historically been used to plug up oil wells so it really does make a waterproof barrier when used in a thick enough layer. You can purchase 50 lb bags at koi or pond supply stores. This is my favorite type as it’s very cheap and doesn’t have any additives. You can also get ‘kitty litter’ bentonite which as far as I can tell is made of the same stuff. Just make sure to read the ingredients. It should say 100% bentonite clay.
As a background, this clay is your best bet. For smaller terrariums that aren’t more than 16″ or so tall, it isn’t too difficult to create a stable background using this material. For terrariums with more height however, it can be challenging to maintain a high enough moisture level for the clay to stay sticky, and it can easily dry out and crumble, loosing it’s structural integrity. The tallest tank I made a background with using this material was 24″ tall, and there were a few occasions where cork or small wood pieces fell off. Others have had some success with bentonite backgrounds too, but I think the biggest concern would be long term viability. For the first few years my background did fine, but after a while it dried out and I eventually took the terrarium apart as I didn’t want falling wood to crush the frogs living in there. Here’s a photo of the bentonite background tank,
Powder type- powder bentonite is used in cosmetics and ceramics, and can be found at many natural grocers in the bulk food section. It is significantly more expensive and does not seem to offer any benefit over the pelleted type.
Powder Red Art and Other Powders
There is an excellent thread on dendroboard about clay based substrates, with some experimentation with powder clays. At one point I purchased a *lot* of powder clays, including red art, laterite, bentonite, kaolin, and one or two others that looked interesting. They come in little bags and were pretty dusty. There are sophisticated ways of creating substrate nuggets using these powders, but ultimately it was not something that I had much success with, and was a little skeptical of how much better these subsrtates were than the clay I could find going hiking. The biggest benefit of this method is that it allows you to add all sorts of cool stuff to the substrate, including calcium, so for Oogphaga especially, this can be enormously helpful.
Parking lot Red Clay
For a brief time in high school I umpired for middle school baseball and softball, and at the end of the season, never received compensation for a few games. So, I took my pay in infield conditioner from a large pile in the parking lot. My mom kept watch and I’d shovel a bag or two of it and off we rode. Unfortunately no bags were ever left, and so I’m not sure what kind it was, but it was unfired, and more brown than a lot of the types out there. It held moisture really well even as a background, higher in the tank. I used it to glue cork to glass a few times and never had any issues. I would guess that most infield conditioners work well as substrates.
This particular clay was collected in Silverton, OR. It seemed appropriate because of it’s consistency, which was very easy to mold, and held it’s shape well. It is a moderately red clay with very little organic matter. I collected a lot, but only ended up using it in two terrariums before the fear of pathogens got the better of me and I stopped using it. It’s near impossible to sterilize clay because of the moisture content, and the risk of baking it and causing a catastrophe in the oven due to stones or rocks has not been appealing.
If you are looking for a clay drainage layer, turface works great. If you are looking for something lightweight, this is definitely not a good option. It is super heavy, as you might imagine, and looks like little red flecks of rock. It might leach iron, but I’m not certain. It usually comes kind of dirty, with red dust all over it. If you’re using it in a fish tank, its worthwhile to give it a rinse first. I’m a little unclear what the benefits are of turface as a primary substrate or as an additive, because for aeration or drainage there seems to be better options out there that aren’t nearly as heavy. It does look pretty, but it doesn’t look all that natural to my eye. The clay below is a mix of turface and infield conditioner.
Valley Athletics Triple Play
This is a fine red clay used in the main infield. It is red, but not bright red. It holds it’s shape well and makes an excellent primary substrate.
Valley Athletics Mound Clay
This clay is a brighter red than the Triple Play, and has a slightly ‘fluffier’ consistency. The color is so bright, at first it was a little off-putting, but out of all the infield clays, this one seems to grow plants the best. This terrarium uses exclusively mound clay. The first photo is when it was setup, and the second is after about six months of growth.
What about Drainage?
There are many ways to create a terrarium, and depending on your micro-climate, misting schedule, and ventilation, your need for good drainage will vary. In a climate where ambient humidity is low, ventilation is high, and misting is done infrequently, the need for a substrate layer that allows for good drainage is lessened. There are many microhabitats within a tropical forest, many nutrient profiles of substrate. In my mind, red clay is ubiquitous with the tropics. The roads are red, your boots get caked and heavy with red clay, rivers run red, water after a storm runs red. It is everywhere.
All my terrariums utilize only clay substrates, with no false bottom or additional drainage layer, and no drain. The substrate is uniform, and misting is done sparingly. The terrariums have been setup for between 1-6 years, and there have been no problems that would lead me to use another method. It’s likely not the best option for those with automated misting or low ventilation, but for some, with infrequent misting and good ventilation, it can definitely work. And for those of you who like to keep it low-tech and low- maintenance, this can be a great option.
For Treefrog species needing a wet period, flooding a terrarium with clay substrate is very easy. After the initial cloudiness, the clay settles and the water clears. Care needs to be taken that foreground plants can tolerate the flooding of course. Some species that I’ve found to be very tolerant of flooding include Biophytum species, Floscopa sp., Solanum ‘hairy monster,’ and many Philodendron species. Surprisingly, there have even been Elaphoglossums, Ground Orchids, and many Begonia species that have also tolerated seasonal flooding (3-4 months), even flowering on a few occasions.
For some reason in all my terrariums there seems to be a significant population of earth worms. I’m assuming they come in with the clay, but am not certain. They do an excellent job of aerating the soil, and thrive in all clay substrate. The biggest downside is that it takes time for them to work the tunnels, so aeration might not be good for the first six months or so. I really only notice how well they are doing at this when I shine a light on the tank bottom at night, and see the reflection of all those pink bodies undulating through the substrate. During the day they make themselves invisible. Somewhat terrifingly, I once saw a 14″ worm in one of my terrariums. The only reason I know it was 14″ is because the terrarium was a 14″ cube and the worm was stretched all the way across the front.
Below are some photos of a clay substrate that was used for about three years. There were a lot of earthworms which did an excellent job of aerating the soil,
Other microfauna such as isopods and springtails also do wonderfully with clay. The best isopod colony I ever had lived in some air pockets between cork bark and clay.
Springtails live in the air pockets of clay, beneath.
Below are some of my terrariums with clay substrate,