Calcined Clay for Terrarium Use

What's in a substrate? - For years, the debate of "What's the best substrate for tropical terraria?" has raged on with opinions as abundant as the hobbyists who hold them. In the early times, pea gravel topped with sphagnum moss was a popular choice. This has been replaced in recent years by some form of drainage layer: hydro-balls, plastic egg-crate false bottoms, filter foam, etc. topped with "ABG mix". 


The term "ABG mix" derives from one of the many tropical soil mixes developed many years ago by Atlanta Botanical Gardens (ABG). The most commonly used recipe in the terrarium hobby is one that ABG formulated to pot their Nepenthes species. It is used because it allows for excellent drainage and aeration at the root zone, which is exactly what this largely epiphytic (growing on trees) genus prefers. The familiar recipe contains the following components: peat moss, fir bark, tree fern fiber, charcoal, and milled sphagnum. 


Through the years, there have been tweaks to this mix for various reasons, some are discussed here. 

1. Ongoing supply issues with quality tree fern fiber 

Years ago, there were questions about the sustainability of the harvest of tree fern. The potential environmental impacts associated with tree fern harvest led many to avoid using it in their "ABG mix". Additionally, for some time, import of tree fern was halted to these same concerns, meaning even if you did want to use tree fern, it simply wasn't available. Today, there is a sustainable source of tree fern fiber, but many recent batches have shown the presence of flower pot fungus. Flower pot fungus has been shown to be an unfriendly fungi to grow in a terrarium based on anecdotal evidence of animal death and plant damage purported from exposure to high spore concentrations.


2. Sustainability and hydrologic issues with peat moss

It is widely known that peat harvest is not a sustainable practice (Note: Peat harvest does not equate to sphagnum harvest.), and it is not a recommended substrate for most, if not all, potting uses. Besides the environmental issues with peat, it is a substance that has unique hydrologic properties in that it is difficult to get to dry, but once it is dry, it is largely hydrophobic (propels water) and is very difficult to re-wet.


3. Cost of individual ingredients

ABG mix can be quite costly to purchase from vendors in small, hobby quantities and can also be quite costly to make oneself unless ingredients are bought in bulk. This leads to some "tweaking" the mix to make it less costly. Peat is a relatively cheap ingredient, so it would be temping to increase the ratio of peat to reduce the overall volumetric costs of mix. However, peat's hydrologic issues, as noted above, can turn a once well-drained, airy mix into an anaerobic swamp. 


4. Highly organic mixes don't represent rainforest soils well

Soils in a rainforest are largely nutrient poor clays with a thin layer of organic material on top that is quickly being turned over. This organic matter is largely composed of leaf litter that falls from overhead trees. This litter is quickly broken down by microfauna and highly microorganism activity at the clay-organic interface. Commonly-used soil mixes lack the inert clay layer and replace it with a fully organic layer, meaning the entire substrate in a terrarium breaks down over time as it is cycled just like the leaf litter layer does in a rainforest. 


So this begs the there a better substrate option that is environmentally friendly, largely inert, and can provide better hydrologic conditions? This is where calcined clay comes in. 

Figure 1. Layer of calcined clay

Like mentioned above, rainforest soils are largely composed of nutrient-poor clays topped with a thin layer of organic material that is replenished when it breaks down. This can be replicated in a terrarium by using calcined clay as the sole substrate and topping it with a healthy leaf litter layer. This provides numerous benefits beyond just "replicating nature".


1. Calcined clay absorbs and holds large quantities of water without getting soggy and anaerobic. Individual calcined clay particles are very moisture retentive, but because the main substrate decomposes very slowly or not at all and there are pores of air between substrate particles, the substrate remains airy to plant roots as long as it is not entirely flooded with water.  

Figure 2. Plant root growth after being planted in clay. 

2. It promotes microfauna production. Because they clay particles do not pack down tightly upon each other, void space remains around each particle. This greatly increases the surface area for small invertebrates, creating a huge refugium for microfauna, and thus, promoting the "bioactivity" of the terrarium. 

Figure 3.  A sample of calcined clay in hand illustrating particle sizes and voids.

3. It is its own drainage layer. Because the particles are virtually entirely inert, a thick layer of clay can also as both the substrate and the drainage layer. For enclosued that are drilled for drainage, nothing additional is needed. Simply fill the bottom of the enclosure above the inlet of the drain and your substrate/drainage layer is done. For undrilled enclosures, simply create a small pool in the corner or use a small diameter piece of pipe where you can lower a siphon hose to drain the water before it reaches the upper surface of substrate. 


4. It is long-lasting. Even with the airiest organic substrates, they will have to be replaced at some point. Because the clay is virtually inert, it maintains its form and qualities for very long periods of time. I have enclosures that have been running for nearly 10 years without a substrate change. Simply top the substrate off with new leaf litter as it breaks down. 


5. It is inexpensive and uncomplicated. Calcined clay for an 18x18" enclosure will cost ~$25 for the lifetime of that enclosure and is as simple to create as washing the substrate, pouring it into the bottom, and topping it with leaves. 


If you have any questions or want to give calcined clay a try, reach out to us or come see us at one of our shows.