The Great Substrate Debate

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Why potting mix is the best option for many reptiles and amphibians —

I want to talk here about why potting mix works so well for so many of our frogs, turtles, lizards and snakes, but I also want to just briefly dispel some of the myths. For a deeper dive into this topic see: Is Miracle-gro Potting Mix Dangerous?

(I use Miracle-gro brand potting mix because of all the varieties Ive tried it provides the most consistent quality. There is no reason to use the “organic” variety unless you simply prefer organic products in general.)

Substrate is one of the 4 key elements of enclosure design – the sun, wind, and water are the others. Substrate is the “earth” for our animals. And as the earth, substrate provides humidity, security, cooler temps, shelter from overhead radiation, exercise and enrichment. It is as important as any of the other 3 key elements of enclosure design, but it is the one most keepers seem to get consistently wrong.

I have found that a good, quality potting mix best meets the needs of many of the species kept. Potting mix is superior to wood shavings, coconut waste products (See: What Wrong with Coconut Husk Substrate?), fancy bioactive substrates (See: Comparing Bioactive Substrates) and any others Ive tried. The only critically important problem it can present is if the moisture content is not managed properly by the keeper. Thankfully, potting mix is more forgiving than the other substrates in this respect. (See: Managing Substrate Moisture)

A substrate should be a humidity sink. (See: The Humidity Mystery) How many times do we hear of a new keeper trying to increase the humidity of their enclosure – often by misting, or closing down ventilation? These are not effective means in most cases. In nature humidity retreats are provided by the earth. Often a layer of forest litter, dried grasses, slabs or bark, flat stones or other surface material creates high humidity refuges that the animals take advantage of when needed. The humidity of the air above the surface really is of secondary importance as long as the humidity layer provided by the substrate is available. (See: Managing Substrate Moisture).

A deep (4-8 inch) substrate of potting mix will be loose enough to allow the animals to burrow through it but will also be firm enough to hold the burrows created. Even species that do not typically burrow will often create depressions under surface cover as retreats. And better than hide boxes, these self made depressions fit the animal snugly, providing the security they need.

The process of creating burrows is perfect exercise for our animals, and provides us with a great opportunity to observe their natural behaviors. Its great to see a hog-nose snake pop up from a vigorous burrowing session with a big dollop of substrate piled on top of it’s head. We know we are doing things right when we see these healthy natural activities.

For some burrowing lizards and species adapted to more arid environments I add up to half course builders sand to the potting mix. (See: Play Sand or Coarse Sand?) The coarse sand drains better and is less likely to compact, so its a better choice for our vivariums.

Species that dont burrow will also benefit from this substrate because of it’s water holding (and releasing) qualities. Dart frogs for example will discover refuges under surface slabs of bark, under low plants or dead leaves that provide high humidity retreats. Understanding how substrate humidity works allows us to avoid the rather silly practice of daily, repeated surface misting in, for example, dart frog enclosures. Research has shown that anurans (frogs and toads) have a mental map of safe retreats in their territory that provide varying levels of humidity. They know where to go to find the environmental parameters they need. (1)

I have watched the same behavior by my dendrobatids. After creating a very simple vivarium for them with deep potting mix substrate and plenty of surface cover, they would explore until they started to identify safe retreats with higher humidity. And just like the various dart frogs I’ve observed in the wild, my captives would move out from these retreats to feed and then return to them as their needs required. With a simple bioactive design like this they could easily go a week or ten days without any sort of misting, just a shallow dish of clean water to re-charge their water balance in.

Finally, lets compare MGPM with the industry standard ABG mix and the bioactive substrates sold by the online retailers (See: Comparing Bioactive Substrates). A typical recipe for ABG (Atlanta Botanical Gardens) mix calls for sphagnum, Tree Fern Fiber, Bark, Peat Moss and charcoal. The bioactive substrates I tested were similar, with the exception of The Biodude’s which was a single ingredient – possibly peat moss.

Miracle-Gro potting mix ingredients are sphagnum, peat, forest products (ground up bark), coir and or compost and perlite. Very similar to the industry standard and very similar to the better substrates sold online, but at a fraction of the cost. The online products also performed rather terribly when I tried to grow plants in them. Because they had none of the fertilizer that the MGPM had, the plants were stunted. Some, like The BioDude’s substrate, didn’t allow the plants to grow at all. The seeds germinated and then the plants literally stopped growing. To remedy this of course The BioDude sells various fertilizers disguised under names like “BioBoost” and BioVibe”. Don’t be fooled.

The key aspect of all these substrates is water balance. The potting mix retains water while allowing for drainage. This brings air into the substrate which is needed for plants and bioactive bacteria. The ABG and similar substrates drain almost completely because they are designed for enclosures that get sprayed or misted repeatedly. As I mentioned earlier, this continual misting is unnatural and detrimental to the animal’s health. Stop over watering and and you will have no need for the expensive designer substrates at all.

1.) https://www.jstor.org/stable/2389843

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2 thoughts on “The Great Substrate Debate”

  1. Hi Joe, thank you for writing these articles! I’m a fishkeeper setting up a terrarium for a gargoyle gecko for the very first time. I was wondering if there’s a way to contact you. I have some questions about setup and care. Thank you!

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    1. If you are on Facebook, join my group: Naturally Keeping Reptiles and Amphibians. There are a number of videos there in the “Featured” section of the group that might be helpful. And of course ask questions for the group and I to try to answer.

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