Managing Substrate Moisture

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A Critically Important and Often Misunderstood Aspect of our Hobby —

The substrate in our vivariums has some important duties. But they can not perform these duties if they have too much or too little moisture in them. Different substrates handle water differently. Some work to our advantage, and some work against us. A good quality potting mix is the best choice for substrate in part because it was carefully designed to handle moisture in the way we, as herpetoculturists, want it to. It both retains moisture, and drains well – apparently contradictory traits! (See: The Great Substrate Debate)

Coconut waste products like Eco-earth, etc are not as useful as substrates because they do not handle water like we ant our substrate to. (See: What wrong with coconut husk substrate?)

The worse substrate is top soil. This comes as a surprise to one keepers who have been told for years to use top soil in their vivarium substrate mixes. This could be because in Europe where much advice about bioactivity originated, top soil is a very different product. In the US top soil is not sanitized and so contains the pathogens – viruses, bacteria, parasites – that we might find in nature. Top soil also quickly compacts, making it totally unsuited for vivariums – even in small quantities.

Simply put, substrates must balance air and water content. Both plant roots and bioactivity are completely dependent on this balance of air and water. when the water content drops too low, bioactivity stops and plants wilt. But just as important, when water content is too high, our aerobic (oxygen using) bacteria are replaced by anaerobic bacteria, and our plants drown, or have their roots rot in the soggy substrate.

An often overlooked, but critically important aspect of substrate moisture is it’s role in providing humidity and humidity gradients. (See: The Humidity Mystery)

Two features of natural set-ups that seem to often promote over watering are misting systems and drainage layers. Drainage layers should almost never be used, and misting / rain systems should only be used when the species or the vivarium benefit from it. (See: Why Drainage Layers Suck , and When and How to Use Mist and Rain Systems)

So how do we maintain the critically important substrate moisture levels? In practice it couldn’t be easier: When the substrate dries down to the point that bioactivity is dropping off and our plants have just begun to wilt, we re-wet the substrate by adding water.

But the devil is in the details. We have to learn what the right substrate moisture levels feel like – even what they smell like. And this takes some practice. The good news it isnt hard and you have some lee-way – its a forgiving system.

Right out of the bag our substrate (Im assuming your using a quality potting mix like Miracle-gro potting mix) will likely be wet.Quite possibly too wet. If this is the case just spread it out across the flor of the cage 3 to 6 inches deep and give it a few days to dry down. Heat and ventilation will aid in this.

You can easily monitor substrate moisture simply by scooping up a fistful and examining it. You want it to be dark and feel cool. It should be slightly damp – not wet. It should not clump when squeezed into a ball, but easily fall apart when pressed on. If it clumps it’s too wet. If you can squeeze even the smallest amount of water out it is too wet.

Our vivariums are integrated systems. The elements of design – the wind, earth, sun and water have to be considered together because they all influence each other. So how long it takes for our substrate to dry down, when and how much water to add, will depend on the elements of this system. All I can do is offer some examples.

A 10 gallon aquarium with 3 inches of potting mix substrate and a screen lid with a 45 watt incandescent overhead may take 2 to 3 weeks to dry down. At that point the top layer of substrate may be dry, while moisture at the lower levels may be just barely detectable. Your plants (I suggest pothos or other hardy species) may be just starting to wilt. The over all humidity of your vivarium will have dropped and even the humidity under surface litter or cover will be reduced to the lower limits f what is acceptable for the species you are keeping.

At this point you may add a half to 3/4 of a gallon of water to this 10 gallon tank. And you can just pour it in anywhere. If you are using a good quality potting mix the water will be dispersed through the substrate evenly in a matter of hours. (Note, coconut waste substrates will not disperse the water well and should be avoided)

Another option is to use a hand pump sprayer to rinse down the walls and plants in addition to adding water every couple weeks. You shouldn’t do this is it isnt necessary and you should do this more than necessary. Ive used this method to rinse animal waste off plants in arboreal snake enclosures, for example. And Ive used it as stimulus or enrichment, like a spring rain.

What you dont want to do is over mist, over water, over use your rain or mist system. The vivarium has to dry down well between waterings. Think of a temperate climate where a rainy day is usually followed by many dry days. The dry, sunny, breezy weather is critical to the health of the ecosystem. The animal’s behavior changes depending on this weather, but they are all adapted to the changes and will respond naturally to them.

Tropical frogs, terrestrial snakes, burrowing lizards, will all use the substrate moisture as needed. A dart frog will quickly establish retreats that remain very humid due to evaporating substrate moisture, even though the surface they are sitting on remains dry. Terrestrial snakes like ball pythons and many Colubrids will burrow thru the potting mix, or curl up under the forest litter you provide, especially when shed cycles approach

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Published by Joe

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