Choosing the right substrate for your snake is a very important aspect of keeping them happy, healthy and thriving. When choosing a substrate, you should first consider the species-specific needs of your snake, such as:
Environment What kind of environment does your snake come from in the wild?
Humidity Does your snake need high, low or moderate humidity?
Climate What kind of climate do you live in?
Enclosure What kind of enclosure am I using? (Tank, tub, rack, PVC, etc.)
Locomotion Is your snake terrestrial (ground-dwelling), arboreal (tree-dwelling), or fossorial (underground dwelling)?
Traits Does your snake like to burrow? Does your snake have an intense food response that could make it more prone to ingesting substrate? Does your snake have heat pits in which certain substrates could become clogged in?
Each substrate has its own pros and cons, and some substrates are more beneficial for certain species than others. For example, fossorial species and terrestrial species with burrowing behaviors will benefit from loose substrates such as aspen or coconut fiber. However, these two substrates are also more prone to molding than others. Some substrates are not meant to retain a lot of moisture, while others are made specifically for this purpose. This is important because a large contributing factor to how humid your enclosure is kept depends on how much (if any) water you add to your substrate, as well as how much ventilation you provide and the climate you live in. The more water you add to the substrate, the more humid it will be; but adding too much can risk an accumulation of bacterial growth which can result in scale rot.
When adding water for high humidity species, you always want to assure the substrate is damp, not drenched. There should be no standing water at the bottom of the enclosure, and you should be mixing the water into the substrate as opposed to misting it. This is because misting only coats the surface of the substrate in water, which often pools and then quickly evaporates. This often leads to sudden spikes and troughs in humidity levels, which is not good for your snake. Therefore, monitor your humidity levels with your hygrometer regularly to decide how often you need to mix water into your substrate in order to maintain the appropriate level of humidity.
For most species of snake, you will primarily be looking at four different types of substrate:
Aspen shavings are a good option for those living in moderate climates with terrestrial or fossorial species that exhibit burrowing behaviors, such as Kingsnakes, Milksnakes and Sand Boas. Aspen is best suited for species that require humidity levels between 30-50%. However, if you live in an excessively humid climate and have a species requiring humidity between 50-75%, aspen may be an option to keep relative humidity within this threshold. That said, aspen is not made to retain moisture and is prone to molding when wet, so keep this in mind when choosing a substrate for your snake. It does have the tendency to stick to things when wet, so be sure to feed your prey items dry to avoid ingestion of substrate. Alternatively, you can clear an area of the enclosure to feed in, as well.
Cypress mulch is a great option for snakes that require humidity levels of 50% and above. It retains moisture well and is not as prone to molding. If you live in an extremely dry climate, you may consider using cypress mulch to keep humidity between 40-50%.
Cypress mulch can contain large wood chips in the mix, so be sure to watch your snake when feeding to avoid ingestion of these chips. Cypress mulch is not the best for burrowing, so it is better for arboreal snakes or terrestrial species that do not exhibit much burrowing behavior. Always use 100% cypress mulch for your snake, never a blend.
Coconut is another good option for snakes that require high humidity. It is comparable to cypress mulch as it retains moisture well and is a popular choice among many keepers. There are two popular varieties of coconut substrate: fiber and chips. Both are available in both loose and compact form. Coconut fiber is comparable to peat-based substrates, while the chips are larger chunks of coconut husk.
While both forms are suitable for many species that require high humidity, the chips tend to be better for snakes with heat pits, as the fiber tends to get stuck in them. Snakes more prone to ingesting substrate should also avoid coconut fiber, as the fiber can coat the feeder and risk impaction.
Potting soil mixes are a popular option for bioactive enclosures, especially for snakes that require moderate humidity and/or exhibit burrowing behaviors.
That said, it is worth noting that it can get clogged under scales and heat pits very easily and sticks to everything when wet. Many keepers will mix the soil with another substrate to create a less dense combination. Always use organic soil -- many soils meant for growing plants often contain toxic chemicals that can be lethal to your snake. Never use a substrate you are unsure about.
In conclusion, the substrate you choose will vary depending on the species of snake you keep and its individual needs. Aspen is a great option for snakes that require moderately low humidity, while cypress mulch and coconut husk are good for species that require higher humidity. Soils are excellent for fossorial species, especially if they require high humidity. There are many factors to consider when choosing the best substrate for your snake, and it may take some trial and error before you find a substrate or combination of substrates you and your snake are comfortable with; but in the end, the best substrate is the one that suits you and your snake the most.