“My wife recently acquired a common southern toad (American toad) and she is curious as to the amount of moisture needed within the confines of the aquarium. I grew up in Michigan and as a child had kept toads throughout the summers w/absolutely no moisture whatsoever (save for a water dish). Exactly how wet/moist should the ground be for a healthy toad? She uses coconut fiber for her frogs but I worry about this toad getting a bacterial infection from excess moisture constantly on its under-belly that it may very well not be accustomed to in its natural habitat. However you are the expert and I must ask what you think? Thanx bunches in advance”
Ideally you want to provide a moisture gradient, so one part of the substrate is wetter than the other. If you are using coconut husk fiber or another soil you can accomplish this by using a good two or three inches of substrate. The top can dry out a bit, while the bottom will normally stay moist, allowing the toad to burrow on its own to stay hydrated and comfortable.
A good way to tell if the coconut husk fiber is too wet is to take a handful and squeeze it. No more than a couple drops of water should come out and if more does then wring it out well in your hand first before adding it to the enclosures.
Toads will also live fine on dryer substrates though, provided there is access to clean water, as you noticed as a child.
A related general sort of rule for keeping amphibians is to keep the enclosure wetter and more humid on hot days and cooler and drier during cooler temperatures or times of the year.