depends on what you have ever put into your garden, if you have used strong fertilizer or pesticides of any kind, I would not advise it.
I did nothing special to get bio active, I shoveled up some nice loamy dirt from out back by the creek that is mostly runoff silt that collects over time forming a clay like dirt that holds burrows well and maintains humidity too.
Fall is coming, so rake up some nice maple/oak leaves.. they go well inside an enclosure.
little bugs and bacteria are beneficial to your enclosure, as thats what makes it bio active.
Varanids (Monitor Lizards) have an immunity system far superior to even humans, as they can find rotting carrion on the ground and welcome it as a free meal with no ill effects at all.
The monitors that get sick and die are a result of poor caging, the heavy stress reduces the ability of their immunity.
However, the brunt of deaths occur from organ failure, as they slowly dehydrate.
This occurs through breathing, since they have no pores, they cannot sweat and they don't urinate. Urates are passed as a chalky white mass in the feces.
Think of it like this, in the winter you can see your breath, that is moisture exiting your body. all Squamates (Lizards and snakes) have specific humidity requirements governed by evolution as to where they are from. Meaning that if you keep a rain forest herp in air thats too dry, it will not survive very long, keep a desert animal in a damp box, same thing.
Savannah Monitors are from a range in Africa that has several months of rainy season, and several months of drought, During the rainy season, there is very little sunlight, and during the dry season there is very little moisture.
This is why deep burrows are so important, it is the necessary tool they use to conserve body moisture, since for obvious reasons there is not much opportunity to drink during the drought season, so they retreat into the burrows and ride it out.
If you have ever noticed, basements are typically dank, damp places even on the hottest summer day, and the air is heavy (suspended water molecules)
this is the air that Savannah Monitors breath most of their lives.
When people keep them in hot cages with air leaks, the humidity burns off and escapes through the cracks, and now your lizard is inhaling dry air, and every exhale releases more of the animals internal water supply.
The process is slow, my last Savannah monitor took five years before his kidneys failed, his uric acid was no longer being filtered out of his blood, gout set in, and he died.
This is the fate of 99% of every Savannah Monitor exported from Africa.
Once an enclosure is sealed up with glue/caulk, rubber gaskets, etc.. and the humidity cannot escape into the room, you will find humidity readings from 60% to 90% without any misting.
Many folks wonder about air flow, so I will say this, how much air flow can there be down in a 3 inch wide hole a yard down? Since the cage door will be opened up daily, the air exchange from that is plenty.
When these conditions are met, the diet becomes a lot less important, as long as it is whole prey, meaning blood, guts, bones and brains, the animal will thrive.
sorry for writing a book, it made me think though.. I should make this into another page for the web site.