I've several dozen of 'em growing.
Some have 0 dirt... just biodegradable bags (and maybe a rag or sponge to help keep the seeds on top.)
How much dirt will be needed in the long term is hard for me to guess, just yet... because I don't know how nutritional the bio bags are for plants? Nor how long it takes 'em to degrade?
Also to consider:
When I recycle the used biofarms... there are left over roots in the bottle... they help give the new plant support... and maybe create their own root mulch?
Soooo... I'm guessing for most plants -- little dirt is needed.
You can see a healthy root structure here.
(This pole bean is currently over 3 feet tall.)
But, some plants like carrots... I haven't figured out how to do it... without creating some sort of sign... like:
"In case of emergency, break glass!"
Soooo... for food that grows underground, I'm working on a separate webpage about potted plants that use a garden sprayer for drip irrigation [I'm testing about 30 varieties with drip... and about a dozen with the bottled reservoir (or lily pad) method]
note: the lily pad method uses a bit more dirt for plants that need it... like popcorn. To do a lily pad pond... you cut the top off a plastic gallon milk jug... fill it over half full with water... place a sponge on top... cover the sponge with as much dirt as it will hold... and add a seed (like a kernal of popcorn from any regular bag of popcorn.)
to date, water management hasn't been a problem... if the right bottle/container is used (and care is taken not to overfilled it with water) the air and water don't really mix. And, everything smells OK.
But, still... I'm thinking why not add a snail or 2? Won't that help keep the water cleaner while adding some brand spankin' new animal fertilizer?
I was going to wait to harvest some free snails in the spring... but, if I see some for sale, I might try them, too.
I just got back from the store... dag nab it... they were out of snails. So, I got some catfish.
2 have been deployed in existing experiments... and I'm building a special garden for the 3 others... to see if they will breed.
The lady at the store said that is a problem with the snails... they overpopulate... and eat all available food... then, they all die.
Kinda sound like humans?