If you'd like to go fully organic you will need to do quite a bit of reading and build your soil to be a full living entity. It's beyond what I know how to do at this point but I do know you will need large pots to hold enough "stuff" for nutrients to be available. The build-a-soil website is quite helpful and there are some really knowledgeable guys on here.
I've spoken with
@Tom Bombadil a couple times and he has a pretty straight forward process. It will take some work and getting the right things at the right time. You could probably fit two large pots with a full organic soil and get some really nice plants in that space. Way beyond 20g/plant.
I think
@Waira also has some really good soil, and I know he has just killer beautiful plants, and I can't think of the company/pack he uses. It escapes me at the moment.
You could also amend the soil you currently have with the right additions from build a soil and be well on your way.
Everything you need to grow no-till with organic living soil. Our mission is to support organic gardeners and to provide the best products and information on how to use them. We carry pre mixed super soil, living soil, organic soil fertilizers, organic water soluble nutrients, soil minerals...
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Read up and good luck! That's about all I can add to the organic discussion.