Setting up a 20L I was thinking about anerobic conditions in the substrate. Having sheets of Red sculpting clay on the base with 1.5in of sifted compost and 1.5in of sand this could take a while for water to leach out tannins.

How do we reduce leaching and promote water flow to the entire substrate. Solution I came up with is sand columning or what ever you want to call it sand spires, etc.
By poking holes with a kabab skewer or in my case sculpting tweezers it would create vertical columns of sand as you remove the skewer.

Since sand has larger and more uniform particle size water flows through it much quicker than finer mixed substrate, such as dirt. This has an added benefit of giving the water column easier access to nutrient transfer and storage via CEC to the dirt and red clay base.

Second, this is inherently adjustable, meaning there is a optimal amount of water "flow" through the dirt and over the clay which is easy adjustable by adding more columns.




What do you think?

As one as you regulate pore size of sand, not smaller than 1mm, and keep to depths you indicated hard anaerobic activity should not be a issue.

Not sure about tannin leeching, but you can get rid of a lot of tannins by pre-soaking your soil and doing a lot of water changes. I am doing a dry start Walstad tank currently, and I pre-soaked my soil for 2 weeks and did water changes in the bucket it was in every day. Cleared up a lot by the end.

If you plant heavily enough the roots themselves release oxygen into the substrate which also helps deter/stop anaerobic bacteria.