Soils are touchy subjects and difficult to agree on, but maybe I can help you reason through by explaining a little about how water retention works.

First, the size of the particles and the particles' porosity determine how much water a soil retains. Soils hold water in the air between soil particles (in vapor form), inside of soil particles, on the surface of soil particles, at the interface where particles contact on another, and if the spaces between particles are small enough, soils will hold water in those spaces instead of air. The least desirable water in soils is the water held in spaces between particles. This is because it deprives roots of the oxygen they need to function normally, which limits growth and vitality, and can even cause the death of roots and fungal root rot diseases.

Fortunately, it doesn't much matter what soils are made of. As long as they hold favorable volumes of air and water, you can grow in them. In fact, if you want to water every hour, you can grow perfectly healthy plants in a bucket of crushed glass.

To make your job easier, you might consider some tips. Soils that hold almost NO water between soil particles (except where the particles touch) and hold almost all water inside of particles are going to be easier to grow in and offer greater opportunity for plants to realize their genetic potential. In order to achieve that end, the particles a soil is made up of need to be fairly uniform in size and larger than about 1/10" in diameter. At a diameter of less than 1/10", soils start holding water between particles, and the smaller the spaces between particles, the more water is held there.

You can take a mixture of particles that hold no water between the particles and destroy the drainage and aeration by adding fine material (smaller particles). Imagine a pint jar of BBs. There might be a half pint of air between the BBs. If you add a half pint of sand to the mix, or coco coir, all of a sudden the larger air spaces are all gone, and the smaller spaces, because they hold water more tightly, are filled with water.

I think you COULD make a very good soil with nothing but pumice and crushed terra cotta, but how well it will perform is going to depend a LOT on the size of the particles. IOW, you'll need to screen them so the particles you use are between .I-.2" in size, and it will be more important to eliminate the fine material as opposed to that material that might be a little more than .2". I don't see the coir as being a plus in the mix I described. Fertilizing can be monkey easy if you get an appropriate fertilizer.

Al

Very informative. Thank you very much