Two alternatives to using sand or gravel in the reservoir layer of a wicking bed are to use coarse organic material like straw or woodchips, or to just fill the whole wicking bed with soil and not have something different in the reservoir layer. I’ve done both and they both work.
Not having different material in the reservoir is the simplest way of making a wicking bed. One criticism of this approach is that, since some of the soil will always be saturated, that it will decompose anaerobically and get smelly. I filled a couple of wicking beds with a mix of potting mix and mushroom compost. They did not smell but after a couple of years the material in the bottom had decomposed to a black sludge. More recently I have been using a 50:50 mix of garden soil and compost and this seems to survive being saturated and is ok after at least three years.
I tested the wicking ability of small woodchips and three soil mixes in 50mm Perspex tubes. The soil mixes were from Corkhill Bros in Mitchell. All three are various combinations of topsoil, sand, manure and compost. Super soil has the most topsoil and vegi mix is much more like potting mix. Garden mix is somewhere between these two.
The woodchips wicked water up 110mm so would be ok in a shallow reservoir layer. Of the soil mixes, super soil wicked up 260mm, garden mix 190mm, and vegi mix 95mm. The vegi mix has quite a few largish pieces of undecomposed woodchips and spaces around these in the narrow tube I was testing in may have hampered the wicking. All the soils would be more compacted in a wicking bed than they were in my tubes so this test may not be a fair indication of their wicking performance.
I was interested to try pure cocopeat. This is often used in hydroponic systems but I have not heard of it in wicking beds. I think it could work if soluble fertiliser was used in the water as it is in hydroponics. However, the wicking ability of cocopeat was not great. At 200mm it was better than the garden mix, but not as much as you would need for a 500mm deep wicking bed. Again, it might worked better if it was more compressed.
I’m beginning to wonder if a mix of sand and compost might be a good medium to use in wicking beds. Sand wicks well, and the compost would provide nutrients to the plants. More research needed…