A few years ago, I was not sure what the best substrate for bonsai would be. As the global consensus is that Akadama is the best substrate for bonsai and root development, I decided to do a little trial. Naturally, not a scientific study, but just for fun. I decided it might be of interest to others what I found so, here goes.
Bonsai substrate test setup
For this trial I used 6 Malus seedlings. In early spring, a few weeks after they had started growing, I bare-rooted the seedlings. After sorting them by size, I allocated them to my regular substrate at the time, or Akadama. I ensured a balanced distribution with the two largest, smallest, and intermediate sizes each in one of the substrates. The roots were clipped to the same length at this time. Then they were potted in a 6-hole container in a mixed pattern. This way the plants are always together, exposed to the same conditions. The container was then placed in full sun and rotated several times over the rest of the year. Watering and fertilizing was as with all my plants.
The mixed substrate used in this experiment consisted of molar clay, mixed with broken expanded shale and pine bark. A few bits of coarse bims are present too (As I re-used some old substrate). I used regular Akadama from a bonsai dealer, sifted, unmixed with other components. (More on substrate for bonsai).
After the leaves had dropped, I inspected the plants. Surprisingly, there was a very clear difference between the seedlings in Akadama and those in my regular substrate. The seedlings in my regular substrate were substantially larger than the ones in Akadama. So substantial that concerns I had at the beginning of the trial regarding equal size distribution across the two substrates had been lifted completely. Every single seedling in my substrate was larger than all Akadama-grown seedlings.
Taking care to not mix things up, I removed each seedling from the container, putting all Akadama-grown seedlings to one side, and the others on the other side. Here we see that the roots from those grown in Akadama (Bottom row) are short and fine with many growing tips. The roots of the seedlings grown in my regular substrate (top row) have fewer active growing tips, and the roots are longer and thicker. The closeup shows this even clearer, where Akadama grown seedlings are on the right.
This trial was done with a very small number of plants. From a scientific perspective the number of samples probably is insufficient for statistically sounds conclusions. That being said, with the differences being so clear, I personally feel confident that the pattern seen here is real, and not a coincidence.
In this experiment, seedlings grown in Akadama showed reduced biomass accumulation. The above-ground parts were shorter and thinner than those in other substrate. For growing bonsai from seed / young material this might mean that Akadama will slow down above ground development. On the other hand, for more mature material it might mean that internodes are closer and fewer extending branches will form when growing in Akadama.
The roots were finer, and with more active growing tips for Akadama. This may assist in developing a better nebari.
Akadama is by most not used pure, but rather in a mix with e.g., bims and lava. As such this could be seen as an unfair comparison. Considering the differences seen in this experiment it is worthwhile to explore further experiments with individual substrate components as well as mixed substrates. If you have done such experiments, please let me know!