Last year I added a large pumice drainage layer to the bottom of my bonsai pot, to see what difference it would make to the root development of some of the Ginkgo bonsai trees. The soil mix used was medium to small graded Akadama with 25% Kiryu, mixed in with a layer of large pumice for (improved) drainage. The results were that the roots within the smaller grade Akadama / Kiryu mix were smaller and denser in formation, creating a very fibrous mat or root plate. Those growing towards the bottom of the pot within the larger pumice layer, were larger diameter roots, with even larger root tips. The health of the tree has been excellent, as previously the root development was all one and an even size. It is amazing to observe the changes to the roots – with only a minor change in soil – how healthy the overall root system is with a free draining mix.
This second example with another Ginkgo, the root size changes were incredible. Such large roots within the pumice layer because of the larger air spaces. Again, good healthy roots in colour, size and density. With no dead roots within the pot or soil. The trees grow rate was very good last year and the root development has been very positive; whilst still retaining a good fibrous root pad above the larger roots within the pumice layer.