If you read any book on Bonsai and it is not long before you will find them recommending Akadama. The usual reason is that this is the best soil to allow free drainage which is critical for healthy tree growth and if you need any more evidence it is what they use in Japan… and they are right when they say this – well the last part at least.
Lets tale a quick look at some simple facts, starting with the average rainfall in Tokyo (taken from en.climate-data.org/location/3292/ )
Okay the numbers here are a bit blurry but the hottest month is August with a balmy 28 degrees Centigrade and the highest rainfall is June with 190 mm. The driest months are December, January and February.
Average annual temperature is 15.4 C and annual rainfall 1435 mm.
Now compare that to London, UK (Taken from the thirty year average for the South East of England) Notice anything? Wettest month is October with 93mm which is only 50% more than August at 60mm and February 55mm, but fundamentally we have fairly steady amounts of rain across the year with a bit less in the spring and summer (although it never actually feels like that!) Our temperatures are a bit lower, but only just.
So study these charts for a moment and it becomes very clear that we are a much much drier country than Japan. Total average rainfall for London is 770mm, nearly half of that in Tokyo!
Is there a reason why they favour free draining soils do you think???
The implications for our trees are that we should not be so focussed on providing free drainage for our trees, but in fact much more critical is to retain some soil moisture so the trees can actually grow. This is why beginners who follow the books can have such a hard time keeping their trees alive and why moderately experienced growers seem to be slaves to watering. The answer is really quite simple: add more organic matter to the soil mix. I always use at least 40% good quality compost, with a bit of grit to keep the mix open and able to drain.
I do throw a handful of Akadama into the mix as well, why? Well because before I realised what was going on I ‘invested’ in a bag and now need to use it up!