Something that drains very well, yet doesn't dry out right away. Watering is one of the most challenging and complicated parts of bonsai, in part because it seems so simple. It is affected by many variables: climate, humidity, season, species, etc. Many people use akadama, pumice, perlite, gravel, different types of bark. It depends on so many things; it's hard to give a definite answer.
I live in the Chihuahua Desert, so I need a soil/substrate that retains more water than you would need in Virginia. I'm still a beginner myself, but I'm reading and learning more and more everyday.
Questions about soil physics come up all the time and I find that I repeat myself a lot on this topic. I'm going to try and write a reference for folks to refer to on this topic. I'm expressly NOT going to offer you any advice on what soil you should use in your garden, whether you should use organics or inorganics, or anything on that line (that's where these soil threads go south). Instead, What I'm going to do is offer some basic information on which you might base your own choices. I'll try to do this at a high level, but this is related to my profession, so it's easy for me to go...
This thread has been invaluable to my understanding of soil physics, water retention, porosity, et
