Pieter Loubser writes:

I am astounded at how complicated we want to make the subject of the medium in which we grow our bonsai or potensai. I cannot call this medium "soil", because, in my opinion you must not have one drop of what is generally called soil, in this growing medium. I have very good reasons for this standpoint, but probably the simplest one is that the particle size of soil is by far too small. You will wipe out all drainage of any significance if you introduce soil into the growing medium. Basically you need some inert matrix that acts only as a matrix (a support structure) to contain the organic material that again contains the moisture (water), that contains the nutrients for the tree. You must have some solid material that acts as the framework. I use silicon sand, which I can buy in determined sizes (I use between 2 and 6 mm particles, in fact I only buy the sizes 2 - 4 and 4 - 6 mm). Should you mix anything smaller than half the size of the larger particle, you negate the size of the larger particle and may as well throw it away. The smaller particles simply occupy the spaces between the larger particles and you end up with inter-particle spaces that are related to the size of the smaller particles only. Simple physics. Then you need to have something in the spaces that will retain moisture. This is as simple as compost. A fine compost, not large pieces. The compost absorbs the water and the water contains the dissolved nutrients. Replenish the nutrients and do so often. If you want to know more ask and you will be given, with pleasure.

George Hefflefinger writes:

You have now had enough replies to your question on soil mixes to know that it all depends on where you live and what is available where you live. Discarding what remains on top of a 1/4" screen and what passes through a 1/8" screen, here in Victoria, given the generous rains of most winters, our mixes are built around this formula

• 30% fir bark
• 25% volcanic rock
• 20% #2 poultry grit
• 25% Forestry sand (somewhere between a fine gravel and a course sand)
• 5% garden compost

Andy Smith writes:

I know that this is going to sound like sacrilege, but my observation has just been that trees grow in dirt. Ordinary, unsifted, filthy dirt. In natural conditions trees are very rarely, if ever, found growing in ideal soil conditions. In fact, a lot of the trees we collect for bonsai got their great character as a result of those "poor" conditions. But no matter what the conditions are the tree usually manages to adapt to them, survive and grow.

Containerized nursery material seems to flourish for years in the stuff it's grown it -which is not bonsai soil.

So, in a bonsai pot, I think almost any soil would do as long as you water and fertilize in a way that complements the specific soil and tree. The main thing being, maybe, that the soil is a homogenous mix so that one part doesn't hold more or less water than another.

All that said, however, what I have slowly learned, or am learning, over the years in regards to transplanting is that what works best for me is to wash away as much of the original growing medium as will easily come off -usually about 70% (and not to leave any areas where air and water can't penetrate). Then I transplant into about 75%-80% of some "grit" (Turface, oil-dry, chicken or turkey grit) and about 20%-25% pine bark. I don't sift anything. The ingredients and proportions vary from time to time. It drains fast, but holds some water. The trees seem to like it well enough.

I could probably do better, but for my purpose -to get a tree established and growing and developing fibrous roots- this seems to work fine.

I met a very knowledgeable bonsai grower in Baltimore several years back who had some beautiful, very old trees. Among them were some Douglas fir, ponderosa pine and Rocky Mtn junipers. Also some other collected material. He'd had these trees for over 20 years and none of them had ever been taken out of their original soil.

"Why would I want to do that?" he asked me, "This is what they grow in."

All the trees looked healthy and vigorous. But since the growing media was consistent -100% natural- I don't think it caused a drainage problem like you expect when a tree is in a mat of it's own dense soil and then dropped into a pot of fast draining bonsai soil. Then, to keep the bonsai soil moist enough to stimulate root growth into it you have to keep the original soil much too wet, possibly killing your tree. Or, if you keep the original soil at the right moisture the fast draining bonsai soil will be so dry that no roots will grow into it.

Anyway, sorry to chatter on so. I don't know anything about soils. But I can't help but notice that trees grow where seeds fall. Not where the soil is right.

Bill Heston writes:

Roots require oxygen to grow, and oxygen travels 10,000 times faster through air than it does through water. "Drainage" means that the water drains out of soil is replaced by air since nature abhors a vacuum; hence, soil contains particles, water, and air. Soil is said to be compacted when there is insufficient space for water and, especially air. Roots are retarded and will eventually die in waterlogged soil. Containers retain more water than the earth, and shallow containers retain more water (hence less air) than deep containers. So, as the depth of the container decreases, the soil needs greater porosity in order that draining water will be replaced by air. What works in a nursery container may well not work in a shallow bonsai pot. The water/air retention ratio is effected both by soil particle size and container depth.

The best, concise discussion of these principles that I have found is in an article by Brent Walston entitled...Why the Earth is Not Like a Pot www.evergreengardenworks.com/earthpot.htm There are many other informative article at this web site www.evergreengardenworks.com

Pauline F. Muth writes:

For general growing of bonsai I use 1/3 turface, 1/3 large "sand" and 1/3 of an organic like bark depending on the species I am growing. For bare root transplant I use 1/2 turface and 1/2 large sand....but I am very good at adding nutrients to my soil: chelated iron, micronutrients and organic fertilizer ( I use Biogold) plus superphosphate for following species. I do use akadama soil in place of turface for my azaleas. Please note I am in Zone 4 and I have the time to check water needs.