Potting 'soil' for our trees is an ever present interesting topic for discussion. Despite the accessible research that's been done enthusiasts entering the field are swayed by the offerings by suppliers of commercial potting media. These products will rarely offer a sustainable option for most growers of most species under most conditions; without some modification.
To start with, calling a potting medium, which is formulated for growing a plant in a pot, 'soil', is just the first contradiction. Then to be confronted with a wide range of commercial products which are primarily composted and decomposed organic matter which is so different from 'soil' as we know it in our gardens, adds to the confusion; along with our trusting acceptance that if a product is offered it must be good at what it claims to do.
Soil, the real stuff in the ground, has a thin layer of topsoil and then subsoil going down to a more rocky geology. The material in the upper layers is basically decomposed rock - inorganic material. In the topsoil biological activity will introduce an amount of organic material. In some of our best cropping soils the organic content will always be less than 10% and often less than 5%. So most of our food is grown in a medium - called soil which is mostly inorganic, clays, sands and degraded rocks. The predominant feature of soil is that the particles are almost exclusively very very fine in size.
This uniformity of fine sizing means that the material generally has good moisture holding ability, which is enhanced by the presence of any clay content. The important feature about soil is that it is in the ground, enabling free moisture to continue to 'fall' down to the water table and so the top soils are rarely water logged. This is exactly why garden soils are less effective for plants in pots; because the lower part of the pot is not in contact with the sponging action of the lower ground and the capilliary action of the uniformly fine particles hangs onto the water and you will regularly get water logging in a pot of topsoil. The capilliary action is the effect you see if you stand a pot in a saucer of water and the moisture will rise up to a certain level above the water level itself. The finer the mix and the more uniform size the particles the greater the capacity to 'wick up' moisture and hold onto it.
Organic matter in soil performs a number of important purposes; it helps hold moisture and nutrients, supplies nutrients as it decomposes, feeds beneficial silo microbes and helps to buffer soil pH (ie keep it neutral). In bonsai potting media we would be looking for the organic component of our mixes to do the same things.
Plant roots have some other particular needs that we must provide for in composing our medium.
First up is getting wet occasionally so we want to make sure our mix is 'wettable' but at the same time should not stay wet. Highly organic mixes when they dry can become hydrophobic - ie water repellent so that once dry they are hard to wet again.
Secondly, healthy root conditions rely on oxygen rich ventilation of the rootspace. The plant's roots transpire releasing CO2 and taking in oxygen. In the absence of oxygen, health will decline and a build up of CO2 will result in a gradual decline in pH, further reducing root health. Additionally good soil microbes are aerobic (breath oxygen) and bad smelly thrive without it. These anaerobic microbes acidify the media, also diminishing root health.
To summarise we want a medium with some organic material for moisture retention, but more inorganic and it needs to be free draining to allow water in to wet the roots and hydrate the organics but sufficiently open to allow good ventilation too. Aeration, drainage and moisture retention suggests a majority of larger rather than superfine materials.
The selection of the exact materials and the proportion of them becomes a matter of personal preference, experience and perhaps more importantly what you can get your hands on.
We are looking for inorganic and organic components.
The inorganic components may be porous (and thus water retaining) or impervious. Water retained in porous particles will be available to the roots but at the same time allows good ventilation of the spaces between the particles.
This is Castle Mountain Zeolite, sizing 1 to 3mm, and no fines. Zeolite is a volcanic material which is hard, heavy and porous. It is milled and so has very helpful sharp edges, good for fine root development and trunk flaring. Zeolite is also cationically active and so works to hold onto some of the positively charged fertilizer nutrients. Until recently I have been using 'Tiler's Sand' sold by the bag in hardware stores. It is coarse and sharp but still very small and I've been wanting to replace it with something larger. This zeolite product fits the bill perfectly.
This is Mt Sylvia Diatomite. It is fossilized diatom skeletons and almost purely silicon dioxide. It is porus and quite soft - chalky in fact. As you can see from the above picture the size range is quite large. The 'oversize' on the right is the material retained on top of a 6mm screen and the material on the left is the 'undersize' which goes through a 'flyscreen' size mesh. The screen used for the oversize is a garden variety one generally available at most hardware outlets, quite cheaply. The quantity of fines isn't enough to justify fine screening but some may chose to take out the oversize. Caution should be taken in screening to avoid breathing the dust.
This is scoria, another volcanic rock. Also very sharp and heavy it would be a very good inorganic component to use if you can get a supply. Depending on where you get it from it may need screening to remove the fines.
Other inorganics which may be good include; coarse sand - nothing less than 1mm and certainly not that fine white sand offered for kids sandpits, deco - decomposed granite or other crushed gravels. Unfortunately many of these products are neither screened or washed and so contain too much superfine, media clogging, dust and mud. They take a lot of work to process them at home, both wet and dry, to get them into a useable state. I have seen recycled teracotta crushed to gravel size for home garden use. If you could get this in the right size range it would be a very useful porous inorganic material. In some places it is also possible to get baked clay particles (made for kitty litter use). I know a guy who raids bull ant nests for the aggregate they put on top of their nests. It's great stuff at 2 to 4mm, and you have to admire his dedication.
At the light weight end of the scale there are pumice, vermiculite and perlite mostly readily available and suitable. Each product has it's own particular values and you need to weigh up your experience and information you can collect in making a choice. We should never forget that one of the prime purposes of a media is anchorage of the tree.
Once again it depends on suppliers available in your area. All of these things are good as long as they are clean, dust-free, not too big, not too small and sharp with a mix of porous and impervious.
And then the organics:
This is 7mm pine bark, slightly composted. It is nicely uniform in size with almost no fines. It is a size which contributes to free draining and at the same time offers the advantages of an organic component.
This is Searles premium potting mix. As you can see from this picture it contains a wide sizing range.
In this picture I've separated the overs and unders. The proportion of fines is large and too much to economically segregate out. The oversize is less and could happily be separated. Seeing the amount of undersize here is a disincentive to use a lot of this in a mix. Some of this fine organic material would however be nicely organically active in a mix as long as there's not too much to create a physical problem.
In this picture I separated the oversize to see what it's made of and you can see pine bark, decomposed timber chips, what looks like fly ash, a few river sand pebbles and a few particles of slow release fertilizer.
There is any number of packaged organic materials to use. Anything that has been composted and thus has a component of humus supplies some useful organic chemicals that promote healthy substrate flora and root growth.
If I could get some small grade charcoal I would use that too. This might be available through an orchid nursery.
Some people advocate using composted manures. These would be good for the nutrient boost and humus but I would be concerned about clogging up the mix and would rather see both nutrients and humus supplied by other means.
In composing a mix the big question is the ratio of organic to inorganic. Experienced long term growers of bonsai line up with agronomic research which suggests that the mix should have no more than 50% organic material. Some take this level all the way down to zero but for the bulk of enthusiasts a workable level would be 25% to 50%, but certainly no more.
Regardless of how much organic material you use the media will never supply all the necessary nutrients and trace elements. Additional fertilization is essential and as you reduce the organic content it becomes more essential, more often.
Some might ask if moisture retention and limiting organic content are not contradictory and in some ways they are.
But it would be better to have to adjust your watering pattern to meet the
needs of the trees in the best mix possible than to compose a sub-optimal
mix to maximise short term water retention.
From materials readily available to me and ones that I can use without too much processing at home, this mix ticks all the right boxes for bonsai in SE Queensland, which has hot summers and warm dry winters:
25% by volume of zeolite
25% by volume of diatomite ( screened to remove the oversize)
25% by volume of the 7mm pine bark
25% by volume of the Searles PPM (screened to remove the oversize)
And this is what it looks like when wet. Using two porous inorganic materials is helpful in our climate where hydration is so important and can be challenging.
A final point to make is a reminder of the value of the beneficial microorganisms in the soil that work with plants for a healthy root system and nutrient uptake. Typical of these is mycorrhiza which is abundantly obvious on pine tree roots. I find that cuttings always take better in media that has prevoiusly grown plants, perhaps an indication of healthy media flora, and so innoculating a new batch of potting mix with a few % of preloved clean material (even some recovered porous inorganic particles) might just give a repotted root-pruned tree a quick start to getting re-established.
The leaves are small and are similar to those of the yew;
strap-shaped, 10-40 mm long and 2-3 mm wide, with a heavy but soft texture. 
