As we said in part 1 of A Good Bonsai Soil Matrix, perhaps the most important aspect of a good soil mix is the size and quantity of the air pore spaces. They must be of the right size and the soil must have enough of them to maintain plenty of air in the mix, as much of the time as possible.
Were we to put very fine particles in the mix with the larger, primary sized particles, they would filter into the air pores and, to some degree, clog them up. The amount of air in the mix would be reduced.
Let it be said again, for it is so important: to maximize the useable air in the mix and optimize the air pore space size, we must have all the soil particles of roughly the same size. *
If some of the particles were much larger, they would simply take up space and reduce the amount of useable matrix that the roots could grow into and, If some of the particles were much smaller, the roots would be able to use that area and grow into them, but the air pores would be so small that they would likely stay filled with water and thus promote pathogenic anaerobic microorganisms.
Air is critical to the roots.
Consider a pile of garbage. When it has been just piled it may still have some air in it, and won’t smell too rank. However, put it in a closed black plastic garbage bag in the sun for a few hours, and, as we all know, it will begin to smell horrible. In fact, it smells horrible because it is horrible. Our noses give us warning that something is amiss – and the gases given off by the rotting vegetative mass in the bag are our warning.
I remember doing a demonstration a few years ago and was brought a plant – quite well styled, actually, that was clearly sick. The symptoms (dying leaves at the end of the branches, especially dying back around the edges and the look of a soil whose particle sizes were way too small – or non-existent – to hold any air).
Immediately, without looking at the plant further I said “it needs repotting and the soil changed
“With such a quick cursory glance that I took, some of the audience members were skeptical. I, across the room, and too far from the object to perceive its odor,nonetheless suggested they smell the root mass. That settled the discussion, for it was fairly rank and clearly the reason for the plant’s distress.
In the same fashion, we can smell a beneficial environment for the roots: the rich loamy soil which by its smell indicates it is filled with healthy root-promoting microorganisms, most usually the mycorrhizae (meaning “fungus root”) which act like an extension of the root system and help to keep it healthy.
In fact, some plants can’t grow (or at least not grow well) without these beneficial microorganisms. One way to show this is to take a plant that seems to require them and dose the root ball with one of several fungicides. Although other plants might thrive, the fungicide, while not damaging the roots directly, will kill the plants because the extensions of the roots – the good guy fungi – will be destroyed.
The best and easiest way to make certain beneficial fungi live in the soil is to deliberately inoculate it with some soil from a healthy plant of the same or similar species.
The Japanese have done this for hundreds of years
with plants of varieties which are known to need these fungi. Very few pines, for example, almost always are treated in this way when being planted or transplanted,
To extend your knowledge of what species of fungi grow well with what species of pine, experiment with different inoculants to see which do best with which trees. You may be surprised by the difference in growth.
*An interesting experiment can be made by placing larger particles of soil at the bottom of the pot. It’s a tradition of long-standing but isn’t healthy for the plant, and proves the need for the soil particles to be of similar size.
The reason is that, at the bottom of every soil layer, there is a saturated layer of soil, called a “perched water table”. It will remain in that soil. Thus there will be even more saturated soil (in two levels) which can’t be made use of by the plant.
Believe it or not, we could even create a third perched water table if we were to add a third layer of a different sized soil particle (not that we would want to, of course).