A good bonsai matrix/soil
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It’s interesting to me how complicated people make the subject of creating a beneficial soil for bonsai, or for any plant in a pot for that matter.  It’s not hard to do, but it must start with the right concepts.

Actually, since a good potting soil is such a specialized product, it would rightfully be called a “potting matrix”. For there is often no natural soil involved in it.

Let’s start at the beginning.  We’ll use the term “soil” since it is the term most people are acquainted with, no matter what the particles are composed of.

To make a very  healthy bonsai soil,

one must keep several  characteristics in mind.   First, it must hold enough water so  as to stay moist from the last watering to the next.
At the same time, it must be able to hold enough air so as to keep it from being waterlogged.  Also, the air must be sufficiently distributed so the air pore spaces  don’t  make such large voids that they can become completely dried out and without access to water.  Thus the air pores must be carefully sized so the edges of the particles around the air pores can maintain a layer of water on them; that is, the water layer can be continuous without filling all the room between the matrix particles and thus – even for a while – fill the matrix so completely with water that there is no room for air.

Spoken differently, it is critical to have, not just enough air in the mix, but to have all the air pores the right size.   To do this we must have all the particles of the right size, and, to whatever degree it matters, the right shape as well.

  The right level of acidity and/or alkalinity

(the pH balance) is also necessary. The correct level  is usually somewhere around a neutral or pH balance, which is called a pH of 7.
This is important, not only for the bonsai, but for all the oher plants growing in the same pot: the moss(if any) and the beneficial organisms growing in the soil which are bacteria and the correct form of fungi which live in conjunction with the plant’s roots.

So if we have the right amount of water, the right amount of air, the correct air pore sizes and the correct ph, we are well on our way to having a very healthy environment for our bonsai.

One question with a non-obvious answer remains: how to get the air pore spaces of the correct size.
The answer lies in the fact that we must have the correct soil particle sizes.

To maximize the air in the mix

we must have all the particles of the same size.  If there are many different soil particle sizes, the small ones will fill in the potential air pores among the other larger particles.  Not to belabor the point, but just in case any confusion remains, consider the following mind experiment:

Fill up a container with large particles (all the same size), pebbles, for example,  and then fill in the remaining room with some fine sand.  Shake it a little bit.  The result will be no growth in the volume of the soil; it will all go into the voids between the larger soil particles, thus only reducing the air in the mix (which we definitely don’t want).

Therefore it becomes obvious that all the soil’s particles must be the same size (approximately) so as to maximize the air and optimize the air pore space sizes (optimized if we start out with the correct soil particle size to begin with) .

That particle size which works best has been found by soil scientists and many bonsai  experts –here, in Japan and in China too,  – to be between ¼” and 1/8” .

No larger particles (which simply take up space and don’t allow roots to grow in that area), and no finer particles (which take up air space and, while allowing roots to grow in those areas, reduce the total air and oxygen available to the plant and thus increasing the possibility of anaerobic microorganisms and disease.

Some people add peat moss or other  fine organic matter, but it increases the likelihood of disease.  There are other organic matter components which we can use that will also hold water, and are much healthier for the plant.

A number of companies, headed by the better bonsai experts, sieve and create such balanced soil mixes, all of which are created  of more or less similarly sieved particle sizes, and we would recommend such a mix for your bonsai.

There are a few other tricks that can help, and here we’d like to toot our own horn a bit, and explain how we have found to make the best soil possible, but we’ll post that in our next newsletter.