C O L I N   L E W I S   B o n s a i   A r t
B O N S A I   S O I L S   P A R T   3

P A R T I C L E   S I Z E   &   R E L A T E D  I S S U E S

Let's look again at the requirements that a good bonsai soil must fulfill and analyze the relevance of various particle sizes to each function.
Nutrients are absorbed while in solution in water, so these two functions are closely linked. They are both influenced far more by the nature of each ingredient than by the size of the particles, with one obvious exception: The smaller the particles, the more water will be held in the inter-particulate spaces by surface tension (sand holds more water than gravel, silt even more). A soil comprising very small particles will inevitable be wetter and thus hold more nutrients than a soil comprising larger particles of the same ingredients.

But retention of moisture and nutrients is only half the story, both also have to be made available to the roots. Roots absorb by direct contact with water or water-bearing solids. Larger particles have larger spaces between them so the roots cling to the surfaces of the particles and are therefore only in contact with a source of moisture along one side - the other side being exposed to the empty inter-particulate space.

This causes the roots to become flattened in order to maximize contact, and to increase in length and girth rather than to ramify. They grow rapidly between the particles until they reach the side of the container, whereupon they begin to spiral around or extend directly downward.

The exception to this is Akadama: Roots are able to grow into and through the grains of Akadama so they can ignore the vacant spaces and ramify freely within the grains, breaking them down into smaller particles as they do so.

In general, the coarseness of the vegetative growth is directly proportional to the coarseness of root growth and thus, in turn, the coarseness of the soil.

Rapidly growing roots produce large amounts of the hormone cytokinin, which stimulates shoot growth.  Heavy extending roots produce heavy extending shoots; finer ramified roots produce finer ramified shoots. If you don't believe me, try tipping a bag of builder's sand on the ground and letting the weeds grow in it for a season. Check out the roots, and you'll wish you could get roots like that on your bonsai - fine, vigorous, white feeders. Then try the same with a bag of gravelů.

For the purposes of this discussion, we'll define sand, grit and gravel thus: Grit comprises grains of mineral between 1.5mm and 3mm. Anything smaller is sand, anything larger is gravel.

The main reason we are so emphatic about good drainage is to ensure there is oxygen in the soil, which  is why these two functions are treated together. The bacteria that cause root death and decay are anaerobic, which means they thrive in soil where there is little or no oxygen. Furthermore, the roots and valuable microorganisms in the soil need a certain amount of oxygen to function.   

Clearly, very fine particles drain more slowly than very large particles, but they do still drain efficiently enough for our purposes (sand drains rapidly, certainly more rapidly than we need a bonsai soil to drain, yet gravel drains even faster). On the other hand, builder's sand would hold more water by surface tension than would gravel. In theory, sand would make a wonderful bonsai soil if you could irrigate it several times a day!

Optimum drainage is achieved through pure non-absorbent mineral with particles of around 4mm. Smaller particles drain just as well, but more slowly. Larger particles add little or nothing to drainage efficiency and become mere obstructions to root growth. One simply has to find a balance between the need for rapid drainage and the need to fulfill all the other functions of a soil.

There is another factor that is a significant influence on the rate and efficiency of drainage: the shape of the pot.  A shallow container, no matter how well endowed with drainage holes, will drain less water than a deep container of similar volume, and is far more likely to become and remain waterlogged. The old "bath sponge experiment" illustrates this perfectly (see below). However, a shallow container will dry through evaporation far quicker than a deep container, particularly when the surface is not dressed with moss
You must have noticed this when you were a kid....  Take a bath sponge and saturate it with water, then lay it flat. The water will drain down but leave a saturated layer about a quarter inch deep. A sponge 5"x4"x1" would drain to five cubic inches of water. 

Now stand the sponge on edge and it will drain more, leaving a similar quarter inch saturated layer, but only one and a quarter cubic inches of water.

This layer of water is called the "perched water table" and is a major consideration in the efficiency of bonsai pot drainage.

Moisture is also lost by evaporation, especially when the soil is fresh and the roots have not colonized the pot. When the soil is warm larger particles will dry quicker than smaller particles because water vapor can travel to the surface more freely. When the soil is too cool to cause evaporation small particles can dry faster than large particles as the capillary action draws moisture to the surface to replace that lost to the wind.

But what about oxygen?
Now, let's be rational here: we're not talking about vast empty caverns between chunks of rock. Look at all those healthy trees around you, in the parks, in the street, even. Their roots have access to oxygen because the soil in which they grow is alive and drains reasonably well. The oxygen is held in countless gazillions of microscopic pores. Granted, they have massive root systems that operate at different levels in the ground whereas, by comparison, our poor trees have very restricted roots and no opportunity to migrate to different soil conditions. But enough oxygen is enough, and over-size air pockets are just as much a waste of space as oversize particles of gravel. If your soil drains reasonably well, it will contain more than enough oxygen for your tree's requirements.
STABILITY Okay, I know, you wire the tree in the pot. But then, when the roots have filled the pot you're supposed to cut the wire off, right? If you don't, you're risking damaging the nebari, and you're preventing the tree from rising in the pot as the roots increase in volume - result: compacted and inefficient roots.

So, assuming you're a conscientious, responsible bonsai grower and you cut the wire from the nebari, the soil has to be such that the roots can bind the particles together to form a cohesive, stable root 'pad'. This should happen with an established healthy tree in one season at the most. If it doesn't, something is wrong with your soil: either it is inhospitable to roots for some reason (see Part 1 and Part 2) or the particles are so large that the roots cannot bind them together.

There is so much discussion about ingredients, sizes, drainage and so on, that there's a danger of ignoring the prime purpose of having soil in the first place, ie: to provide a healthy environment for roots to grow in. And based on the premise that healthy roots mean a healthy tree, this is the most important function of all.

Akadama, the best medium on the planet for maples, at least, is often criticized for breaking down after a year. So what? It still drains well enough - just a little slower maybe; there are still oxygen pores; and the growth of fine feeder roots is phenomenal. Why is this? Because, unlike other mineral ingredients, roots can grow into and through the particles which means that they have virtually 100% of the pot volume in which to grow unimpeded. It also means that the roots are in contact with moisture-bearing matter over their entire surface, so they are finer. Plus, the ease with which they can grow means they ramify prolifically.

With solid mineral ingredients, of course, we want as much inter-particulate space as possible for roots to occupy. However, the size and number of the individual spaces is at least as relevant as the total volume. Smaller particles do not mean less space, they mean smaller spaces - the volume of space remains the same

Both illustrations are 50% screen: there are equal areas of black and white. Imagine the black as mineral and the white as space and you will see that the proportion of space remains constant regardless of particle size

Since we have learned that smaller particles encourage finer, more ramified feeder roots, and larger particles encourage heavier, more structural roots, we can adjust the particle size according to the stage of the tree's development without affecting the amount of space available.

Note: To compensate for the effect of surface tension, the smaller the particle size of your soil, the greater the proportion of
drainage mineral you should add to the mix. 

Our task is to arrive at a mixture that offers close to 100 percent of the pot volume for root growth, drains well yet holds water at 'field capacity', retains oxygen and provides stability for the tree. How you achieve all this and what ingredients you use is up to you, but here are a few general tips....

In many cases the particle size available to you will depend on the products you acquire. Lava, Akadama,  pumice, Turface, etc., all have a specific size range, and grading it for the perfect mix can take some time. Standard Turface is a good size for general purposes, but lava tends to be larger. I sift out the dust and the larger particles of lava and try to match the Turface. Similarly, Haydite has a larger aggregate, so that should also be sifted to size. Both lava and Haydite can be crushed and re-sifted with care, but it's hardly worth the effort.

If you choose a large particle soil (4mm or larger) please don't use gravel for drainage. The inter-particulate spaces between lava, Haydite, pumice or whatever will provide enough drainage (provided they are not clogged with nasty bark) and the gravel will merely use up room and will contribute nothing. I have little experience with pumice, but from what I have observed, its soft surface tends to induce finer root growth, which may excuse the use of slightly larger particles.

If you use organic matter, please don't use commercial bark, and please, please sift out all the fines. Organic particles can be a little larger than other ingredients, but anything less than 3mm - or soft unstable particles - should be discarded.

Important note: Add organic matter to your mix immediately before use, not in advance. Ingredients such as Turface, Akadama, Haydite, pumice and even sand, will suck all the moisture out of the organic matter, making it easy to crumble to dust and difficult to re-wet.

My preferred particle size for drainage material is 1.5 - 3mm (#1 filter sand or starter grade chicken grit), no larger. In theory 3mm (#3 filter sand) particles would be ideal, but when you consider that smaller spaces induce finer roots and reduce evaporation, the inclusion of smaller grains of grit to increase the number of smaller spaces can be a benefit. I commonly use a mix containing at least 65% grit (as defined above) for pines and some junipers.