Soil Concepts

Abstract

This is an article describing how soil and water interact in the kind of substrate you might use in a bonsai pot. The objective is to help build your intuition for drainage and aeration, what the terms mean and what factors in your growing media control it. It is written in layman’s language and I avoided most aspects of soil chemistry. It is meant to describe the important physical processes as they might be taught in a basic horticulture class. The concepts are not hard, but some of the terms might be unfamiliar. But if you familiarize yourself with the concepts, you’ll be well equipped to make informed choices about your bonsai substrate and answer questions like, “What grain size should I use?”, or “What does sieving do?”, or “Do I really need a drainage layer?”, or “What does ‘fast-draining’ mean anyway?”. The concepts aren’t magic and you won’t need some sage expert to tell you what to do – you can figure it out on your own. If I can do it, so can you!

Introduction

Questions about soil physics come up all the time and I find that I return to these concepts fairly often. Rather than continuing to churn, I thought we all might benefit from a short lesson so folks might refer to it when questions about the issue inevitably come up. I’m expressly NOT going to offer any advice on what substrate you should use with your trees, whether you
should use organics or inorganics, or anything along that line (that’s where these substrate discussions go south). What I’m going to do instead is offer some basic information one can use to base informed choices. We’re going to talk about what happens when we irrigate our bonsai and how the substrate properties can impact the amount of air and water that is trapped in the pore space. We’ll talk about what air-water ratios are healthy and what are not.
I’ll try to provide references that support qualitative distinctions such as “good” and “bad” ratios. But in truth there is optimal, less effective but workable, and really truly bad. I’ll try to make these distinctions where appropriate and known.
The reason I don’t like to make specific recommendations is because there is no single ideal substrate for everyone. It’s your garden and you are the one doing the watering so you’ll have to make your own choices. There are so many trade-offs – you can increase the air-filled porosity (more on this later) and you can get healthier growing conditions, but you have to water more often. It’s easy to underestimate how much more water your trees will need if you increase it too much. AND the plants needs change over time depending on humidity, season, temperature, size and shape of the pot, and many other factors. Hopefully at the end of this folks will be able to make some educated choices about the substrate they wish to use.

Summary

The main message is that there are a few factors controlling how much air and water are in the pore space of your substrates:

• Grain size, shape and composition
• The depth of the container, the thickness of the drainage layer and the properties of the drainage layer material.

If we use a fine-grained substrate in a shallow containGrain size, shape and compositioner we’ll have water-logged conditions in at least part of the pot for some time after watering. Not good, but we can mitigate it by adjusting some of the factors above. You’ll find that deep pots give you lots of options for optimal growing mediums, but those same substrates are sub-optimal or even deadly in some of the shallow containers we use for bonsai. Why? What’s the solution? How fine-grained is too fine? What are the trade-offs? Let’s get started with the basics.

If you use a fine-grained substrate in a shallow container you will have waterlogged conditions in at least part of the pot for a period of time after you water. You can mitigate it by adjusting the factors in the list above. There are lots of growing mediums that can work effectively in a deep pot, but many of those same substrates are not optimal in a shallow container. We
choose the container depth based on aesthetics. So what strategies do you have at your disposal to maintain optimal growing conditions due to that choice? It’s only one:

• Change to a substrate size, shape, or composition that improves the aeration

The drawback is that you’ll have to water more often because there is a tradeoff between air-filled porosity and water holding capacity. Higher AFP is clearly good and when the AFP drops below 10% it’s clearly bad. In between “high” and 10% there is a wide range of less effective, but workable AFP. You’ll have to find your own suitable conditions in your back yard for trees
under your care. But the theme of this resource is to give you the tools and knowledge you need to make informed decisions about what the trade-offs are.

It’s all about capillary action…

but what the heck is capillary action anyhow? even if you don’t think about capillary action very much, it is happening all the time. Capillary action describes the behavior of fluids due to the forces of adhesion, cohesion and surface tension and it’s happening all around us. It’s the movement of water within the spaces of a porous material due to adhesion, cohesion and surface tension. It’s easier to understand than you might
think. I’ll try to help build some intuition on how this works because it ties together everything else we’re going to talk about. First, we’re all familiar with the picture to the left, aren’t we? We probably all did this in our high school physics
class.
In the experiment, water is drawn up the capillary tube against the force of gravity. The height it reaches is related to the diameter of the tube – it goes further the narrower the tube. Why? Well, because water is sticky, thanks to cohesion (water molecules like to stay next to other water molecules) and adhesion (water molecules like to stick to other substances). See how
the water curves upward where it comes in contact with the glass? That’s due adhesion of water and the tube – it causes an
upward force where the water comes in contact with the glass and so the meniscus turns upward. That force is higher in narrower tubed. The water travels until the upward capillary action due to adhesion is exactly balanced by the force of gravity pulling the water down.
(Credit – Age of Rieson, http://rieson.blogspot.com/2012/12/capillary-action.html ).

Not only does water tend to stick together in a drop, it sticks to glass, cloth, soil, tree fibers, and the fibers in a paper towel. Dip a paper towel into a glass of water and the water will climb onto the towel. In fact, it will keep going up the towel until the pull of gravity balances the capillary forces, just like in the capillary tube experiment.
In some materials the tendency for water to adhere to the surface is stronger than the tendency for water to stick to itself. Water will readily invade the pore space of these materials. In other materials, however, the tendency for water to stick to itself is stronger than the tendency for water to adhere to the material. In other words, water will tend to not wet the surface, but to remain beaded up like a little ball. Lot’s of plants have leaves that are non-wetting – that way when it rains the water just runs off rather than sticking to the leaf surface weighing it down..

Watering your bonsai trees would be a lot trickier without capillary action. Your trees put down roots into the soil which are capable of carrying water up from the soil into the plant. Water, which contains dissolved nutrients, gets inside the roots and starts climbing up the plant tissue. As water molecule #1 starts climbing, it pulls along water molecule #2, which of course, is dragging along molecule #3, and so on

How does Capillary Action work in Soils?

Capillary action works in the soil the same way it works in the capillary tubes. Water moves upward through soil pores (the space between the soil particles) because it has the tendency to adhere to the solid grains the same way it has the tendency to adhere to the glass in the capillary tube. The height to which the water rises is dependent on the pore size. The smaller
the pores, the higher the capillary rise.

But soil doesn’t have just one pore size – it has a range of pore sizes. So it’s more complicated than water rising through one constant diameter capillary tube – it’s like ALL the capillary tubes are there at the same time! Near the base of the soil column water will invade all the pores sizes – large and small – the same way it can get into all of the capillary tubes. As a result, the average saturation there will be quite high. Near the top of the soil column, the larger pores will be air-filled and only the smaller pores will have water in them so the average saturation will be lower. Make sense? Water won’t be distributed uniformly in the pore space – the soil will have higher saturation deeper and lower saturation shallower. We’ll look at this in more detail, but here’s another good reference for further reading.

Let’s define some terms

1. The porosity is the fraction of your soil that’s air. So if you have a liter of soil and 50% of it is air space, it’s said to have a 50% porosity. Most soils have 50% to 80% porosity.
2. The air-filled porosity (AFP) is the portion of that porosity that is air after the soil has been irrigated and the gravitational water has drained away. In other words, if you completely saturate the soil and then let it drain, some of the pore space will be occluded by water due to capillary attraction binding the water to the soil against the force of gravity. So it’s a number less than the total porosity and is an average for the whole volume of soil.
3. The water-holding capacity (WHC) is just the opposite – it’s the portion of the porosity that is water after the soil has been irrigated at the gravitational water has drained away.
   AFP + WHC = total porosity. You’ll also hear this number referred to as the “field capacity” (when you’re talking about soils in the field) or the “container capacity” (when you’re talking about soils in a pot). This is the water available to the plants after you turn the hose off – so it’s what the plant has to live on until the next time you water.
4. The water saturation is the fraction of the pore space that’s occupied by water. This number changes over time – it’s high when you water and get’s lower after the gravitational water drains away.

Microporosity vs macroporosity

There’s an additional concept I’d like to introduce while we’re on the topic. I want to talk now about “dual porosity” systems. The phrase refers to soil medium or geologic formations that have more than one population of pore sizes – like a population of large pores AND a population of small pores. There are lots of ways that this can happen in nature, but in a bonsai pot we’re typically thinking about the difference between INTER granular porosity (the pore spaces between the grains) vs INTRA granular porosity (the pore spaces within the grains). This graphic will help:

See? There are large spaces between the grains (the intergranular macropores) while
there are small pores within the grains (the intragranular micropores). Now that we’ve
established the basic physics, we know what this means – if there are two populations of
pore sizes, then there are two populations of water saturations as well. The macropores will have high AFP and low water saturation whereas the micropores will have low AFP and high water saturation. This is a good thing! We can have a high AFP (that the plants like), but store additional water in the micropores which will increase the humidity of the system for a longer period of time than we might otherwise expect.

Let’s look at a couple of examples of microporosity. The picture on the left is a photograph of a piece of wood taken through a
microscope. Look at all the microporosity in there! What you’re looking at are the multitudinal units called cells which make up wood. Lot’s organics look like this – it’s one of the reasons that they hold so much water. But inorganics can have a high microporosity too. Take a look at the image on the left – that’s a picture of a piece of pumice.

Pumice is a naturally occurring rock erupted from a volcano. It’s erupted as a liquid (molten rock) that ascends very quickly through the earth. As it ascends, gas (water vapor, CO₂, etc) evolves from the liquid and forms bubbles. Then when it erupts the liquid quenches forming a glass and preserving the
bubbles – that’s what all those holes are. Scoria (we call it lava rock) has the same thing. The
spaces the bubbles formally occupied are also intragranular porosity and can hold water the same as the cells in the piece of wood. That’s partly why different material – even if it’s the exact same size and shape – will hold a different amount of water!

What are we aiming to achieve?
Good potting mixes generally have an AFP of at least 15-20% and a water holding capacity of at least 30%. And you don’t have to take my word for it – there are several decades of horticultural and agricultural research that will tell you the same thing. Waterlogged soils are a bad thing for almost all plants. You want good aeration in your soil mix. Personally, I like to keep the AFP quite high – 25% or more. Of course, that means you have to water more often, but that’s just the price you pay. Here
are some facts:

• As a general rule, less than 10% AFP and your soil is waterlogged – plant growth suffers. Too long under these conditions and plants may die (minus a couple of notable exceptions that have adapted to surviving these conditions). You don’t have to take my word for this – there are a ton of studies that have demonstrated that this is the case. There are tons of reference about this – I’ve included a bunch in the references, but there are a lot more.
• Increase the AFP and increase the growth rate, but you’ll need to provide more water and nutrients. Increase it too much and the trees will not have access to enough water when you take the hose away. Decrease it too much, and plants will stay wet and growth rates slow down dramatically. As AFP approaches that magic 10% number, growth rates are minimized and plants can easily succumb. I like mine high and I just water more often. But we all make our own choices.
• The optimal AFP can be pretty species specific too – some plants (maples, pines) grow best with a higher air-filled porosity. Others can tolerate a much lower AFP than most plants. Bald Cypress and Water Elm, for instance, can tolerate long periods under water logged
  conditions, but they are specialized to do so. In general though, if your soil stays too moist it encourages soil pathogens such as phytophthora and soil collapse making it difficult to care for your plants.

I hope what you appreciate is that the two most important physical properties of a potting mix are AFP and WHC. When we say “good drainage”, what we really mean is “good AFP”. It’s the single most important physical measure of a good soil mixture. We say “good drainage” because AFP is hard to see when we water, but we can see when a soil allows water to pass through rapidly. Soil which has this characteristic typically has a high AFP. But “good drainage” is not a measurable quantity and AFP is. In fact, you can do it at home on your own. It’s easy, but that’s a different topic.

So What Factors Control AFP?
AFP and WHC are controlled by the capillary properties of the soil. They are a result of the small pore spaces and due to the fact that soils particles are “water wet”. In other words, water tends to adhere to the soil particle. When you irrigate, all the pore space is temporarily filled with water. When you stop, the water that’s not held by capillary forces drains out of the pot due to the force of gravity. When it stops, there is still water held in the pore spaces due to these capillary forces (we’ll touch on this several times). The key factors that control AFP are:

1. Grain shape and size

• Rounded particles have lower porosity, angular fragments have higher porosity. However, angular grains tend to have a smaller pore size so tend to stay wetter and have a lower AFP than rounded grains. Compare the angular and rounded grains in the figure on the
  right. 200 micron rounded grains have a porosity of about 46%. 200 micron angular grains have a porosity of about 55%.
  Porosity tends to go down a bit as the grain size increases (counter-intuitive, right?). But the effect is not huge. In fact, for the grain sizes we’re dealing with it doesn’t change much at all. What’s more important is that the size of the pore space is strongly effected by the grain size. So as you increase the grain size, the pore size goes up significantly, so the AFP increases and the water-holding capacity goes down.

2. Sorting

• A non-uniform grain size has low porosity and small pore sizes because the fine-grained material tends to fill in the pore space between the larger grains. Therefore if you make your soil more uniform in size, it tends to have a higher AFP, and lower water-holding capacity.

• If you choose to use a non-uniform grain size for your soil mix (i.e., you don’t sieve) be aware of the issue of stratification. Here’s a nice experiment performed by Davetree. See what happened? Davetree mixed a range of grain sizes and through successive waterings the fine grained fraction migrated down to the bottom leaving the coarse fraction at the top. If this happens in your pot, it will influence the water saturation profile you should expect.

“Finer, heavier grains may sink to the bottom of the pot while lighter, larger grains can float to the top upon successive watering. This may impact the growth of your plant and the amount of water and air available.”

So what happens when you water?
So when you put your soil in a pot and add water, the water will first fill the pore space in the soil (as long as you can add water faster than it spills out the bottom of the pot). For all intents and purposes, the pore space is fully saturated as in the figure on the right. All of the available pore space is filled with water. When you stop watering, a bunch of water will continue to drain out the bottom of the pot. That’s called gravity drainage – the capillary forces in the soil are just not strong enough to hold that much water in the pot. But at some point, the capillary forces and the gravitational forces will be in balance and gravity drainage will stop.

Once the gravity drainage stops, there will still be water trapped in the pore space – that’s the capillary-bound water and the soil and water are in capillary equilibrium. At this stage the soil is at “field capacity” (if you’re in the field) or “container capacity” (if you’re talking about potted plants). It’s the maximum amount of water that the soil can hold by capillary forces alone.

In between waterings, the water saturation in the soil continues to change and the water in the soil will fall below container capacity. There is water loss through the soil surface due to evaporation and there is additional water loss through the plant due to transpiration. It will continue to fall until all the capillary bound water is gone. But the water saturation is not zero. There is still a small amount of water in the pore space held by capillary or electrostatic forces so strong that the plants can’t access it. This is called the “wilting point”. The remaining water in the pore space at the wilting point is called “hygroscopic water”.

Too long at the wilting point and plants will not recover turgidity when you water them again. So your goal is to maximize the time the plant spends at the optimal air-filled porosity above the wilting point without killing yourself by watering every 15 mins. Tricky balance.

But is the water saturation the same everywhere in the pot?
In a word, no. Remember those capillary tubes? They’re back. Right after you water, the saturation is high at the bottom of the pot and decreases upward until it reaches what’s called the irreducible saturation – that’s the amount of water bound to the soil grains by capillary forces. Since the profile is
controlled only by gravity, the only thing that matters is what the soil is and how high above the bottom of the pot you are. The water saturation increases downward until it’s almost 100% saturated.

Does the height of the pot matter?
In a word, yes. Look at the figure to the left. The taller pot has a lower average water saturation and higher air-filled porosity than the shorter pot. The saturation curves are identical and the equilibrium saturations are dependent only on the height above the bottom of the pot and the type of soil that you are using. When you switch to a shorter pot, you just lop off the lower water
saturation part of the soil profile and keep the higher saturation part on the bottom. So shorter pots tend to have a higher average water saturation than taller pots. Weird, but the bottom line is that if you’re using finer grained, more angular, or more poorly sorted soil components, you’ll get a taller saturated zone at the bottom of the pot and higher water

saturation throughout the soil. And if you use a shallower container it will have lower AFP and higher water saturations than if you used the same soil in a deeper pot.

Barboza, SA, 2016, “Introductory Soil Physics 1.2”, Journal of the American Bonsai Society,visualization 17/7/2024
https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.bonsainut.com/threads/introductory-soil-physics.25258/

Barboza, S.A., 2017, Soil Concepts: Part Four – What’s Behind the Curtain?, Journal of the American Bonsai Society, V. 52, N. 1, pp. 20-25, visualizzato 17/7/2024