Introductory soil physics

[Ujjawal Roy]

Hi Scott, I read most of the messages and comments in the previous 14 pages of this article and I have this question that I would like to ask you. So, once the top soil dries up (and I'm talking something about turface or calcined or any particle that can absorb and hold water inside itself in general) or starts drying up due to surface evaporation, what happens to the perched water table? Does the water from saturated zone travel up (and by that I mean gets absorbed into the drier particles) or does it just drain out overtime or does the excess water just straight-up evaporate? I feel the latter is kinda not possible. Can there be a substrate that has high AFP as well as WHC? And if there is such a substrate is there a chance that roots will drown even though they are getting oxygen through the pores and water out of the particles? From what I understand, if there are air gaps between the wet particles (absobtion and not ADsorption) there shouldn't be any root rot as they are getting oxygen as well as water in the form of moisture. I would appreciate if you could elaborate on this topic. Thank you!

 

[63pmp]

Perched water table will only rise as high a capillary action will allow, depending on particle size. You do not want a perched water table in your pots as it will become a low oxygen zone in wet weather and harm roots. This often happens in root bound pots, the tree is OK while it can suck up the perched water table during routine transpiration, but once you get a week or two of wet weather the lower roots start to suffocate and pathogen attack can occur.
Better to water twice a day than run risk of perched water table.

My understanding is that water in porous particles is unavailable to plants because as soon as the water film between roots and particle is broken, water can longer be transfered to the roots, the water in the particles does help maintain soil humidity.

 

[Ujjawal Roy]

Perched water table will only rise as high a capillary action will allow, depending on particle size. You do not want a perched water table in your pots as it will become a low oxygen zone in wet weather and harm roots. This often happens in root bound pots, the tree is OK while it can suck up the perched water table during routine transpiration, but once you get a week or two of wet weather the lower roots start to suffocate and pathogen attack can occur.
Better to water twice a day than run risk of perched water table.

My understanding is that water in porous particles is unavailable to plants because as soon as the water film between roots and particle is broken, water can longer be transfered to the roots, the water in the particles does help maintain soil humidity.

I've already encountered root rot and top growth death on my plants because of overwatering. Now, what I mean to ask is - let's consider my soil is coarse and granular and also absorbs water inside each particle, will there still be a perched water table in the lower layers on my soil if I see the top soil going dry? People say one should water his trees when the top layer looks dry but i fail to understand what happens to the lower layers as it's out of sight. What if the lower layer is still saturated with water but the top soil is dry. How to know what going on in the lower layers?

 

[63pmp]

If your soil is composed of coarse particles it would be unlikely to have a perched water table, even directly after watering. However, this depends on what you call "course", as I might think that your soil is composed of only medium sized grains. You can check for perched water table by getting a clear container, putting drainage holes in it, fill with potting mix and pour water over it. You should be able to see where the water pools, if it does.

Also, I'm not sure what you bonsai containers are, if the are wide with a single, centred drain holes, like Italian terra cotta pots, then it is likely a puddle will form, even if completely level. Pots need to drain from the edges. Will post a photo later to help with this.

I've already encountered root rot and top growth death on my plants because of overwatering. Now, what I mean to ask is - let's consider my soil is coarse and granular and also absorbs water inside each particle, will there still be a perched water table in the lower layers on my soil if I see the top soil going dry? People say one should water his trees when the top layer looks dry but i fail to understand what happens to the lower layers as it's out of sight. What if the lower layer is still saturated with water but the top soil is dry. How to know what going on in the lower layers?

 

[63pmp]

If your soil is composed of coarse particles it would be unlikely to have a perched water table, even directly after watering. However, this depends on what you call "course", as I might think that your soil is composed of only medium sized grains. You can check for perched water table by getting a clear container, putting drainage holes in it, fill with potting mix and pour water over it. You should be able to see where the water pools, if it does.

Also, I'm not sure what you bonsai containers are, if the are wide with a single, centred drain holes, like Italian terra cotta pots, then it is likely a puddle will form, even if completely level. Pots need to drain from the edges. Will post a photo later to help with this.

I've already encountered root rot and top growth death on my plants because of overwatering. Now, what I mean to ask is - let's consider my soil is coarse and granular and also absorbs water inside each particle, will there still be a perched water table in the lower layers on my soil if I see the top soil going dry? People say one should water his trees when the top layer looks dry but i fail to understand what happens to the lower layers as it's out of sight. What if the lower layer is still saturated with water but the top soil is dry. How to know what going on in the lower layers?

 

[63pmp]

This is a Chinese pot with poorly formed base, it has a big hole, but doesn't drain well with water ponding at the edges. It needs at least 4 new holes for it to drain better.

The next pot is a handmade pot, which unfortunately cracked during firing, and extended with use. It is 9 inches wide with a 3/4 inch central hole. The bottom of the pot is perfectly flat so that if the pot is only slightly tilted it will cause water to pond. Also it is a very small drainage hole preventing air flow into the potting mix. 3 additional holes were added after firing to aid drainage.


Of course, the more holes the better!

 

[63pmp]

Comparing with a Japanese pot. This pot was made in the seventies, and has interesting features. It's 21 inches long.
And 3/4 of an inch deep at the edges.

The bottom of the pot is completely flat.

But the inside is tapered towards the centre to aid in drainage.

Note that the outside holes are close to the edge, this pot is designed so the it can tilted to assist with draining after watering or during wet weather. Shallow pots struggle with perched water tables, these design features overcome that problem.

The shallowness of the pot means aeration is not a problem here, adequate air movement will occur with water pulling air into the soil after watering. Additionally, only a madman would modify a pot like this.

 

[Wires_Guy_wires]

People say one should water his trees when the top layer looks dry but i fail to understand what happens to the lower layers as it's out of sight. What if the lower layer is still saturated with water but the top soil is dry. How to know what going on in the lower layers?

This a mechanical story of cohesion, adhesion and capillary action. Water adheres to the soil, but it also coheres to itself. A well draining pot will draw water out by cohesive force, like straining tea of coffee, simple gravity. The water that stays behind is locked inside the particles, adhesion, if those particles make contact with each other, water can cohere to itself again; capilary motion takes place. Cohesion and gravity act like connecting two droplets on a foggy window; they make a bigger droplet and gravity starts working. In the capilary sense: a sponge can fill itself to the top if you put the bottom in a cup of water, just like our soil particles act. But this capilary action only works if all particles make contact and if the soil particles have a stronger adhesive force than gravity (more pores = more force.. Water doesn't stick to metal, it does stick to rock, and it sticks even better to sponges).

If the particles don't make contact, every soil particle has it's own water table inside of itself, if water can enter. If they don't make contact, this means they're surrounded by air and they will start evaporating water, creating a damp environment around them.

The Japanese took this physical behavior of water and soil particles to the next level: coarse particles on the bottom so cohesion will occur and draw out most of the water AND let a lot of air in by replacement of substance (if you remove water, you get air, to get air out, just add water). The smaller particles up in the top layer are there to actually hold water for roughly a day. We give our plants a multitude of 'total pot volume' of water, just so those cohesive forces can work their magic. My pots are 250mL in some cases, I water them with roughly 1000mL.

If a soil is well composed - and it rarely is perfect - then it should dry out on the bottom by drawing in air and by the forces of gravity, and it should dry on the top because of evaporation (and adhesive forces from the water leaving the pot on the bottom). So a well composed soil should roughly dry as fast on the top as it dries on the bottom.

Our plants have less issues in colanders because here the evaporation takes place on six sides instead of one (and the pot holes). Only the center of the pot stays damp a bit longer than the rest.

Unglazed pots for conifers are not just used because they look good, the lack of a glaze also helps water evaporate on all sides, albeit a bit slower compared to a colander. Everyone eventually gets salt buildup on their unglazed pots because of this.

If your lower layer is saturated with water, it shouldn't pool! If it pools, then gravity doesn't work and you'll end up with a dead tree. That's why we all want holes in our pots. Pooling = lack of air = suffocation and fermentation = anaerobic microbe secretions and anaerobic pathogens = dead plant.
The whole "fast draining but water retentive" phrasing is kind of a mind fuck, but once you understand the mechanics, it starts getting fun to play with it.


Some of these forces can be bypassed by surfactants too! A super dry soil will be water repellent, a drop of soap will fix that issue right away.

 

[Ujjawal Roy]

Comparing with a Japanese pot. This pot was made in the seventies, and has interesting features. It's 21 inches long.View attachment 346040
And 3/4 of an inch deep at the edges.
View attachment 346041
The bottom of the pot is completely flat.
View attachment 346042
But the inside is tapered towards the centre to aid in drainage.
View attachment 346043
Note that the outside holes are close to the edge, this pot is designed so the it can tilted to assist with draining after watering or during wet weather. Shallow pots struggle with perched water tables, these design features overcome that problem.

The shallowness of the pot means aeration is not a problem here, adequate air movement will occur with water pulling air into the soil after watering. Additionally, only a madman would modify a pot like this.

This pot looks great for bonsai, it meets most of the good horticulture needs of the tree!

 

[Ujjawal Roy]

This a mechanical story of cohesion, adhesion and capillary action. Water adheres to the soil, but it also coheres to itself. A well draining pot will draw water out by cohesive force, like straining tea of coffee, simple gravity. The water that stays behind is locked inside the particles, adhesion, if those particles make contact with each other, water can cohere to itself again; capilary motion takes place. Cohesion and gravity act like connecting two droplets on a foggy window; they make a bigger droplet and gravity starts working. In the capilary sense: a sponge can fill itself to the top if you put the bottom in a cup of water, just like our soil particles act. But this capilary action only works if all particles make contact and if the soil particles have a stronger adhesive force than gravity (more pores = more force.. Water doesn't stick to metal, it does stick to rock, and it sticks even better to sponges).

If the particles don't make contact, every soil particle has it's own water table inside of itself, if water can enter. If they don't make contact, this means they're surrounded by air and they will start evaporating water, creating a damp environment around them.

The Japanese took this physical behavior of water and soil particles to the next level: coarse particles on the bottom so cohesion will occur and draw out most of the water AND let a lot of air in by replacement of substance (if you remove water, you get air, to get air out, just add water). The smaller particles up in the top layer are there to actually hold water for roughly a day. We give our plants a multitude of 'total pot volume' of water, just so those cohesive forces can work their magic. My pots are 250mL in some cases, I water them with roughly 1000mL.

If a soil is well composed - and it rarely is perfect - then it should dry out on the bottom by drawing in air and by the forces of gravity, and it should dry on the top because of evaporation (and adhesive forces from the water leaving the pot on the bottom). So a well composed soil should roughly dry as fast on the top as it dries on the bottom.

Our plants have less issues in colanders because here the evaporation takes place on six sides instead of one (and the pot holes). Only the center of the pot stays damp a bit longer than the rest.

Unglazed pots for conifers are not just used because they look good, the lack of a glaze also helps water evaporate on all sides, albeit a bit slower compared to a colander. Everyone eventually gets salt buildup on their unglazed pots because of this.

If your lower layer is saturated with water, it shouldn't pool! If it pools, then gravity doesn't work and you'll end up with a dead tree. That's why we all want holes in our pots. Pooling = lack of air = suffocation and fermentation = anaerobic microbe secretions and anaerobic pathogens = dead plant.
The whole "fast draining but water retentive" phrasing is kind of a mind fuck, but once you understand the mechanics, it starts getting fun to play with it.


Some of these forces can be bypassed by surfactants too! A super dry soil will be water repellent, a drop of soap will fix that issue right away.

So, basically, what I am understanding is we let the top soil dry out so that the saturated zone atleast has a chance to get unsaturated and get filled with air for some time and then we water again. Am I right? I feel saturatared zone mostly stays wet even then. Why do some people say, "so and so particle is too water retentive" do they mean they hold too much moisture inside themselves or do they mean inter-particular water? If a particle is holding too much moisture within itself then isn't that great as water isn't pooling and also there's latent moisture trapped in those particles; I've read that trees take up water vapour and not suck out the water, so, isn't having too much moisture inside the soil particle (intra-particular moisture) a good thing? The spaces between every such particle is still open for air to fill in...roots can take that air and use the evaporating water out of such a substrate.

 

[63pmp]

Plants only take up liquid water, they cannot absorb water vapour.

 

[Wires_Guy_wires]

I'm breaking up the questions for clarity.

So, basically, what I am understanding is we let the top soil dry out so that the saturated zone atleast has a chance to get unsaturated and get filled with air for some time and then we water again. Am I right?

Correct. But the bottom should 'breathe' too.

I feel saturatared zone mostly stays wet even then. Why do some people say, "so and so particle is too water retentive" do they mean they hold too much moisture inside themselves or do they mean inter-particular water?

It's both. If a particle is too water retentive, it's usually in relation to the tree, watering habits and the environment. This is a balance between inner- and interparticular water. Deciduous trees can handle way more moisture and will likely need more moisture than most conifers. Akadama is too water retentive for my conifers, in my climate, with my watering habit. I water too often, my temperatures are too low to cause enough evaporation and my conifers don't like soils that stay wet for too long. It would be better for me to use some other component like perlite, pumice, lava rock, these however balance on the edge of being not water retentive enough, so I add some pine bark to end up somewhere in the middle. My beeches however love water, and they do well in akadama for instance. Only when a particle is fully saturated, it will leave inter-particular water. When it dries out, water is drawn to the center of the particle. That's where the roots need to grow towards, and if that happens our soil starts breaking down into smaller particles.

If a particle is holding too much moisture within itself then isn't that great as water isn't pooling and also there's latent moisture trapped in those particles

This is what we want, because on the outside of the particle, there's air. It gets tricky when both inter- and intra particular water is both on the higher side. While trees have a hard time sipping water from the air, the damp environment and the net negative energy of evaporation (evaporation lowers the temperature) of a soil will cause some condensation of water below the surface. The process behind it is two folded; the roots grow into the particle pores and the particles themselves hold liquid water and the particles produce some condensation where the rest of the roots can drink from. As the roots suck one particle dry, neighboring soil particle might lose water too because of the interparticular forces; suck on one side of a piece of watermelon, and the rest will lose water as well. If the soil is too coarse, this doesn't work because the particles don't interact. If the soil isn't coarse enough, it takes too long to 'suck it dry' and fermentation can happen because there's not enough air replacement per X-amount of time.

In other words: if either one is out of balance, for instance because a particle both holds a lot of intraparticular water and interparticular water (like akadama does for me with my watering habit) then there's barely any air flow, the forces of gravity aren't strong enough to pull water out of the bottom of the pot (in other words: gravity isn't strong enough to overcome the adhesive forces between akadama and water). Then my soil stays too wet (3-8 days), even though there is no pooling of water there's also no new air flowing in either. In general, the more organic material, the more adhesive forces there are. There's a cool discussion going on in the world thanks to hydroponics and aquaponics, which boils down to the fact that just "too much water" isn't the problem. I have a juniper growing in 100% water. It's the lack of fresh air that causes issues.

What you'd want to look for is a balance. That's why we have soil wars on this forum and that's why everyone has their own "best soil". Boons mix wouldn't work for me, with my watering habits and my climate, but it does work for people in warmer regions who can control themselves when watering. Using different blends of soil components, with different ratios, can help you find a balance between the air, gravity, intra- and inter-particular forces. This is something we can only find out ourselves by trial and error.

I hope this makes sense!

 

[Ujjawal Roy]

I'm breaking up the questions for clarity.

Correct. But the bottom should 'breathe' too.

It's both. If a particle is too water retentive, it's usually in relation to the tree, watering habits and the environment. This is a balance between inner- and interparticular water. Deciduous trees can handle way more moisture and will likely need more moisture than most conifers. Akadama is too water retentive for my conifers, in my climate, with my watering habit. I water too often, my temperatures are too low to cause enough evaporation and my conifers don't like soils that stay wet for too long. It would be better for me to use some other component like perlite, pumice, lava rock, these however balance on the edge of being not water retentive enough, so I add some pine bark to end up somewhere in the middle. My beeches however love water, and they do well in akadama for instance. Only when a particle is fully saturated, it will leave inter-particular water. When it dries out, water is drawn to the center of the particle. That's where the roots need to grow towards, and if that happens our soil starts breaking down into smaller particles.


This is what we want, because on the outside of the particle, there's air. It gets tricky when both inter- and intra particular water is both on the higher side. While trees have a hard time sipping water from the air, the damp environment and the net negative energy of evaporation (evaporation lowers the temperature) of a soil will cause some condensation of water below the surface. The process behind it is two folded; the roots grow into the particle pores and the particles themselves hold liquid water and the particles produce some condensation where the rest of the roots can drink from. As the roots suck one particle dry, neighboring soil particle might lose water too because of the interparticular forces; suck on one side of a piece of watermelon, and the rest will lose water as well. If the soil is too coarse, this doesn't work because the particles don't interact. If the soil isn't coarse enough, it takes too long to 'suck it dry' and fermentation can happen because there's not enough air replacement per X-amount of time.

In other words: if either one is out of balance, for instance because a particle both holds a lot of intraparticular water and interparticular water (like akadama does for me with my watering habit) then there's barely any air flow, the forces of gravity aren't strong enough to pull water out of the bottom of the pot (in other words: gravity isn't strong enough to overcome the adhesive forces between akadama and water). Then my soil stays too wet (3-8 days), even though there is no pooling of water there's also no new air flowing in either. In general, the more organic material, the more adhesive forces there are. There's a cool discussion going on in the world thanks to hydroponics and aquaponics, which boils down to the fact that just "too much water" isn't the problem. I have a juniper growing in 100% water. It's the lack of fresh air that causes issues.

What you'd want to look for is a balance. That's why we have soil wars on this forum and that's why everyone has their own "best soil". Boons mix wouldn't work for me, with my watering habits and my climate, but it does work for people in warmer regions who can control themselves when watering. Using different blends of soil components, with different ratios, can help you find a balance between the air, gravity, intra- and inter-particular forces. This is something we can only find out ourselves by trial and error.

I hope this makes sense!

Thank you so much for breaking it down and explaining in detail. I'll try to implement these rules for my substrates.

 

[cornfed]

I just finished reading through this entire thread. Thank you for all the information! It's funny, the method must've spread around the community because I was already testing my own soils (I'm brand new to bonsai) after a member of my local club shared a presentation he made on the subject last fall. I have a spreadsheet that looks exactly like #137.

What was most enlightening to me was the affect of pot height on the air-filled space of various substrates.

I wanted to share a graphic I found in another paper by Heltsley on the internet. I have been using it as a guide as I test different blends of substrates available to me.

Note: we have no idea the height of the container he used to test these, and we know that impacts the number, so the more important of the two graphs is the one on the left. ​

I am currently putting one-year old nursery trees into 5-gallon air-pruning pots. I need 55 gallons of soil to complete the project, so I've been thinking a lot about soils, hence my stumbling across this thread. I tested the mix I am using at 33% AFP & 22% WHC, and I was stressing myself that I needed to increase both numbers with a different blend of substrates to get in that sweet spot in the above graph. I have four components I am working with that I have tested separately and use a spreadsheet to average a blend so I can hypothesize before actually mixing and testing.

My bonsai club says 25% is a good target for both AFP & WHC.
Heltsely says 30-40% for AFP & 20-30% for WHC.
In this thread the only target I saw was for AFP to be at least above 10%, ideally above 20% and that the thread author uses a mix with 40% AFP (which seems to jive with Heltsley's graph).

I guess my question is how do we know what the right percentages are? You provided good sources for AFP, but is there a consensus for WHC? But I'm sure that is different depending on the tree species, and maybe even the climate and watering habits and container proportions, so maybe the question is irrelevant???

Also, is only the water in between substrate particles useful? Is any moisture absorbed by a substrate like Diatomaceous Earth or Turface useable by the roots? For example, In this excellent video by Ryan Neil, he says that he doesn't like Turface because, "it's not how much water it holds, it's how much water it gives [...] it doesn't give the water back."

 

[hemmy]

I guess my question is how do we know what the right percentages are? You provided good sources for AFP, but is there a consensus for WHC? But I'm sure that is different depending on the tree species, and maybe even the climate and watering habits and container proportions, so maybe the question is irrelevant???

If you are developing stock in nursery containers then you should look up nursery guidelines for these parameters. There is a lot published. It is completely different than what we need in our shallow containers. Especially if you don’t want to be watering nursery stock multiple times a day.

 

[cornfed]

The broad theme of this thread is understanding the science behind the soil. I'm not necessarily asking for advice on what to put in my mix, just what numbers to shoot for in my measurements. I meant to add information to the thread by asking a new question. And hopefully adding something new, since that video and graphic are younger than this post.

 

[Mikecheck123]

The broad theme of this thread is understanding the science behind the soil. I'm not necessarily asking for advice on what to put in my mix, just what numbers to shoot for in my measurements. I meant to add information to the thread by asking a new question. And hopefully adding something new, since that video and graphic are younger than this post.

I don't think this is information that you can derive analytically. I.e. you just have to try it and see how it goes.

Also, fwiw, five gallons sounds too big for one year old stock. One gallon sounds better.

 

[cornfed]

In the articles that @markyscott published in the American Bonsai Society Magazine after starting this thread (found here), I found this: (Anybody new to the soil physics, just start with that link instead of reading the whole thread like I did)

​

Where he says sets a good goal for WHC at 30%. That is a little higher than Heltsley recommends in that graphic I linked above, but pretty close. Anyways, it seems like AFP might be the more important factor, since it dictates how often you water and how fast the tree grows.

I don't think this is information that you can derive analytically. I.e. you just have to try it and see how it goes.

So yeah, I think you're right. It might not even be fair to ask for a goal number for AFP or WHC since it is so dependent on an individual's situation & goals. It's enough to know the limits (like no less than 10% AFP).

Since this is the first time I've tried to grow a tree in a container I am just trying to at least understand the repercussions of my soil decisions and how pot size and format come into play in making those decisions. (if you any more input on my up-potting, I'd welcome it on my tree journals).

Then for my second question:

Also, is only the water in between substrate particles useful? Is any moisture absorbed by a substrate like Diatomaceous Earth or Turface useable by the roots? For example, In this excellent video by Ryan Neil, he says that he doesn't like Turface because, "it's not how much water it holds, it's how much water it gives [...] it doesn't give the water back."

Scott answers it here. I believe he is saying that only the water in between the grains is usable by the tree, but that the micro-porosity OF the grains will help increase humidity of the system. That's why Pumice and Scoria are so useful. Whether or not other materials have the same beneficial micro-porosity is probably best suited to it's own thread since this one has so successfully avoided the soil wars for so many years.

 

[Mikecheck123]

I am just trying to at least understand the repercussions of my soil decisions

Here's one lesson I've learned many times (because I'm a slow learner): thinking about adding some potting soil because it's cheap? Don't do it! In some respects, adding potting soil to a bonsai mix is worse than just using straight potting soil.

 

[cornfed]

Here's one lesson I've learned many times (because I'm a slow learner): thinking about adding some potting soil because it's cheap? Don't do it! In some respects, adding potting soil to a bonsai mix is worse than just using straight potting soil.

Ha. It was this very question that brought me to this thread in the first place. I googled it AFTER I added some fine material (20%) to my mix in an attempt to raise the WHC and planted four trees in it. Then I second guessed myself and came here. I think the trees will be fine, it's not a bonsai pot. But at the same time, the next six will be planted in uniform substrate. Thanks!