Introductory soil physics

[Waltron]

Scott, below is a quote from Nick Lenz, my question is about how do you know which plants prefer fine soil, and which plants prefer course soil? I just want to give the plant whichever it prefers on inital collection.. but it seems somewhat tricky deciding which prefers which..

AR: What is your soil mixture of choice?

NL: Again, how many grains of sand are there on the beach? Collected plants may go immediately into very coarse or fine soil depending on the hunch about their survival reality. Drier type plants are grown in coarse soil in large containers, although they would prefer the fine. Fine soil plants are sometimes transferred to very coarse soil to develop stringy roots of great length for special purposes, and they don't like that at all. Medium tends to be the grade for mature plantings, but wee things may be planted in either coarse or fine. You must know each plant intimately. No rule holds for more than a growing season. Next spring, I am going to repot a Rocky Mountain juniper from coarse into very fine soil to see what that does. This is a topic for several lectures, and no one probably wants to know when you can buy one grade, the grade from the local supermarket below the birdseed.

 

[markyscott]

Scott, below is a quote from Nick Lenz, my question is about how do you know which plants prefer fine soil, and which plants prefer course soil? I just want to give the plant whichever it prefers on inital collection.. but it seems somewhat tricky deciding which prefers which..

AR: What is your soil mixture of choice?

NL: Again, how many grains of sand are there on the beach? Collected plants may go immediately into very coarse or fine soil depending on the hunch about their survival reality. Drier type plants are grown in coarse soil in large containers, although they would prefer the fine. Fine soil plants are sometimes transferred to very coarse soil to develop stringy roots of great length for special purposes, and they don't like that at all. Medium tends to be the grade for mature plantings, but wee things may be planted in either coarse or fine. You must know each plant intimately. No rule holds for more than a growing season. Next spring, I am going to repot a Rocky Mountain juniper from coarse into very fine soil to see what that does. This is a topic for several lectures, and no one probably wants to know when you can buy one grade, the grade from the local supermarket below the birdseed.

There is no single ideal soil for everyone, that's why I make no recommendations. There are so many trade-offs - increase the AFP 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. I haven't run across the same sensitivity with collected trees as Nick has (most of mine go into a fairly coarse mix), but I don't have as much experience as he does with collected trees either. Hopefully at the end of this folks will have a good reference to know what it means when they change their soil or the depth of their pot or the like.

Scott

 

[Solange]

Let's think about this more because it is a bit counter intuitive. Let's first define some terms.

1. The porosity is measured when the medium is completely dry. It's 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. 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.

So now to your question. For a given soil medium the porosity is constant. Let's say 50% for the sake of argument. Doesn't matter how tall or short the container is - it will always be 50%. After you water, you'll be left with a profile that looks like this (depending on the size of the pot):

View attachment 118925

See? 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. As you can see, the figure on the left (the taller pot) has a lower average water saturation (and higher air-filled porosity) than the shorter pot on the left. I integrated the hypothetical curves above and got 26% average water saturation (or 24% average AFP) for the pot on the left and 32% average water saturation (or 18% average AFP) for the pot on the right. In other words, a shorter pot is wetter than a taller pot. Weird.

Scott

That is how I understood it to work. Which, correct me if I'm wrong, is the exact opposite of the section I quoted earlier and why I posited the question to begin with.

 

[markyscott]

That is how I understood it to work. Which, correct me if I'm wrong, is the exact opposite of the section I quoted earlier and why I posited the question to begin with.

I see the inconsistency now - it's a typo in my earlier statement. Thanks for the correction.

Scott

 

[Solange]

No problem! I appreciate the clarification and this is a great refresher

 

[markyscott]

OK - so here's another fun fact. We talked about what water saturations look like in the pot immediately after watering. What happens during the rest of the day?

To talk about this, we need to introduce another concept - hygroscopic water. Here's what we know so far - when we water all the pores are filled and the soil is close to saturation. When we stop watering, the gravitational water drains away and the soil is at field capacity - the so called "irreducible water saturation" (except it's not irreducible). This capillary bound water is what is available to the plant. So during the day the plant will draw water from the pore space and the water saturation will drop below field capacity. 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 is called "hygroscopic water". Here are the three saturation states; 1) fully saturated, 2) at the field capacity, 3) at the wilting point (aptly named).



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.

Scott

 

[markyscott]

Can you please share how container shape changes these figures as well? There are some physics there I'm not understanding

Let's talk about container shape. I think the easiest way to talk about this is by analogy. Consider a sponge. Saturate it with water and lay it flat side down on the table. Look past the fact that the gravitational water will make a mess of your table and think about what's left in your sponge. Youll end up with high water saturations at the bottom of the sponge and water saturation will decrease upward. The water will drain away until it reaches the sponges "field capacity".

Now, what happens when you take that same sponge and tip it on its side? Like this:



Well, a bunch more water will come out. Why? Because when you lay it flat side down, you have a large volume of the sponge at the high water saturations found at its base. When you tilt it on its side, a much smaller volume of the sponge is at high water saturation, so the sponge is (temporarily) above field capacity. You know what that means - another mess on the counter. But think about what that means for your bonsai pots - a shallow wide bonsai pot holds more water at higher water saturations than a tall narrow pot even if the soil volumes are exactly the same! In fact, it holds a lot more water. Pretty cool - and you can test this by making a mess of you own counter to see if this works.

Scott

 

[Owen Reich]

Great explanations Scott. The drainage layer also allows for "show potting" a tree more easily; a more shallow container is possible just by removing that coarse layer and setting the tree in for the exhibition.

I drove my soil science professors crazy with questions about container growing and soil profiles. One suggested an undisturbed organic layer (O) much like a non-tilled soil, and then A,E,C, etc. His idea was dust all the way to quarter size. If you had worms in the pot, it may work effectively.

 

[markyscott]

Great explanations Scott. The drainage layer also allows for "show potting" a tree more easily; a more shallow container is possible just by removing that coarse layer and setting the tree in for the exhibition.

I drove my soil science professors crazy with questions about container growing and soil profiles. One suggested an undisturbed organic layer (O) much like a non-tilled soil, and then A,E,C, etc. His idea was dust all the way to quarter size. If you had worms in the pot, it may work effectively.

Thanks Owen and a great point about show potting. This is a tough topic to reduce to level that's useful without boring folks to tears, but we'll give it a go. Thanks again for checking in.

Scott

 

[A Random User]

This is a very good thread Scott, and I thank you for posting it!

I think it is awesome having this as a discussion, for it allows folks to brain storm and ask questions about things, and bring up points that often are not considered... Wirh that said, I too second the motion, that after a really nice thorough discussion is had, that perhaps you really consider pulling parts out of the overall thread and make a tutorial. This way you have all of the info, without some perhaps overlapping.

A couple of thoughts I would be interested in asking questions about would be the following...

If I understand that adding a drainage layer raises the level in the pot of the water retention area, and if I am to understand that a shallower pot actually retains more moisture due to parcement layer as shown with your sponge example.

I am curious to know if you think other factors could possibly play a role in both case scenarios...

In your duscussion so far you have examined the effects of gravity... and have said, if I recall correctly that it makes no difference in the size of drainage holes or the amount.

Yet, I am stuck... wondering if this would be the case? Drainage holes yes, let water out... but they also let air in. Often in the process of putting our trees to grow on benches, we are elevating them off of the ground and to a height that recieves much more wind, than the ground does...

So my question would be, how then does this effect, or does it have an effect on what you have stated thus far?

Also, the sponge diagram shows the shallower pot holding more moisture, but I am curious, does it actually hold more moisture, or does it hold the same?

Lastly, I think this has been quickly touched on, but curious it's effect as well... some substrate obviously holds water better... Lava or pumice for example actually hold water within its pores and sometimes one will often find small feeder roots growing into these pores to obtain this. So, if you have a larger pot with more of this substrate, could you also say that the larger pot might as well retain more moisture? Verses a smaller pot with the same substrate? I guess what I am getting at, is do you think that this somehow changes this or keeps it the same?

Not trying to obviously disagree with what you have said or the science behind it... just trying to understand the overall picture.

Part of the reasoning for me asking these questions, is that if air has an effect on the drying out process, and air is able to go through the drainage holes, in effect drying the soil out from below at the same time... then would this also act like the drainage layer and help move up the area in which moisture is retained?

Then what about things like air pots or colanders, seeing we were just discussing their effect.... These seem as though, they would not only raise the moisture retention level up and down, but also make it move vertically in, or out as well... towards the center or towards the walls of the pot, depending on if you used them or not.

Sorry everyone for being so long... just thought it was worth asking.

 

[A Random User]

Sorry... one last question I just thought of as I re-read this...

If you place a pot on the ground, as I often do to sometimes grow out a piece of naterial, yet keep finer roots inside the pot...

Or sometimes I will place shohin on a larger pot full of substrate to allow for them to retain healthy and continue to grow, in such a small pot.

What does this do to the mix?

Is the pot, still maintain it's same water retention level, is it lowering, raising, etc?

 

[markyscott]

This is a very good thread Scott, and I thank you for posting it!

I think it is awesome having this as a discussion, for it allows folks to brain storm and ask questions about things, and bring up points that often are not considered...

Hi Sawgrass - thanks for your questions. I've been meaning to do this for a while, but there is something about soil that gets peoples bonsai hackles up. This'll be good as long as we can keep it civil. As for your questions, there are a few here and I don't know the answer to all of them, but I'll take as stab at it. Let me see if I can break them down.

Drainage holes yes, let water out... but they also let air in. Often in the process of putting our trees to grow on benches, we are elevating them off of the ground and to a height that recieves much more wind, than the ground does...

So my question would be, how then does this effect, or does it have an effect on what you have stated thus far?

Much of what I talked about is the water saturation in the pot right after you stop watering. The gravitational water drains away and the water saturation quickly (in seconds) reaches the field capacity. I touched on what happens after that in Post #26, but in reality, as you anticipated, it's really complicated. First, there are evaporative losses - this effects the soil at the top of the pot most rapidly and is our biggest enemy. After all, we're trying to develop surface roots and if that layer keeps getting dessicated, roots won't grow there very well. We'll talk more about this later. I suppose you'll get more evaporative losses through the drainage holes, but I imagine that this will be a small fraction of what is being lost from the top of the pot. And second, there are transpiration losses as the plant takes in water to maintain itself. So starting with the field capacity, the saturation profile in the soil will slowly decay toward the wilting point. The saturation at the top of the soil will drop most rapidly followed by the saturation of the soil in the root zone. The soil at the bottom of the pot will drop most slowly - if we've got a good medium there are fewer roots there to transpire and evaporative losses are at a minimum.

Scott

 

[Eric Group]

@markyscott
http://www.bonsainut.com/resources/

Do it!

 

[markyscott]

Also, the sponge diagram shows the shallower pot holding more moisture, but I am curious, does it actually hold more moisture, or does it hold the same?

The sponge laying on its side definitely holds more water. This is an easy experiment to try at home and a great learning exercise. Let's stick in some numbers though and work it through. A typical sponge is about 12.5cm x 9cm x 2.5cm. That's a volume of about 280ml. Let's assume a porosity of 50%, so 140ml of pore space. Let's further assume that the the pore space in the bottom 1.25 cm is 100% saturated and the top 1.25 cm is 40% saturated (that's an AFP of 30%).

So for the sponge lying flat:

• The bottom 1/2 of the sponge has 12.5cm x 9cm x 1.25cm x 50% porosity x 100% saturation = 70 ml of water
• The top 1/2 of the sponge has 12.5cm x 9cm x 1.25cm x 50% porosity x 40% saturation = 28 ml of water
• That's a total of 98 ml of water

And for the sponge standing on it's side:

• The bottom 1.25cm of the sponge has 12.5cm x 2.5cm x 1.25cm x 50% porosity x 100% saturation = 20 ml of water
• The top 7.75cm of the sponge has 12.5cm x 2.5cm x 7.75cm x 50% porosity x 100% saturation = 48 ml of water
• That's a total of 68 ml of water or about 2/3 the water that the same sponge holds when it's lying flat.

Scott

 

[markyscott]

@markyscott
http://www.bonsainut.com/resources/

Do it!

Alright - I'll boil it down, but let's enjoy a nice conversation first.

Scott

 

[A Random User]

Hi Sawgrass - thanks for your questions. I've been meaning to do this for a while, but there is something about soil that gets peoples bonsai hackles up. This'll be good as long as we can keep it civil. As for your questions, there are a few here and I don't know the answer to all of them, but I'll take as stab at it. Let me see if I can break them down.



Much of what I talked about is the water saturation in the pot right after you stop watering. The gravitational water drains away and the water saturation quickly (in seconds) reaches the field capacity. I touched on what happens after that in Post #26, but in reality, as you anticipated, it's really complicated. First, there are evaporative losses - this effects the soil at the top of the pot most rapidly and is our biggest enemy. After all, we're trying to develop surface roots and if that layer keeps getting dessicated, roots won't grow there very well. We'll talk more about this later. I suppose you'll get more evaporative losses through the drainage holes, but I imagine that this will be a small fraction of what is being lost from the top of the pot. And second, there are transpiration losses as the plant takes in water to maintain itself. So starting with the field capacity, the saturation profile in the soil will slowly decay toward the wilting point. The saturation at the top of the soil will drop most rapidly followed by the saturation of the soil in the root zone. The soil at the bottom of the pot will drop most slowly - if we've got a good medium there are fewer roots there to transpire and evaporative losses are at a minimum.

Scott

I totally understand about differing types of soil... and I agree that is best trying to stay as far away from an actual comparison as possible... and sticking to perhaps more of just the actual Science of what is happening. The only reason, why I brought up things like a lava or a pumice is mainly just because these type of substrates I believe act similar to a pot itself... this is to say they don't completely shed water like perhaps a normal let's say stone might , but sometimes cup it as well.


But, yes... no soil debates! Strictly plutonic here!

I also understand, and thought that other factors might play a role... this is the main reasoning for me asking, I wanted to make sure that I was on the same page... what you were saying was what I thought it was... that being this was just the basic fundamental properties of what is happening.

Sorry, to interupt! Keep it up... a very good discussion to be had.

 

[Eric Group]

My contribution:
Dirt don't hurt!

 

[markyscott]

Back to it - here's a nice graphic I made for a lecture some time ago. It's a good visual to help you remember how the different aspects of soils impact AFP and WHC. Remember that this is a trade off - higher AFP is generally good. We like aeration in our soil. But the tree lives on the capillary bound water between waterings. So you can maximize the AFP, but it will definitely impact your responsibility in keeping the plant alive. Optimal soil conditions are preferable, but only if you can keep the dang thing watered. But these are the main variables you can adjust to increase or decrease the AFP.



Scott

 

[markyscott]

Here's a question that comes up all the time - should I sieve? Well, in the spirit of what we're trying to do here, I'm not going to answer that. But I will tell you what are the consequences of your choice (at least the consequences I know about).

First, there is the issue illustrated in the previous graphic. If we use a mix of grain sizes, we'll have a relatively lower porosity than we would if we used a single grain size. We all now know what that means - our soil will have a lower AFP and higher water saturation. This is absolutely clear, but it's not a problem unless your AFP drops below 15-20%. Below 10% and it's definitely a problem.

Second, there's the issue of stratification. Here's a nice experiment performed by Davetree. This really stuck with me - I contacted Davetree shortly after he made this post and he was kind enough to give me permission to use this image in one of my presentations.


This is really interesting - 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. So here's a pop quiz - what do you think this will do to the saturation profile in the soil?

Scott

 

[c54fun]

One thing I would think is that saturation zone goes up with coarse on the bottom so I would thing the saturation zone would go lower.
Love this thread. Well done and thanks.