Introductory soil physics

[fredman]

Yep I have to second that. I read quite a few substrate threads, but this one towers out. Thanks for the way it was presented Scott. It certainly gives me a new perspective on some things. I'll have to refer back to it from time to time though. Very good input from all others also...Thank you

 

[markyscott]

OK back to it. Here's some more results. Let's think back to our previous discussion about how air-filled porosity (AFP) should decrease with decreasing pot height and water-holding capacity (WHC) should increase. We talked a lot about capillary forces and why we should expect this is the case - and can be very counterintuitive to folks who don't work in a weird field and have to deal with this kind of stuff every day like I do. AND we talked about drainage layers and how they act to decrease the height of the soil and therefore increase the WHC and decrease the AFP. We talked about other factors that control these important variables such as grain size, grain shape band sorting. And we looked at some simple experiments demonstrating these relationships.

So let's explore a little bit more.

Scott

 

[markyscott]

Recall our turface experiment we discussed previously -

Here's the data for Turface:

View attachment 119395

Volumes are in milliliters, Phi is porosity, AFP is air-filled porosity and WHC is water holding capacity. As you can see, total porosity is insensitive to grain size, but AFP and WHC are pretty sensitive. The finest grain fraction has the highest water saturation (about 49%) and the lowest AFP. As you increase grain size the AFP goes up and the WHC goes down - exactly what we anticipated. Here's a graphical expression of the data.

View attachment 119394

Also remember. If you use this in a pot, you should expect higher WHC and lower AFP - these are tall graduated cylinders. As you decrease the height of the soil, we should expect saturations to go up. The smallest size fraction has 28% AFP even in a very tall container - we're looking for more than 20% so we'll need to be mindful when were dealing with shallow containers.

Here are the water saturations (remember this is just the percent of the pore space occupied by water - or WHC/Phi)

View attachment 119397

Scott

One thing we noted was that even the smallest size fraction we tested (1/16"-1/4" or 1.6mm - 6.4mm) had adequate AFP (28%). But this experiment was conducted with 500ml of soil. In the beaker I use, that equates to a 10.7" (or 27.5 cm) soil level. But how does the AFP for this size fraction change with soil height? When growing bonsai, this is a very important question because we want to maintain optimal growth conditions in shallow containers.

So let's think about how to look at this issue.

Scott

 

[markyscott]

So, here's the setup.



Five tests with different soil volumes (we looked at 500ml last time). In each successive test I decreased the soil volume by 100m. So we ran 500, 400, 300, 200, and 100ml cases. In these beakers that equates to a heights of 10.7", 8.7", 6.6", 4.5", and 2.4". In metric units, that's 27.5cm, 22.2cm, 16.8cm, 11.5cm, and 6.1cm. The soil I used was turface - specifically the 1/16"-1/4" size fraction. Here are the results:



Somewhere around 2.5" the soil became waterlogged. As we discussed before, below 10% AFP and plant growth suffers. Too long under these conditions and plants will die (minus a couple of notable exceptions that have adapted to survive under these conditions).

And one thing to remember - these are average AFP's. The saturation profile in the soil is variable with higher saturation on the bottom and lower saturation toward the top. So remember that when the AVERAGE is 20%, parts of the s0il can be much higher. For instance, if you have a 5" pot it may be true that the average saturation is 22%, but the bottom 1/2" will be waterlogged, according to these results.

Scott

 

[Austrian]

Very interesting! Basically directly after watering most of our bonsai pots are rather waterlogged. So to me it seems choosing a substrate is more a matter of controlling how long after watering that remains the case.

 

[markyscott]

Very interesting! Basically directly after watering most of our bonsai pots are rather waterlogged. So to me it seems choosing a substrate is more a matter of controlling how long after watering that remains the case.

Hello Austrian - the message I'm trying to convey is this. There are several factors that control how much air and water is in the pore space of your soil:

1. Grain size
2. Grain shape
3. Grain composition
4. The depth of the pot

If you use a fine-grained soil in a shallow container you will have waterlogged conditions in at least part of the pot for some period of time after you water. You can mitigate it by adjusting the factors in the list above. There are lots of thing that can work effectively in a deep pot, but many of those same soils are not ideal in a shallow container. So what strategies do you have at your disposal to maintain optimal soil conditions when you move to a shallow container?

1. Increase the grain size
2. Change to a substrate composition that drains better
3. Some combination of 1 & 2

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. You'll have to find your own goldilocks zone in your back yard for trees under your care. But the theme of this thread is to give you the tools and knowledge you need to make informed decisions about what the trade-offs are to your choices.

I'm just proud that we've managed to go nine pages into a soil discussion without a flame ware.

I've discovered that I can still be amazed.

Scott

 

[Austrian]

Yes, but if your measurement shows that under 2.4" height and under 1/4" grain size the substrate is waterlogged, surely the majority of bonsai pots/soils fall under that. That's what I meant.

 

[markyscott]

Yes, but if your measurement shows that under 2.4" height and under 1/4" grain size the substrate is waterlogged, surely the majority of bonsai pots/soils fall under that. That's what I meant.

Darn it. There's a typo - and no way to go back to fix it. I apologize that I didn't catch it when I posted it.

In the experiments above it should read 1/16" - 1/8". Not 1/4". This was a fine-grained soil mixture, but it is a size fraction that many include in their substrate.

Based on our discussion here, and at the very least, I'd encourage folks to sieve out the <1/8" size fraction. In my garden I mix three substrates with different size fractions for different purposes - a 1/8"-1/4", a 1/4"-3/8" and a 3/8"-1/2".

Scott

 

[Austrian]

Ah, alright, now it makes sense, thanks!

 

[markyscott]

One other thing - I've pretty much exhausted the list of topics that I had planned for this thread. I wanted to again thank everyone for the civil discourse and good discussion. I've really enjoyed it. At the moment I don't have plans for introducing any new topics unless something comes up in discussion that could add to the conversation.

Probably the main topic we did not delve into was soil chemistry. I understand the basics and we touched on the edges of the issue, but it is not an area where I have special expertise. So I wanted to keep this directed at physics and water retention.

I'll try to synthesize the key points of our conversation and build a resource (or resources) for people to refer to (That'll give me a chance to fix the dang typos too). But this thread will always be around for discussion and to ask questions.

Thanks to everyone who participated.

Scott

 

[coh]

Good work Scott!

I'd suggest, for completeness, a couple of additional experiments to go with the last one. I'd pick a different substrate of your choice - maybe pumice, maybe akadama. Sift it to the same size as the turface and repeat the experiment. How much difference do you get?

My biggest issue with turface has always been the particle size (too small) and to a lesser degree, the shape (not rounded, so it packs more tightly than rounder particles). Both would contribute to excess water holding. I think one can still use it successfully (many have/do), but more care might be needed in watering.

Then repeat with that other substrate, but the next size (larger) particle. Ideally, if you can sift out enough larger size turface particles, you'd repeat with the next size of that.

Easy for me to spend your time! But I think that would give us more context.

Thanks for doing this,

Chris

 

[0soyoung]

Not meaning to demonstrate that no good deed goes unpunished, @markyscott, but is there any chance of you doing the tests to determine the saturation zone depth for all the other substrates you've tested in this thread? That would make for the first thorough catalog of substrates. Man, what a resource your data would be!

 

[wireme]

Someday I'd like to compare the WHC results of pumice by in one sample submerging it and other sample just pouring water through. When I first started using pumice I had some trees that seemed to suffer from chronic under watering that season. I feel like when you just water through all those pores in pumice don't necessarily fill up. If they don't the dry interior can remove moisture quickly. Before using pumice it was mainly granite, no pores, different behaviour. I water a bit differently now, quick pass followed by a more thorough drench. Anyways I'd like to someday do a test to see what it takes to fully saturate all the pores in a pumice substrate.

 

[markyscott]

Not meaning to demonstrate that no good deed goes unpunished, @markyscott, but is there any chance of you doing the tests to determine the saturation zone depth for all the other substrates you've tested in this thread? That would make for the first thorough catalog of substrates. Man, what a resource your data would be!

That's a big job Oso. Do you have a favorite that you'd like me to try?

Scott

 

[markyscott]

Someday I'd like to compare the WHC results of pumice by in one sample submerging it and other sample just pouring water through. When I first started using pumice I had some trees that seemed to suffer from chronic under watering that season. I feel like when you just water through all those pores in pumice don't necessarily fill up. If they don't the dry interior can remove moisture quickly. Before using pumice it was mainly granite, no pores, different behaviour. I water a bit differently now, quick pass followed by a more thorough drench. Anyways I'd like to someday do a test to see what it takes to fully saturate all the pores in a pumice substrate.

That's exactly right, wireme. Recall the picture of the pumice island I posted? There are pores inside of pumice that have no access to water. It has a high microporosity but part of it is not effective. They will never fill. If you look at the experiments, for a given size fraction there it has a relatively high AFP and low WHC. So you will need to water more often when you use a substrate high in pumice fraction.

Now that said, pumice is a natural substance and there is a great deal of variability in it's properties. So there's pumice and there's pumice. But I imagine that not all of it will behave the same as that which I tested here.

Scott

 

[markyscott]

Good work Scott!

I'd suggest, for completeness, a couple of additional experiments to go with the last one. I'd pick a different substrate of your choice - maybe pumice, maybe akadama. Sift it to the same size as the turface and repeat the experiment. How much difference do you get?

My biggest issue with turface has always been the particle size (too small) and to a lesser degree, the shape (not rounded, so it packs more tightly than rounder particles). Both would contribute to excess water holding. I think one can still use it successfully (many have/do), but more care might be needed in watering.

Then repeat with that other substrate, but the next size (larger) particle. Ideally, if you can sift out enough larger size turface particles, you'd repeat with the next size of that.

Easy for me to spend your time! But I think that would give us more context.

Thanks for doing this,

Chris

Ok Chris - I'll probably have to use a different size fraction because only turface (and maybe DE) has a significant size fraction that small. My pumice source now has almost none in the 1/16" -1/8" size range.

And another thing about grain shape - it's kind of interesting and also a bit counterintuitive. Angular grains tend to have a higher porosity than rounded grains. See?



http://homepages.see.leeds.ac.uk/~earpwjg/PG_EN/CD Contents/GGL-66565 Petrophysics English/Chapter 2.PDF

I'm not sure what it does to the average pore size - in the end, that's what is most important to the properties we're discussing here. Consider the plot closely - it's very informative. As you increase grain size, but the porosity drops but the actual pore size increases!! I believe that the same thing happens when you change grain shape - more angular grains have a higher porosity, but a smaller pore size distribution. So the big deal with angularity is not the porosity - it's probably the lower AFP.

Scott

 

[petegreg]

Thanks for this thread and valuable info, Scott. I'd have a Q... the sponge example quoted...
You are always talking about shallow pots (the sponge example) and waterlogging. If I say that a bottom area (size/dimensions) of a container is important regardless its hight, am I wrong and why? I mean if I have two pots with the same bottom area dimensions but one is shallow and the second is deeper...will be the height of water column different?

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:

View attachment 118936

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

 

[0soyoung]

That's a big job Oso. Do you have a favorite that you'd like me to try?

I understand; I do. We just need some 'students' to submit their homework

How about the be-all end-all, Acadama?

 

[markyscott]

Thanks for this thread and valuable info, Scott. I'd have a Q... the sponge example quoted...
You are always talking about shallow pots (the sponge example) and waterlogging. If I say that a bottom area (size/dimensions) of a container is important regardless its hight, am I wrong and why? I mean if I have two pots with the same bottom area dimensions but one is shallow and the second is deeper...will be the height of water column different?

Hi petegreg -

The height of the saturated zone is a property of the soil, not the pot. In any soil medium the water saturation will increase toward the bottom of the pot. If you have a wide shallow pot, there's a lot of water tied up in the saturated zone at the bottom - the volume is the length of the pot (L) times the width of the pot (W) times the height of the saturated zone (H) characteristic of the soil medium you are using. And that's it - using a deeper pot won't change this at all. It will have the same H as the shallow pot because that's a characteristic of the soil not the pot.

So you are correct that it matters because the bottom dimension of the pot will help determine the total amount of water held in the soil. And shallow pots will have a higher water saturation overall and deeper pots lower.

Scott

 

[markyscott]

I understand; I do. We just need some 'students' to submit their homework

How about the be-all end-all, Acadama?

All right - that's two requests for akadama. I'll give it a go. I'm curious too as I like to use it as a soil component. Votes for a size fraction? I'm not sure I have enough to run the 1/16"-1/8" fraction - most of my sources hardly have any in that range. How about 1/8"-1/4"?

Scott