Introductory soil physics

[It's Kev]

@markyscott great thread, I’ll give it an in depth read later.

 

[ralf]

I do appretiate guys that are sharing their knowledge with us. A great thread. Thank you for this, Scott.

 

[markyscott]

I ran some additional experiements that those of you who follow this thread might be interested in. First, I changed my experimental method a bit. I found it very difficult to measure properties of small soil volume by measuring the volume of water drainage. So I switched to weight - I think it’s more accurate all the way around. I just measure the dry soil weight and then tare my scale. Then I measure the weight of the water that I add to the soil to completely saturate it. Finally, I drain all of the gravitationally held water and weight the soil again. The difference in weight between the saturated and drained measurements should be the weight of the capillary bound water. And the weight of the water that I add to the dry soil to completely saturated it is a measure of the porosity. It all works out pretty easy. Just a lot of this:



Scott

 

[Gary McCarthy]

I ran some additional experiements that those of you who follow this thread might be interested in. First, I changed my experimental method a bit. I found it very difficult to measure properties of small soil volume by measuring the volume of water drainage. So I switched to weight - I think it’s more accurate all the way around. I just measure the dry soil weight and then tare my scale. Then I measure the weight of the water that I add to the soil to completely saturate it. Finally, I drain all of the gravitationally held water and weight the soil again. The difference in weight between the saturated and drained measurements should be the weight of the capillary bound water. And the weight of the water that I add to the dry soil to completely saturated it is a measure of the porosity. It all works out pretty easy. Just a lot of this:

View attachment 170789

Scott

That's pretty much the method Brian uses in his article and how I've been doing my testing.

@markyscott two quick questions. When you add the water to the soil do you let it sit for a little bit before you drain off the water? And then when you drain off the water do you then let that sit for a little bit and then drain off the water again?

Thanks.

 

[markyscott]

So heres the set up. I ran a series of experiments on three soil types - calcined clay (turface), akadama and pumice. I seived each of the three soils to the 1/8” to 1/4” size fraction. For each grain type, I measured the AFP and Container Capacity of the soil as a function of the height of the soil in the graduated cylinder. I ran tests on 50ml, 100ml, 200ml, 300ml, 400ml, and 500ml of soil. The smallest volume of soil was equivalent to a ~3cm (~1.2”) soil column and the largest volume was equivalent to a ~27cm (~10.7”) column.

Here are the results:


The cyan line is akadama and the dark blue line is pumice. Turface is the gray line. Each circle represents an individual measurement.

On average, pumice had the highest porosity (61.9%). Akadama had the next highest porosity (58.1%) followed by calcined clay (57.4%). The AFP for pumice and akadama was identical within experimental error across the range of heights tested. However, since pumice has a slightly higher porosity, its container capacity is slightly higher. Calcined clay had significantly lower AFP - a full 15% lower for each of the soil heights tested. For the 50ml soil volume (an ~3cm or 1.2” column in this experiment), pumice and akadama had about 25% AFP whereas calcined clay was 12% and close to waterlogged conditions (10% AFP).

Scott

 

[markyscott]

That's pretty much the method Brian uses in his article and how I've been doing my testing.

@markyscott two quick questions. When you add the water to the soil do you let it sit for a little bit before you drain off the water? And then when you drain off the water do you then let that sit for a little bit and then drain off the water again?

Thanks.

Yes - I let it sit for about 5 min and then add additional water if I need to before measuring the saturated weight. When I drain, I just let it do so until the water stops dripping out of the bottom - usually a couple of minutes.

Scott

 

[Gary McCarthy]

Yes - I let it sit for about 5 min and then add additional water if I need to before measuring the saturated weight. When I drain, I just let it do so until the water stops dripping out of the bottom - usually a couple of minutes.

Scott

That's what I've been doing as well. Thanks Scott!

 

[River's Edge]

I do tend to use a drainage layer, especially in deeper pots. I find it easier to keep the plants watered in our climate.

Scott

When screening pumice, the larger particles in the bagged product I normally use are just larger than the openings in the large bonsai seive 1/4". for practical purpose say 1/4" to 3/8". The next size down seive retains what i refer to as medium size particles 3/16 " to 1/4 " . Can you suggest what the effect is when the larger particles are placed in a layer two particles deep and then covered with a Bonsai mix of medium size particles. I am referencing the normal three screen bonsai seive.
Is there a perched water effect with these dimensions and the small difference in particle size between the thin drainage layer and the rest of the Bonsai Soil Mix particle size?

 

[markyscott]

When screening pumice, the larger particles in the bagged product I normally use are just larger than the openings in the large bonsai seive 1/4". for practical purpose say 1/4" to 3/8". The next size down seive retains what i refer to as medium size particles 3/16 " to 1/4 " . Can you suggest what the effect is when the larger particles are placed in a layer two particles deep and then covered with a Bonsai mix of medium size particles. I am referencing the normal three screen bonsai seive.
Is there a perched water effect with these dimensions and the small difference in particle size between the thin drainage layer and the rest of the Bonsai Soil Mix particle size?

The perched water effect is real, but the effect is biggest when you have a pronounced difference in pore size in the two layers. As the pore size difference decreases, the height of the saturated zone above the drainage layer gets smaller. If you’re using the 1/4” to 3/8” size as your drainage layer and 3/16” to 1/4” as your soil, I doubt that the effect is large, but I’ve never seen any experiments demonstrating or refuting this supposition.

FWIW, I use bigger particles for the drainage layer - 3/8” to 1/2”. For most plants I use 1/4”-3/8” for soil and the small stuff - 1/8”-1/4” I use as a top dressing. Anything small than 1/8” I throw away.

Scott

 

[River's Edge]

The perched water effect is real, but the effect is biggest when you have a pronounced difference in pore size in the two layers. As the pore size difference decreases, the height of the saturated zone above the drainage layer gets smaller. If you’re using the 1/4” to 3/8” size as your drainage layer and 3/16” to 1/4” as your soil, I doubt that the effect is large, but I’ve never seen any experiments demonstrating or refuting this supposition.

FWIW, I use bigger particles for the drainage layer - 3/8” to 1/2”. For most plants I use 1/4”-3/8” for soil and the small stuff - 1/8”-1/4” I use as a top dressing. Anything small than 1/8” I throw away.

Scott

Thanks. I also discard the fines, unless needed for cuttings and seeds. I would postulate that the slightly larger drainage particle may allow for a better air/moisture mix across the interior of the pot bottom than if the medium particle size was throughout. However, i would not wan to be the person trying to devise that experiment and measure the difference,

 

[markyscott]

Thanks. I also discard the fines, unless needed for cuttings and seeds. I would postulate that the slightly larger drainage particle may allow for a better air/moisture mix across the interior of the pot bottom than if the medium particle size was throughout. However, i would not wan to be the person trying to devise that experiment and measure the difference,

Here it is for Turface


Height of the column is the total effective porosity. Height of the blue column is the container capacity. Height of the green is the AFP. This is for an 11.1 cm soil column.

S

 

[0soyoung]

I ran some additional experiements that those of you who follow this thread might be interested in. First, I changed my experimental method a bit. I found it very difficult to measure properties of small soil volume by measuring the volume of water drainage. So I switched to weight - I think it’s more accurate all the way around. I just measure the dry soil weight and then tare my scale. Then I measure the weight of the water that I add to the soil to completely saturate it. Finally, I drain all of the gravitationally held water and weight the soil again. The difference in weight between the saturated and drained measurements should be the weight of the capillary bound water. And the weight of the water that I add to the dry soil to completely saturated it is a measure of the porosity. It all works out pretty easy. Just a lot of this.

I'm confused. Originally your cylinder had a hole in the bottom that you covered with finger (in honor of all thumbless primates!). Then you measured the volume of water that drained out after you removed your digit.

How are you plugging/draining with this method?????

 

[markyscott]

I'm confused. Originally your cylinder had a hole in the bottom that you covered with finger (in honor of all thumbless primates!). Then you measured the volume of water that drained out after you removed your digit.

How are you plugging/draining with this method?????

I made a plug! It sounds obvious when I say it, but it took me an hour or so to figure out something that would work. I ended up wrapping electrical tape around the end of a chopstick and pushing it into the hole. I then tared the scale with the plug and the empty cylinder and filled it up to the desired soil volume. Then I tare the scale again with the dry soil in the cylinder and add water till it’s even with the top of the soil - that gave me the pore volume. I then removed the plug and re-weighed after the water drained to get the AFP. Easy peasy.

Scott

 

[Gary McCarthy]

I bought one of these to do my soil testing - https://www.amazon.com/dp/B004WMOR6...t=&hvlocphy=9005502&hvtargid=pla-318190381386

Works GREAT!

 

[0soyoung]

I understand where your confusion comes from. My apologies. Although both points are correct, my original statement is misleading as written. Please accept this as a correction:

Grain shape - rounded particles have lower porosity, angular grains have higher porosity. Although in general higher porosity tends to mean higher AFP, angular grains have a smaller pore size, hence the lower permeability and higher average water saturation

Thanks for pointing me to a much needed clarification.

Scott

I'm going back through this thread. Funny how I see things later.

Specifically regarding this grain shape stuff. Doesn't post #225 demonstrate that grain shape is irrelevant for the size stuff we're concerned with??

If so, when we encounter AFP > 40% we know that the particles are porous, like lava - right?

Further, post #225 would seem to say that impervious particles (of the size we care about) will always have a porosity very close to 40%. Hence AFP less than 40% would reflect water held by surface attraction (capillary action) --> higher CEC should have lower AFP (for impervious particles, of course).

 

[Clicio]

This is surely the MOST incredible thread about soil I've ever read!
Thanks a lot to all of you guys, specially Scott.

 

[AlainK]

Hi Scott,
I don't have time to read the 14 pages right now, but I've bookmarked the thread.
Thanks for posting.

 

[markyscott]

I'm going back through this thread. Funny how I see things later.

Specifically regarding this grain shape stuff. Doesn't post #225 demonstrate that grain shape is irrelevant for the size stuff we're concerned with??

If so, when we encounter AFP > 40% we know that the particles are porous, like lava - right?

Further, post #225 would seem to say that impervious particles (of the size we care about) will always have a porosity very close to 40%. Hence AFP less than 40% would reflect water held by surface attraction (capillary action) --> higher CEC should have lower AFP (for impervious particles, of course).

Good question Oso.

I wish that graphic extended to a size range more relevant to us. I think that you’d find is that the total porosity is more or less independent of grain size in the size range that we care about. But different grain shapes will result in different theoretical limits of uncompacted random packing with angular grains having a higher porosity limit than spherical grains. Similarly, well-sorted grains will have a higher porosity than poorly sorted grains. But also remember, it’s not the total porosity that really matters to much for AFP. It’s the shape and size of the pores that counts. Two impervious materials - one comprised of angular grains and one comprised of spherical grains - can have identical porosity. However, the pore size associated with the angular grains will be smaller than that associated with the spherical grains and therefore have a higher equilibrium water saturation.

I wish I had more information about the experiments behind that graphic, but I don’t have the info on the sphericity of the grains used. I believe that they were synthetic and built to a pretty tight tolerance, so I would imagine that natural material would have quite a bit more variability than that exhibited in that chart. But it makes the point about the trends. In theory, if we could know the correct theoretical limit of uncompacted random packing for the material we were working with, we could do exactly what you suggest and determine intra- vs inter-granular porosity.

S

 

[KillerButts]

Thank you very much Scott, you made a few chapters in my Soil Science Simplified book much more comprehensible. Looking forward to reading more of your posts.

 

[markyscott]

Thank you very much Scott, you made a few chapters in my Soil Science Simplified book much more comprehensible. Looking forward to reading more of your posts.

Thanks. I’m glad you got something out of the discussion.

S