Introductory soil physics

[fh05]

Great post Scott. It takes a lot of time and effort to do what you are doing. Appreciate your work, thanks.
Did you get the same WHC for these three groups?

 

[markyscott]

Here's the data for Turface:



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.



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)



Scott

 

[markyscott]

Great post Scott. It takes a lot of time and effort to do what you are doing. Appreciate your work, thanks.
Did you get the same WHC for these three groups?

I just have that much love for the nutters. At least 25% of the time . Glad you feel as though you're getting something out of the discussion.

Scott

 

[markyscott]

So, there are the numbers. Now I have questions for you:

1. Was it what you expected?
2. Was there anything that surprised you?
3. Did the results make you think of additional questions you hadn't thought of before?

Scott

 

[0soyoung]

1. yes
2. no
3. yes: if 'we' repeated these measurements, but each time with smaller volumes (say, 250 ml, then 125 ml, then 60 ml, etc), we should be able to then figure out what the saturation layer depth is for each category of grain size; seems like a good number to know - maybe there's an easier way.

 

[markyscott]

1. yes
2. no
3. yes: if 'we' repeated these measurements, but each time with smaller volumes (say, 250 ml, then 125 ml, then 60 ml, etc), we should be able to then figure out what the saturation layer depth is for each category of grain size; seems like a good number to know.

I'm always surprised when I do these that changing grain size has zero impact on the porosity. I know that it's relatively insensitive, but zero impact (or at least within measurement error)? It's fascinating to see that.

Good question Oso - we'll add that to the list. There's a bit of a practical limit with the way I set up the experiment, though. It'll get harder and harder to measure values the smaller the soil volume we use. But that is what it is and I agree that's a good question. We'll have to pick a soil to use though. Let's think about that next.

Scott

 

[coh]

Have you ever tried these measurements with a clear container (so you can see any perched water) shaped like a bonsai pot? It's something I've been meaning to do but haven't gotten around to it. Maybe this winter...

 

[0soyoung]

Have you ever tried these measurements with a clear container (so you can see any perched water) shaped like a bonsai pot? It's something I've been meaning to do but haven't gotten around to it. Maybe this winter...

I've tried to with clear plastic orchid pots and it is a problem very similar to seeing it in a sponge, if you've ever tried that. The layer of water is just not that distinct visually.

 

[coh]

I've tried to with clear plastic orchid pots and it is a problem very similar to seeing it in a sponge, if you've ever tried that. The layer of water is just not that distinct visually.

Thought that might be the case, what if you add some color to the water...I'd also be interested to see how the numbers for AFP and WHC change in the shallower container.

 

[markyscott]

Have you ever tried these measurements with a clear container (so you can see any perched water) shaped like a bonsai pot? It's something I've been meaning to do but haven't gotten around to it. Maybe this winter...

I never have, but it's a great idea. Let me think about that a bit.

Scott

 

[markyscott]

Thought that might be the case, what if you add some color to the water...I'd also be interested to see how the numbers for AFP and WHC change in the shallower container.

I've got some food coloring. Do you have a color preference?

Scott

 

[markyscott]

OK - let's talk about some other results.

This time, let's stick to the 1/4"-3/8" fraction and change grain compositions. Here are a bunch of different soil components.



Seramis - Pumice - Haydite - Turface

 

[markyscott]

Granite - Boon Mix - Akadama

 

[markyscott]

So 7 different commonly used inorganic soil components. Before I present the results,

1. Which do you think has the highest/lowest porosity?
2. Which do you think has the highest/lowest AFP?
3. Which do you think as the highest/lowest WHC?

Scott

 

[0soyoung]

So 7 different commonly used inorganic soil components. Before I present the results,

1. Which do you think has the highest/lowest porosity?
2. Which do you think has the highest/lowest AFP?
3. Which do you think as the highest/lowest WHC?

Scott

NO!!!!!!!!!!!

Don't open the door for soil wars!!!!

DATA --> insight.

Granite is low porosity, but ...

 

[markyscott]

NO!!!!!!!!!!!

Don't open the door for soil wars!!!!

DATA --> insight.

Good point forget the question. I'll just post the results!

Scott

 

[markyscott]

Here are the numbers.



T = turface
P = pumice
H = haydite
BM = Boon mix
G = granite
A = akadama
S = seramis

The pumice came from Wee Tree. Seramis is no longer available in the US. Boon Mix is approximately equal parts pumice, scoria and akadama. It has a small percentage of horticultural charcoal and granite in as well.

Scott

 

[coh]

I'm surprised how different turface and haydite are in terms of WHC and water saturation. Do you have lava to test? I did some very simple tests of water holding capacity a while back and think they (lava and pumice) scored quite differently, though I'm not sure where the numbers are. Also interesting that turface and akadama scored similarly. Wonder what the variance/standard deviation is for these kinds of tests.

 

[markyscott]

Here are the plots.

• Granite had the lowest porosity, the lowest WHC and the highest AFP.
• Turface and Seramis had the highest water saturations
• Boon mix, Akadama, and Seramis had the highest porosity
• Pumice had the highest AFP
• Seramis had the highest WHC, followed closely by akadama and turface (which were exactly the same)

There you go. I don't know if Andrew is still following along, but I'd be curious to know what he thinks about the pumice results.

Scott

 

[markyscott]

I'm surprised how different turface and haydite are in terms of WHC and water saturation. Do you have lava to test? I did some very simple tests of water holding capacity a while back and think they (lava and pumice) scored quite differently, though I'm not sure where the numbers are. Also interesting that turface and akadama scored similarly. Wonder what the variance/standard deviation is for these kinds of tests.

Hi coh - I don't have any scoria on hand, but I think I can get some. I'll give it a go. I believe that scoria and pumice will have very different results.

I can't claim that these are scientific results - they'd have to be repeated many times to be sure because there'll be some variability - especially with the imprecise tools I'm using. Also pumice and scoria are natural materials and there's quite a bit a variability in their texture and intragranular porosity, so I'd expect a fair amount of variance batch to batch and supplier to supplier. That said, these numbers are in line with what I've gotten when I did them before, so I think that they are directionally correct.

One of the keys to getting reproducible results is to carefully screen out the dust. That can skew your results significantly toward lower porosity and higher water saturation. There's a lesson there too, I'm sure.

Scott