Author Topic: Substrate and water retention  (Read 15139 times)

Markyscott

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Re: Substrate and water retention
« Reply #15 on: November 30, 2012, 11:44 AM »
Are you sourcing seramis locally or do you order it online?

Ordered it online from the website above.  Shipping costs are reasonable as it's pretty low density (similar to pumice).

Scott
 

0soyoung

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Re: Substrate and water retention
« Reply #16 on: November 30, 2012, 01:54 PM »
Thank you Markyscott. Well done, IMHO.

It seems to me that the depth of saturation zone ought to be independent of pot shape. This could be simply and easily demonstrated to be true/false by using another column with a significantly different diameter than the one you've been using. One would observe the depth of water retained in each column - is it or is it not the same.

Again, kudos, for demonstrating a little basic scientific analysis. Most refreshing!

I hope to see more.

 

Markyscott

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Re: Substrate and water retention
« Reply #17 on: November 30, 2012, 02:11 PM »
Thanks for the post! very interesting.  I'd imagine Double Red Line Akadama would hold more and more water as it breaks down-but that's just a guess.  I would have also guessed that fresh Akadama would have still held more water than turface.  I'd be interested to see where lava compares with the rest. 

So can you not set your sprinklers to water another time during the day when your at work?   

I think it does break down a bit over time in the pot.  Fine-grained akadama behaves like fine-grained pumice - it becomes more water retentive.  So my intuition is aligned with yours - it should probably become more retentive over time.  Here that might be a good thing as a more mature root system has higher water demands.

In this test, Turface was consistently a bit more water retentive than akadama. The issue I had when preparing the material was the grain size - Turface mvp is pretty fine grained compared to the other material, so to get 500ml of 3/8-1/4" turface, I had to do a lot of sieving and there were a lot of fines that ended up getting discarded.  I hate doing all that manual labor only to have 1/2 of it end up in the compost heap.  Oh well - my aspidistra looks great!

Re the sprinklers - the answer is yes there is some optimization to do there.  Good thing I like projects.

Scott
 

Markyscott

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Re: Substrate and water retention
« Reply #18 on: November 30, 2012, 02:53 PM »
Thank you Markyscott. Well done, IMHO.

It seems to me that the depth of saturation zone ought to be independent of pot shape. This could be simply and easily demonstrated to be true/false by using another column with a significantly different diameter than the one you've been using. One would observe the depth of water retained in each column - is it or is it not the same.

Again, kudos, for demonstrating a little basic scientific analysis. Most refreshing!

I hope to see more.


Thanks 0soyoung - I appreciate your feedback.  The soil saturation profile is pretty well understood and I think you're right in that the profile is independent of the shape of the container.  What it is related to is depth (saturation increases with depth) and porosity (saturation increases with decreasing porosity).  Porosity is proportional to grain size.  So as you move up from the soil at the base of the pot, the saturation is initially very high and then drops off as you move toward the soil surface.  You might be able to see the bright band at the bottom of the beaker in one of my previous posts.  Within that zone the saturation was close to 100%.  The zone was taller with finer grain sizes.  I don't expect that to change with the diameter of the beaker as it's related to the capillary properties of the soil.

But here's how the field capacity (the amount of water in the soil after all the gravitational water has drained away) is related to the shape of the pot.  As I pointed out above, the saturation at the bottom of the pot is high and drops off with distance from the bottom. If you only have a little way to go (in a shallow pot, say) the saturation doesn't drop off much before you reach the soil surface.  If you're looking at a deeper pot you can go up further and the saturation can drop off considerably more.  So in the beaker (think of it as a very tall pot) only a small fraction of the 500 ml of soil has that very high saturation.  If I spread that 500ml of soil out into a thin layer, the entire volume of soil would have have a very high saturation, so field capacity is much higher.  Visualize a sponge standing on end vs a sponge lying flat.  The flat-lying sponge will hold more water then the sponge standing on end because of this same effect.

Here's an interesting aside.  What does the drainage layer do?  Essentially, it makes the pot shallower, moving the higher saturation zone normally at the bottom of the pot shallower as it will sit on top of the "drainage layer" - it makes a perched water table, so to speak.  So there is no increase in drainage with a drainage layer (the 500ml of soil has the same field capacity whether it is sitting on a drainage layer or not.  What it does is move the wet zone off the bottom of the pot.

Scott
« Last Edit: November 30, 2012, 03:35 PM by Markyscott »
 

0soyoung

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Re: Substrate and water retention
« Reply #19 on: November 30, 2012, 03:48 PM »
Right on - I agree whole heartedly and as you've laid it out for all to see, it is easy to demonstrate these facts.

So when one has an anchient and prized tree, in a hot climate like the environs of Edo and Kyoto in the summer time, one puts coarse soil at the bottom of the pot so the roots along the bottom of the root pad aren't sitting in saturated soil. Then Owen, for example, puts finer soil around the upper edges of the pad so that there is ample water for roots near the surface that also have to contend with evaporation loss. I do believe this is why the misnamed/misunderstood "drainage layer" appears in his videos. It is not for drainage, per se, and calling such just obfuscates what it is about.

Again, thanks for the expose and discussion. I love science and glad you do too!

What's next?
 

Markyscott

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Re: Substrate and water retention
« Reply #20 on: November 30, 2012, 04:12 PM »
Right on - I agree whole heartedly and as you've laid it out for all to see, it is easy to demonstrate these facts.

So when one has an anchient and prized tree, in a hot climate like the environs of Edo and Kyoto in the summer time, one puts coarse soil at the bottom of the pot so the roots along the bottom of the root pad aren't sitting in saturated soil. Then Owen, for example, puts finer soil around the upper edges of the pad so that there is ample water for roots near the surface that also have to contend with evaporation loss. I do believe this is why the misnamed/misunderstood "drainage layer" appears in his videos. It is not for drainage, per se, and calling such just obfuscates what it is about.

Again, thanks for the expose and discussion. I love science and glad you do too!

What's next?

Thanks again 0soyoung.  For me it's helpful to develop a physical intuition about what's going on so that when recommendations such as these are made based largely on experience, I have some capacity for reasoning the physical basis for them.  

A question struck me as I was reading your inference about the effect of the drainage layer.  Does it keep the roots at the bottom of the pot out of the highly saturated zone, or does it encourage more feeder root development above the drainage layer and away from the pot edge where it is cooler and the feeder roots are less susceptible to damage from contact with the heated pot?  You've given me something to think about.

In the short term I'll post some results of the tests on other substrates and soil compositions.  Then I want to think about "soil pH" - another factor I hear a lot of conflicting information about.

Scott
« Last Edit: November 30, 2012, 04:27 PM by Markyscott »
 

Markyscott

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Re: Substrate and water retention
« Reply #21 on: November 30, 2012, 06:52 PM »
Here's an interesting plot.  It compares the water retention capacity of four baked clay products: Seramis, Turface, Akadama, and Haydite as a function of grain diameter.

 

John Kirby

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Re: Substrate and water retention
« Reply #22 on: November 30, 2012, 06:59 PM »
Haydite isn't baked clay, it is expanded shale (think popped popcorn). I guess the surface adherance of water is having a greater pact than substrate, in general terms. Now replicate 5,000 times, Owen is waiting with bated breath.
 

Markyscott

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Re: Substrate and water retention
« Reply #23 on: November 30, 2012, 07:11 PM »
Haydite isn't baked clay, it is expanded shale (think popped popcorn). I guess the surface adherance of water is having a greater pact than substrate, in general terms. Now replicate 5,000 times, Owen is waiting with bated breath.

Thanks for the correction, John.  And I have all my bonsai minions feverishly working on the 5000 replications.

Scott
 

Markyscott

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Re: Substrate and water retention
« Reply #24 on: November 30, 2012, 07:25 PM »
Same plot as before, but with my version of "Boon mix" added.  Houston, not being in an important volcanic province, is poorly supplied with scoria.  So scoria is replaced with haydite in my version to save cost a bit: equal parts akadama, pumice, and haydite along with a bit of granite and charcoal.  Note that the water retention is a little better than any of the components alone.  Not as high as turface or seramis, but higher than akadama, haydite, or pumice.  Here are a couple of possibile explanations:

1) Grain shape is an important control on equilibrium saturation.  It's possible that the mix of grain shapes lowers the porosity somewhat and, as a result, increases the capillary held water.

2) I was pretty thorough about removing the dust from the substrate I was using for the experiments as I rinsed it all and then dried thoroughly.  I seived the mix to remove the dust when mixing the soil.  I seived again before the experiment, but did not rinse and dry as it takes several days.  This may have skewed the results somewhat to higher water retention for the mixture.

Other ideas?
 

0soyoung

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Re: Substrate and water retention
« Reply #25 on: November 30, 2012, 07:47 PM »
Suppose that each element of the mix was as its own layer 'cake' of a soil. Isn't it obvious that the total retention would be the weighted average of the individual components, where the weight is the relative proportion of the total. At a glance I would say that is what you've got - the weighted average. This would simply say that the extra stuff like charcoal was negligable in this case and that there's nothing 'exotic' going on. Simiarly if it is homogenous (i.e., well mixed) the saturation depth would be the weighted average, wouldn't it?

On the other hand, if something exotic was happening, the total volume of your soil would be less than the sum of the component volumes. Alcohol in water is like this. Alcohol molecules fit in spaces between water molecules so that 1 volume of water and an equal of alcohol makes less than 2 volumes mixed.
« Last Edit: November 30, 2012, 07:49 PM by 0soyoung »
 

John Kirby

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Re: Substrate and water retention
« Reply #26 on: November 30, 2012, 07:54 PM »
Remember, two points make a straight line, so the straight lines may or may not have any relevance. But, what couldn be interesting would be to see the variance at each of the common mean particle sizes- from 3-4 measures per point. My students often get irritated when I ask them if two measurements are actually different. Based on what you have shown, I am not sure that you can actually say that any of the means are actually different using inference. But I certainly like your approach.

One more thing, with measures such as depth and temperature and vapor pressure and flux, I would bet a Pepsi Cola that you are not looking at a simple linear equation.

But I am just a grumpy old cuss.
« Last Edit: November 30, 2012, 07:57 PM by John Kirby »
 

Markyscott

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Re: Substrate and water retention
« Reply #27 on: November 30, 2012, 08:18 PM »
Remember, two points make a straight line, so the straight lines may or may not have any relevance. But, what couldn be interesting would be to see the variance at each of the common mean particle sizes- from 3-4 measures per point. My students often get irritated when I ask them if two measurements are actually different. Based on what you have shown, I am not sure that you can actually say that any of the means are actually different using inference. But I certainly like your approach.

One more thing, with measures such as depth and temperature and vapor pressure and flux, I would bet a Pepsi Cola that you are not looking at a simple linear equation.

But I am just a grumpy old cuss.

All good points GOC - in my defense I pointed out that it was semi-scientific at the beginning.  Not ready for a Nature paper yet, to be sure but when it is, I'm looking for a good co-author.  Here are some thoughts:

Point 1 - I actually expected the curves to behave more linearly with porosity than grain size, but they don't for some reason. 

Point 2 - You're right that the experiment needs to be repeated for the different points to establish the variability.  I'll continue to run these and I'll post updates occassionally when I have additional data.  But it can take several days to thoroughly dry the soil between experiments, so there are some practical limits to how fast I can turn around the data.

Point 3 - Probably not a linear equation as equilibrium saturation is strongly related to angularity, sorting, and porosity as well as average grain size.  But for this system and the data I've collected so far, it appears to behave fairly linearly with respect to grain size over the range of sizes that I tested.

Cheers
Scott
 

Markyscott

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Re: Substrate and water retention
« Reply #28 on: November 30, 2012, 08:31 PM »
One more observation before bed time.  When testing Turface I had an abundance of fines, so I tested a cut with about a 1/8" average grain size.  The results were pretty interesting - when I poured the water through, there was a significant amount of channelization.  Once a saturated path was connected through the beaker, most of the water flowed through the channel leaving portions of the substrate unwetted.  Pouring more water through did not significantly wet the dry spots.  Even after pouring several times the soil volume of water through the beaker dry spots remained.  I've attached a photo (sorry for the picture quality) taken after having poured 2000ml of water through the 500 ml of soil.  The light areas are dry spots that remained.  I poured 4000ml of water through the 500ml of soil and the dry spots remained.  Increasing the average grain size seemed to eliminate this problem.

Having dry spots in the substrate after watering is probably something to be avoided.  I can imagine that this might kill feeder roots and weaken the tree in the long run.  If you leave fines in the substrate, be aware that this can happen and take care with the watering.  It might take much more than you think to thoroughly wet the medium.

Cheers
Scott
« Last Edit: November 30, 2012, 08:33 PM by Markyscott »
 

John Kirby

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Re: Substrate and water retention
« Reply #29 on: November 30, 2012, 08:45 PM »
Well, you can rationalize just about anything, especially if you can make a reasonable graphical representation!

As I inferred, I salute your efforts. However, I wouldn't draw many conclusions other than stuff in pots hold water. It is kind if like recording the temperature on a monday, tues,wed,thurs, fri, sat and a sunday. Then, making the statement "On Monday the temperature was 70 F, on Tuesday the temperature was  72,etc.,and then concluding "Tuesdays are warmer than Mondays."

Great little book out there that helps with these kinds of things, called "How to lie with Statistics", great examples of how graphs and figures can unintentionally lead to unusual interpretations.

Thanks Scott for stimulating a fun conversation!