Introductory soil physics

[Orion_metalhead]

In a grow out setting it should retain enough moisture lower down to work. For example, small red stone chips dont absorb water but they maintain moisture when laid in a 3 or 4 inch depth. Pea gravel would do rhe same. Maybe combine it with a top dressing of landscape fabric cardboard to raise humidity.

 

[markyscott]

Scott, question. I was using mulch to fill my above ground beds in the previous home with great results, but when I had to pull the trees out to move I realize that the escaped roots actually grew into the mulch making it very hard to pull. I since have the idea to freight some pumice in, probably 1-2 cu Y to fill all beds, but at the price + freight I think I will be well over $1k. I am stopping by the local Napa to take a look at the size of their 8822, and see if it would be a cheaper option. I also researched a little on expanded shale, but couldn't find a supplier with smaller particles.

What other substrate/stone would you consider if you were doing the same thing? I'm not looking for a material to add into the pot, but a loose substrate that the roots won't grow into, will help retain moisture on the grow bags, and allow the roots to grow between the particles (not something that would compact hard like decomposed granite).

I still have around 5-7 months as they are supposed to break ground in the next 2 weeks with a build time between 4-6 months. I'm even considering taking a road trip with a trailer to Arizona, I think Acme Sands is the closest place to us that sells in bulk. That would be around half the price including a sleep over night to wait for them to open.

Have you considered perlite? I like using a perlite/coco coir 70/30 mix for my seedlings and growing out young plants. I think DE would be a good option, but 8822 is far too small for my taste. I'd much prefer a good coarse grained perlite and it's pretty inexpensive. My instincts are to avoid pea gravel. No secondary porosity for moisture retention. It would be just an inert space filler. But let me know your experience if you give it a go.

- S

 

[Maiden69]

Have you considered perlite?

I bought a 4 cuF bag of perlite to place all the trees in pouches in pot-in-pot panting. I think it would be a good option but it would be a mess with the winds and rain here in central Texas. Definitely not a fan of 8822 if the particles are too small… I guess I will have to do some more research.

 

[Dadayama]

Have you considered perlite? I like using a perlite/coco coir 70/30 mix for my seedlings and growing out young plants. I think DE would be a good option, but 8822 is far too small for my taste. I'd much prefer a good coarse grained perlite and it's pretty inexpensive. My instincts are to avoid pea gravel. No secondary porosity for moisture retention. It would be just an inert space filler. But let me know your experience if you give it a go.

- S

O'Reilly's Autoparts sells an EP Mineral company product called Optisorb that is listed as large course DE. I have not tried it so I don't have a testimonial for the it. https://www.oreillyauto.com/detail/...-suv-2020-subaru-forester?q=floor+sweep&pos=6

Question though, so I understand how DE absorbs but does it give up water back up to the plant? I keep wondering if it would suck the moisture out of the roots. If you or someone has an answer I am all ears.

 

[Maiden69]

O'Reilly's Autoparts sells an EP Mineral company product called Optisorb that is listed as large course DE. I have not tried it so I don't have a testimonial for the it. https://www.oreillyauto.com/detail/...-suv-2020-subaru-forester?q=floor+sweep&pos=6

Question though, so I understand how DE absorbs but does it give up water back up to the plant? I keep wondering if it would suck the moisture out of the roots. If you or someone has an answer I am all ears.

Thanks for posting this, I found a few places that sell EP products and thought about it but considered the 8822 as it was "cheaper". The particle size seems in the 1/16" - 1/4" from the picture in their website. As far as your question, I know that a lot of people use it in their soil, I think @sorce is or was one of them. In my case, I never let the soil dry out entirely, so I don't see an issue with the De particle absorbing moisture from the roots.

 

[Maiden69]

For the EP Minerals, the coarse particle is their 8925 product, they have a 9825 that is medium sized. They do look slightly similar in size... they also have a monto clay / DE mix that is sold as thrifty-sorb that surprisingly looks similar to Bonsai Jack's Bonsai Block, with the difference that Bonsai Jack Monto and Block have a rounder, more desirable shape.

8925


9825


Thifty-Sorb

 

[Bonsai Forest]

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

Do these values change between organics (such as Lowe's average run-of-the-mill potting soil) vs. inorganics (Akadama, Pumice, etc.)? And is this one of the many major distinctions between Development vs. Refinement? I've heard that soil such as Akadama has tubercles throughout the interior of each grain/particle: Does that play a role in the AFP/WHC?

 

[Bonsai Forest]

Hi c54fun - that's the right idea, but it was fines migration that Davetree noticed. The fines moved down to the bottom of the cup with watering leaving the coarse grained fraction on top. Take a look at the picture again - see how much coarser the soil is on top of the cup than on the bottom? Stratification through fines migration would enhance the downward increase in water saturation resulting from gravity drainage. So even higher water saturations and lower AFP at the bottom of the pot than the top due to fines migration in a poorly sorted mixture.

So now we know. If we don't sieve we'll get higher water saturations and lower AFP overall. And due to fines migration we may end up with a more pronounced water saturation profile than we would otherwise expect.

Scott

An increased potential for root rot?

 

[markyscott]

An increased potential for root rot?

Yes. Fungi responsible for root rot are encouraged by high water saturation.

I.e.:

What happens when soil gets too much water

Saturation can lead to crown and root rots long after the rain has stopped

californialocal.com

S

 

[markyscott]

Do these values change between organics (such as Lowe's average run-of-the-mill potting soil) vs. inorganics (Akadama, Pumice, etc.)? And is this one of the many major distinctions between Development vs. Refinement? I've heard that soil such as Akadama has tubercles throughout the interior of each grain/particle: Does that play a role in the AFP/WHC?

Let’s define terms. “Macropores” or “mesopores” are the large spaces between the individual grains that make up the soil. “Micropores” are the tiny spaces within the grains themselves. All soils - whether organic or inorganic - have at least some degree of microporosity. Organic soils tend to have a lot. Inorganic soils have some and it’s pretty variable as to the amount. As the size of the pore decreases, the capillary and electrostatic forces binding the water to the soil surface increases dramatically. As a result, water contained in the macropores is generally plant available, but not all water contained in the micropores is, as the binding forces can exceed the plant’s ability to access it.

With that in mind, there are three saturation states that matter. “Saturation” happens when you are watering and all the available macro and microporosity is completely filled with water. The second you stop watering, the water drains away by gravity until the capillary forces holding onto the water are equal to or greater than the force of gravity. This state is called the “field capacity”. It’s the water available to the plant until the next time you water. Think of it like a battery. It’s fully charged at field capacity.

At field capacity, some of the pore space is filled with air, and some is filled with water. That’s the air-filled and water-filled porosity (AFP and WFP). How much of each is a function of the soil type, as discussed at length in this thread. Generally, the macropores are partially saturated and the micropores are fully saturated at field capacity.

Once you walk away, stuff happens to the remaining water. Some of it gets used by the plant. Some of it evaporates. The amount of water in the soil goes slowly and inexorably down over time just like draining the battery in your cell phone. At some point, the water saturation gets so low, the only water that’s left is bound to the soil by forces that exceeds the plant’s ability to access it. This point is called the “wilting point”. Most of the bound water at the wilting point is in the micropores.

All this is to say that yes. Micropores make a play a role in afp and wfp. All soils have micropores. Organic soils tend to have a lot of microporosity. Less of the water in micropores is plant available than that in mesopores.

S

 

[Bonsai Forest]

Let’s define terms. “Macropores” or “mesopores” are the large spaces between the individual grains that make up the soil. “Micropores” are the tiny spaces within the grains themselves. All soils - whether organic or inorganic - have at least some degree of microporosity. Organic soils tend to have a lot. Inorganic soils have some and it’s pretty variable as to the amount. As the size of the pore decreases, the capillary and electrostatic forces binding the water to the soil surface increases dramatically. As a result, water contained in the macropores is generally plant available, but not all water contained in the micropores is, as the binding forces can exceed the plant’s ability to access it.

With that in mind, there are three saturation states that matter. “Saturation” happens when you are watering and all the available macro and microporosity is completely filled with water. The second you stop watering, the water drains away by gravity until the capillary forces holding onto the water are equal to or greater than the force of gravity. This state is called the “field capacity”. It’s the water available to the plant until the next time you water. Think of it like a battery. It’s fully charged at field capacity.

At field capacity, some of the pore space is filled with air, and some is filled with water. That’s the air-filled and water-filled porosity (AFP and WFP). How much of each is a function of the soil type, as discussed at length in this thread. Generally, the macropores are partially saturated and the micropores are fully saturated at field capacity.

Once you walk away, stuff happens to the remaining water. Some of it gets used by the plant. Some of it evaporates. The amount of water in the soil goes slowly and inexorably down over time just like draining the battery in your cell phone. At some point, the water saturation gets so low, the only water that’s left is bound to the soil by forces that exceeds the plant’s ability to access it. This point is called the “wilting point”. Most of the bound water at the wilting point is in the micropores.

All this is to say that yes. Micropores make a play a role in afp and wfp. All soils have micropores. Organic soils tend to have a lot of microporosity. Less of the water in micropores is plant available than that in mesopores.

S

Thank you for this thorough response.