Since turface particles have a different size and moisture holding capacity than sand, I would not consider them a "replacement" for each other.

Porosity is an important factor to consider, which you touched on when considering the water and oxygen between the particles.

Still not convinced that sand isn't useful in almost all C&S mixes?

We've discussed this a number of times, and, while graphs are useful, they don't take the place of actual in-the-field experience.

Turface is useful as part of a mix (and sometimes, it's all you need for rooting cuttings), but when you have access to pumice, why use Turface? IME there's simply no need to.

I too am curious as to why these questions keep coming from you. If my posts aren't credible (and sometimes they're deliberately not so, though not in responses to you), surely some of the other folks who have taken the time to respond in great detail to you are.

@cactusmcharris, some of my plants have thrived in container soils with significant sand content and struggle (without constant watering) in gritty mix. I am trying to understand the science behind why that might be happening.

@Gabby C, I never said Turface substitutes for sand in terms of function. I asked a simple question: *IF* I replace sand with Turface, what do I gain and what do I lose? "Replace" in this context did not mean functional substitution. Replace meant using 1/3 Turface instead of 1/3 horticultural sand.

@tropic I thought that most of the perched water table was about the water that is *outside* of the particles in the soil? I thought it was the forces of adhesion, cohesion, and capillary action that mostly determined where the perched water table would settle. The whole point of using some soil components with high water retention is to store water inside particles in a way that benefits the plants over the next few hours or days without robbing the soil of oxygen in between particles.

If you want to take the position that water retention also determines the perched water table, I would want you to show some studies that quantify how much this is so. I am not questioning the idea, but definitely I would want to know in the big picture how important is that idea relative to adhesion/cohesion/capillary action.

I like your approach to putting smaller particles in the topsoil of a container that has both shallow and deep-rooted plants. I had a similar idea because I have been filtering out the fines from Turface using a 1/20" sieve. I have boatloads now of this Turface "fines" that is all pretty much over 1 mm (and visually they appear to be over 2 mm). I am going to experiment with doing a perched water table experiment using a soil that is:

a) 1/3 coarse sand, 1/3 peat, 1/3 pumice

b) 1/3 large particle (> 2mm) Turface, 1/3 peat, 1/3 pumice

c) 1/3 Turface fines (< 2mm), 1/3 peat, 1/3 pumice

I will report back with PWT results that I get from those combinations.

Westes, yes some plants do OK with the excess water retained by sand. I grow several subspecies of Salvia Guaranitica in pots. Due to their very dark leaves and being in direct sun for 15 hours of summer sun they use A LOT of water in transpiration, and do poorly in straight 5-1-1.

In 90+ F weather they will dry out and wilt if not watered every single day. I started to adjust the mix to make it more water retentive. I'm not too far off of some of the "premium" bagged soils for the Salvia ONLY, and have used 1/16 to 1/8 (2-4 mm-ish) turface as TOC stated above (because I had it on hand as a byproduct of making bonsai soil) with success. I also grow Cardinal Lobelia in a pot in a 4" pan of water. Definately got some Perched water in that one! If I put it in 5-1-1, it will grow and flower very poorly. My others do fantastically well in 5-1-1.

Westes it is not easy to decipher what you want unless you provide a full description of what other components of your soil are. Horticultural sand is different from normal sand since they are usually over 2mm and are somewhere between 2-5mm usually. That will behave exactly like grit in gritty mix. Although chicken grit as available tends to be more like 4-5mm in size and has very little in the 2mm size and so will retain less water. All the water retained in sand or grit is at the surface only. That is why the water retention is so low for them. Smaller the particle size, the greater is the surface area available for retaining water for them.

Which one you are talking about sand or horticultural sand?

Gritty mix as known here IS composed of grit, turface and bark. One can adjust the relative proportion of grit vs turface to increase/decrease water retention without compromising air porosity. Sand and turface are wildly different components - so Gabby is quite right asking the question. Turface and similarly porous substances hold water internally as well as at the surface. The amount held at the surface will be somewhat similar to sand and grit. But water held internally depends on the pore structures. Whether that water is available to the roots is more complex. Turface gives it up easier than Pumice.

I also suggest not to focus too much on those water release curves. That is only part of the puzzle. Trying to interpret the way you are doing is not going to be very helpful. If you want to then please read and understand from the original article very very carefully. I think they were posted by another member here many years back.

TOC, you wrote: But water held internally depends on the pore structures. Whether that water is available to the roots is more complex. Turface gives it up easier than Pumice.

Any idea how Diatomaceous Earth stacks up? I switched to DE because the particle size has a better % of useable than turface and supposedly keeps critters at bay and price.

I do not know the characteristics of DE. I would think it will be fairly close to turface - may be even better. So far I have used it one bag as sub for turface and I cannot tell any difference. When I can remember I put a layer of sifted DE at the bottom to discourage ants to enter from the holes and start a colony (happened a few times).

@tropic I am talking about horticultural sand, so presumably 2mm particles.

If I want to consider the characteristics of sand alone, why am I going to confuse the issue by specifying all the other components at the same time? All you are doing is turning a single variable experiment into a multi-variable experiment that has so many permutations it cannot be well understood in this kind of discussion format. We cannot even get people in this discussion to focus and have a clear discussion about sand. Everyone wants to twist words and find some way to restate the problem so they can say "No that's wrong".

I considered water retention for sand against other common mix components because I wanted to understand this characteristic of sand alone, to understand what it contributes by adhesion and to better understand that it retains almost no water. Basically, it looks like sand is providing a very short-term intense water saturation - via adhesion of water between sand particles - and that quickly disappears over the first day or two of drying (depending on conditions), and after that sand contributes very little to the roots.

In considering substitution of Turface *fines* (2mm to 4mm) for horticultural sand, I was simply asking what do I give up and what do I gain. What I give up is a little bit of water adhesion, which some might argue is actually a benefit. What I gain is a lot of extra water retention in the turface, which releases water on a different schedule than the peat. I also gain the ability to quickly rehydrate the mix because Turface never becomes hydrophobic. Peat - when completely dry - is hard to resaturate.

I created some test soils and I will observe how those behave over the next week of drying. It was too late in the day to clearly see the PWT location, but I will try again tomorrow.

Reading back your original post: it was not obvious to me that you were talking of horticultural sand and not plain sand. Two very different things. As I said particle size is crucial to any discussion of soil. Are you even sure that the graph of sand you presented is representative of horticultural sand? Second, focusing on one component and making conclusions out of it is not worth much. Third, I did not realize you were considering peat as organic component. If peat is involved then it is not a structured soil. Peat particles are so small that including it in a mix will throw off any water/air properties of the other components out of the door.

As I said many times, particle size and range is important. When you mix two components with differently sized particles water/air properties changes.

I have attempted to describe and inform as much I am willing at this point. Seems like you have drawn your conclusions regarding your focus - sand. Good luck with your quest.

Looks like one of your earlier posts just showed up in my feed now. I am afraid I am going to basically repeat myself but here it goes. You asked:

"I thought that most of the perched water table was about the water that
is *outside* of the particles in the soil? I thought it was the forces
of adhesion, cohesion, and capillary action that mostly determined where
the perched water table would settle. The whole point of using some
soil components with high water retention is to store water inside
particles in a way that benefits the plants over the next few hours or
days without robbing the soil of oxygen in between particles."

PWT is solely determined by the GAP between particles. Smaller the gap higher is the PWT. The gap is related to particle size and size ranges. If the particles are large enough the gaps are also large and vice-versa. If most particles are largish and a fraction of the particles are smallish then the small ones will fill the gaps between larger ones and make the gaps smaller on the average. It takes relatively little of the small ones to kill the gaps created by larger ones. So you see if you add peat to any medium it will not matter because it will kill all the air gaps. That is also why I keep insisting on knowing what other components are used in the mix.

Surface tension as the name implies refers to the water at the surface of the particles. Surface tension tries to pull up the water through the gaps while gravity tries to pull it down. Smaller the gap higher is the force pulling the water up. That is because the amount of available surface becomes relatively larger than the air gap between particles. Sort of like a wick. And so the PWT is higher. Within PWT region every (most) gaps are saturated with water. There is no air in PWT region. Above PWT the gaps will be composed of air and water. Higher you go the proportion of air increases. That is what your graphs are telling you.

The value at 0kPa is the total capacity also called total porosity. For peat it is 90% and for sand it is 30%. Means that is the maximum space available for water and air to occupy. At 0 kPa every space is occupied by water. At 1kPa it is about 80% for peat and 20% for sand. 1kPa = 4inches of water. Means if you go up 4inches in the container then peat will have 80/90 = 88% of available space filled with water. For sand it will be 20/30 = 66% of available space filled with water. The rest of the available space is air. But you have to know particle sizes too to make sense since those curves will be will be wildly different. The amount of available space will vary with particle size ranges. For a well made gritty mix the curve will be very steep and quickly fall to to a steady number (like pumice) within a fraction of kPa.

Total water retention is sum of water within and outside the particles. So is the total air in the mix. A good structured mix primarily relies on water held inside particles to provide water and the gaps between particles to provide air. A much smaller amount will be held at the surface of the particles also available for plants. See the pumice curve where it flattens out. That is the amount of water held within pumice. For sand it is negligible amount held at the surface.

Peat has some great properties but has no place in a structured soil. If you want to use peat then keep it below 20%. Use some combination of sifted pumice, turface, grit etc for the rest.

@tropic, regarding your shorter message:

1) If you want to show water retention curves for sand of different sizes, you are welcome to do that. I agree that the water retention curve I grabbed is probably using smaller diameter sand than the sand I am using. That said, I doubt that water retention curves for different types of sand used in soil applications will show dramatically different results. In any case, I am using the kind of sand that is best suited to horticultural applications: coarse 2mm washed sand.

2) You say "...focusing on one component and making conclusions out of it is not worth much " To see why that is wrong let's consider that wonderful new structured soil component named "rose thorns". Everyone online is posting how their plants grow great in rose thorns. Using your approach, I will add rose thorns to a structured soil with nine other components. Then I can show moisture retention curves for the soil. And I post photos showing how great my plants do in soil amended by rose thorns. I guess rose thorns must be great?

The problem with that is that you created an experiment with 10 variables instead of one. In my approach, I will show a water retention curve for rose thorns. I will measure a perched water table for rose thorns alone. My experiment will quickly show rose thorns do not hold much water and the perched water table is very low. I conclude a lot more about that component by analyzing it on its own than I ever would with your approach of creating a soil that mixes it in.

Once I understand the behavior of the component on its own, then maybe I can build soils that use it and do further experiments.

3) Regarding peat: use structured bark that 1/8", 1/4" or any size you like. This post is about sand, and I was just grabbing a placeholder for the organic component of a mix. I understand that the mix will have its own characteristics, and you need to consider all of the components for a given mix.

@tropic, Regarding the second longer post, you had a nice formula at the bottom for how to calculate the amount of air in the soil based on porosity and the soil pressure. Are there any rough guideline targets for what percentage of the soil in a root zone should be air? Obviously this might be different based on the kind of plant, so what is the typical range from something like grass on one end to cactus on the other?

Regarding what makes the perched water table, I understand the capillary action idea based on gaps. But equally important would be that - for a given volume - smaller particles will have dramatically more surface area than larger particles. The adhesion force of water will therefore have much more surface area to stick to with smaller particles. These ideas go hand in hand, since the smaller particles with greater surface area will also leave less space between particles, thus creating a capillary action force to oppose gravity.

Westes: "Regarding the second longer post, you had a nice formula at the bottom for how to calculate the amount of air in the soil based on porosity and the soil pressure" Thanks and it is water pressure not soil pressure.

"Are there any rough guideline targets for what percentage of the soil in a root zone should be air? Obviously this might be different based on the kind of plant, so what is the typical range from something like grass on one end to cactus on the other?"

There are many articles written on total/air/water porosity of various mediums - mostly peat based and some bark based too. I do not have any good reference handy. But the numbers are all over the place depending on who is doing the research. Of the top, minimum air porosity should be around 20% of total volume. Water porosity of 30-60 again depending what material they were studying. Also, I do not know any reference on preferred range by plants of different kinds.

@tropicofcancer A minimum of 20% is a valuable target to work against, thanks.

Regarding the water saturation of peat in the water release curve I linked here: to be fair the 90% is not all water outside of the peat? Some of that water is held inside like a sponge would hold water, so that part of the water retention would not steal air from the soil mix?

Something that is difficult for me to understand, as a novice to all of this soil science, is what percentage of a given water release curve is held by adhesion of water to the outside of the soil substrate, and what percentage is held internally and does not affect the air in the soil. Your formula may already take that into account, but I need to understand that better.

Saturated water condition always includes the external and internal water. Any soil mix (even if it is single component) the total porosity is the combined internal/external space available. Like water, air can also occupy some of the internal pores (eg when the soil is very dry) the sum of water and air porosity is equal to total porosity. Usually the internal pores are small enough that after watering thoroughly they are occupied by water. The external spaces can be occupied by water and air both.

My formula does not take into account any difference between water at surface of particles vs internally held water. There is no clear way to distinguish them. Check this thread out for getting a better understanding: https://www.gardenweb.com/discussions/4061703/porosity-water-holding-capacity-cost-of-common-soil-components?n=35

It is also important to remember that peat moss expands as it absorbs water. And that expansion DOES affect pore space or the availability of air surrounding the individual particles. That, together with the fact that peat moss is excessively water retentive as well as a very small particle size to begin with, is one of the primary reasons the PWT in a peat based soil will be higher than in one with a minimal amount of peat included.