just wondering.. is fir and pine bark the same in WHC etc?
what % are we aiming for with the gritty mix and the 511? there has got to be a big difference between the WHC of the two mixes right? but how about for the AS & TP, not much?

also, what about those people growing in pure pumice. if say you want to increase its WHC, or of any medium, do you just add a fraction of smaller particles?

I would imagine they are quite close. Most of the discussions I've seen is that fir is a bit more durable but I've never tested it myself.

With gritty mixes, it really depends on your growing conditions and what exactly you are growing. Each grower will have a different requirement and customizing/experimenting with mixes can be very rewarding.

As you mentioned, 511 will hold a lot more water than most gritty mixes. Air space may be higher? Depends on how big the pieces are and how many fines are included in the mix.

For pumice you are also correct. Smaller pieces will enable the mix to hold more water. Which is why I tend to use smaller pumice for rooting/smaller plants and bigger pumice for well established plants.

Thank you mayo!

soo... i got really curious about rice hulls. do you think it would work for the mix?

I've seen much discussion for their use with succulents from more tropical countries as rice hulls are easier to get there. It should work but I have no experience with them.

I can't remember the name, but there is a nursery in CA using rice hulls in their container medium.

Is there a reference that would show how quickly water evaporates at different temperatures and humidity? What I am finding during my first Winter with Al's gritty mix is that the mix can remain quite moist at the base of a pot for more than a full month. In 40F to 50F air temperatures with humidity around 40% there appears to be very little evaporation after you get more than about six inches down into a gritty mix soil.

If you grab a clear plastic cup around the size of your pots, then you can watch the rate it dries top to bottom. Not exact, but it's close.

@ewwmayo so the question is whether any online reference would give us approximate evaporation rates. I want to plug in my temperature and humidity, and then maybe the pot height, and what does theory say about how quickly a container of water that height should evaporate.

What I am realizing is that one of the reasons plants do not need watering in climates where the temperature stays under 50F - but above freezing - during the Winter is that water is staying around in the ground for a very long time. What meager water needs the plant has are probably being met from an unwatered container.

@ewwmayo do you any data on what the water holding capacity of clay soil would be, to compare that against turface?

Sorry, the closest I have is akadama (clay soil) as above in the table.

Never bothered to check the clay soil in my backyard (very hard and frozen) and I don't think it's very good for growing anyway.

I found this video and although it is targeted at bonsai soils I still found it useful.

Here

@ewwmayo in your formula above you have:

Total Porosity = weight of water needed to fill pot / empty pot volume * 100.

I do not have a direct measurement of the weight of the water needed to fill the pot. I have a direct measurement of the weight of the water that drained from the pot. I could calculate the weight of the water that filled the pot as:

Weight of water-filled pot - Weight of pot with dry soil

Is that what you meant by "weight of water needed to fill pot"?

To get the weight of water needed to fill the pot, have the dry pot full of dry mix on the scale. You will need to temporarily seal the hole on the bottom of your container.

Note that weight and then add your water. Take the new full weight then subtract the dry weight.

The result is the weight or mL of water needed to fill the pot. This weight will be greater than the weight of the water that drains out.

@ewwmayo so that was a yes. :)

Haha, yes!!

Westes: I am confused. How did you do the experiment? Did you seal the drain hole of the pot before filling the pot with water? Without that this experiment cannot be conducted.

I used duct tape. If you use plastic cups you can a cup-in-cup method if you account for that in the way you measure.

@tropicofcancer there is no experiment yet. I was asking @ewwmayo to define the words in his text. I supplied the answer to my own question, which he confirmed was correct.

@ewwmayo, where in your formulas are you using the measurements for step 3 and step and step 12 below? Also, the volume of the "empty pot volume" would be up to the soil level in the pot? I do not understand how you are canceling out the volume unit of measurements in these formulas?

1. Weigh the dry empty tub.
2. Use a small piece of duct tape and seal the drain hole of one pot. It must be water-tight!
3. Weigh the dry empty pot.
4. With the pot on the scale, fill it with water and record the weight.
5. Empty the pot and dry it.
6. Fill the dry pot with your soil mix to the brim.
7. Weigh the dry pot with your soil mix in it.
8. With the pot with soil on your scale, fill it with water and record the weight. If the ingredients take some time to absorb water, you may need to wait a few minutes and top up the water to the top of the soil mix level.
9. Carefully remove the pot and put your dry empty tub on the scale.
10. With your pot over the tub, carefully peel a corner of the tape to let the water drain into the tub. If your hole is too big the mix will fall into the tub and skew your measurements.
11. Weigh the water drained with the tub.
12. Weigh the drained but still wet mix and pot.

Formulae:

• Total Porosity = weight of water needed to fill pot / empty pot volume * 100.
• Air Space = weight of water drained from pot / weight of water needed to fill pot * 100.
• Water Holding Capacity = (weight of water needed to fill pot - weight of water drained from pot) / empty pot volume * 100.

Sane countries use SI units in which water density is 1 gram/cm3. So it works out fine.

@tropicofcancer The SI unit for volume is cubic meters. Should we convert the volume measurements to cubic meters in his formulae? It would be much easier if he might just give an actual calculation for one of the formulae that involves empty pot volume.

Regardless of what units you use there, what I do not see is how the units are getting canceled out in the calculation. Total Porosity - for example - is a percentage calculation. In his formula, we have weight in grams, divided by volume in cubic meters (?) times 100. That seems to give a result in grams/meter^3, and how do the units cancel out to derive a percentage?

You can do it in cc and grams. The value is the same in grams and cc for water.

bump

@ewwmayo, I am wondering if you ever got measurements for peat?

I did not! The only thing I need to measure but I haven't yet is Leca balls. They are under my counter and I just didn't get to it. If you are able to get some, I'm happy to include into the above.

Do you want to measure Peat? Do you have a local supplier who can give you a sample?

I would measure it out of curiosity. We don't really have suppliers like that I don't think here, so it was just be totally out of pocket. I'm curious but not enough to buy a big bag of peat!

I have peat, a big bag. It is years old, so very dry, but I don’t think that would be a problem.

I found some very nice data on water holding capacity of different grades of peat moss.

Is water holding capacity (WHC) the best metric for determining any detrimental water holding characteristics of soil? It seems to me that what we would really care about is the fraction of the water being held that is due to adhesion and cohesion, versus being absorbed into the substrate. A water holding capacity of 60% might not be harmful to a plant, if all of that water is held inside the substrate. On the other hand, if much of that 60% is water being held to the outside of microscopic particles (e.g., compost and manure) then the roots are drowning.

Is there any way to explicitly measure this idea? In other words, can we measure the amount of water holding capacity that is attributable to adhesion of water to the outside of the substrate? Does @tapla (mid-Michigan, USDA z5b-6a) have any insights on this issue?

@ewwmayo One of your formulas is:

Water Holding Capacity = (weight of water needed to fill pot - weight of water drained from pot) / empty pot volume * 100.

Shouldn't the denominator be a weight and not a volume? Dividing a weight by a volume does not give a sensible result. And it should be the weight of the water and the soil together in the saturated soil? In other words:

Water Holding Capacity = (weight of water needed to fill pot - weight of water drained from pot) / weight of water and soil in saturated pot * 100.

And we calculate the weight of water and soil in the saturated pot from the weight of the saturated pot after it drains MINUS the weight of the empty pot.

@westes Zone 9b California SF Bay - Weight and volume of water doesn't really matter to since 1mL = 1g of water? At least, that is my understanding.

Westes, from units point of view it may not make sense dividing volume by weight or vice-versa. But for calculations sake it does not matter here since density of water is 1. Which is nice since you could either measure weight and/or volume whichever is more convenient.

"In other words, can we measure the amount of water holding capacity that is attributable to adhesion of water to the outside of the substrate? Does @tapla have any insights on this issue?"

I keep chewing on this issue since I do a lot with EC metrics and whatnot. It's not the same, but logic still applies here.

And the answer is...not easily, but I don't think we have to. Because not very much is due to adhesion for very large particles. For soil-size particles, it's much more and quite important, but in that case it's becoming a bath around the soil particles because the water is interacting with itself--adhesion, yes, but influenced by porosity, and so on.

Try the following experiment. Take those small pebbles above and fill a small pot, glazed on the inside, with them. A nice three incher. Then take 30 ml (about 1 ounce) of water, placing a catch glass below the pot. Pour the water in the top. Measure the water in the glass as it comes out the bottom, including gently tilting the pot back and forth to pour out any water that's on the bottom but not pouring out due to slight pitching of the pot base.

You're probably going to get back 22-26 ml depending on pebble size. I only had larger pebbles and got back more. Perlite is smaller and has more cracks, so you'll get back much less water. The water can form bridges more easily between the particles and hang onto both itself and the perlite.

24 cc of sand would hold about 6 ml of water, packed loosely, but I used a very slight base of soil to hold the sand in without spilling all over the place, so my number was slightly high.

Miracle-Gro Moisture Control sometimes feels like it won't stop absorbing water. Half its own volume in water? That's probably high, but it sure feels like it if it's just slightly damp. If dry, it has to be dampened first to absorb the most water.

As the particle size shrinks, the water starts to interact with itself in the interstitial spaces between the particles, so the soil will hold more water due to the water molecules "holding hands" in the spaces between. Compaction of the soil, charge regimens, and half a dozen other things will come into play here. So good, spongy organics will certainly tend to hold more water...because they have lots of holes and lots of exposed charges for one of water's charge "hands."

Conversely, a dry organic can be a bit waxy and repel water until it gets moist, so it can be something that refuses water and has little adsorption nor absorption for a while. In this case, the water adsorption is going to vary by current moisture content.

We sometimes use--not that I recommend it here for cacti or succulents--surfactants to influence the soil's water absorption abilities after a drought to overcome the tendency repel water at first. In this case, we're directly altering the surface tension of the water to force the first part of the rainfall to absorb rather than run off as it would otherwise do.

You can use either weight or volume, but for ease of understanding, I would suggest you keep everything in volumes or everything in weights, and not mix weight and volume together in the same calculation (unless you are willing to invest in unit math to prove the correctness).

For simplicity of calculation, volumes are the way to go.

Today I took measurements for Peat and Leca, which are added below for reference. It took a very long time to get the peat measurement because wetting a hydrophobic pot of peat was quite the endeavour.

The Leca was left sitting for around 15 minutes in water but not much was absorbed, as reflected in the numbers. It was much smoother and harder than I would have imagined, even after being soaked in water.

Many thanks to @gdinieontarioz5 for loaning me the peat and Leca to do these tests!

@westes Zone 9b California SF Bay - If there are different formulas to be used please post and update the table accordingly.

@ewwmayo So the basic problem is that porosity is the sum of the air and water space in the soil. Therefore, Air Space + Water Holding Capacity = Porosity. Since your numbers do not keep that identity, I suspect that what is happening is your calculation of WHC is correct, but the calculation of Air Space is actually giving us the percent of the Porosity that is occupied by air. Your calculation is probably correct, but that number is not the useful number.

Fortunately, you can convert all of this pretty easily. Take any column you like. I will take the Lechuza Pon. You have:

Porosity = 55%

Air Space = 69%

WHC = 17%

What you actually want to report is that percent of the soil volume that is occupied by air. So multiply 55% by 69% and you get approximately 38%. And - sure enough - 38% + 17% = 55%! And isn't that the more useful idea?

55% of the soil substrate/mix was occupied by air or water.

38% of the soil mix is occupied by air.

17% of the soil mix is occupied by water.

So I would have reported your results as:

Porosity = 55%

Air Space = 38%

WHC = 17%

@ewwmayo Because of the unclear terminology and so many conflicting references, I had to bang my head against the wall periodically for a few months before any of these ideas became clear.

There are still many aspects of this I do not understand well. One key idea that all of these calculations seem to just miss is what part of the water is the adhesion of water to particles and what part of the water is held inside large particles? Theoretically, you could have a soil mix with small particle sizes and very high WHC, and we would probably criticize that mix because most of the water is stealing the air space and drowning roots. But what if that water was held tightly inside of particles and had the same WHC? Wouldn't that soil potentially be leaving plenty of air between particles? Yet we seem to have no way to measure this. Or at least I never see the texts talking about it.

All of these formulas seem to be making the assumption that all of the porosity is water and air between soil particles, which is not a complete picture. It is still all useful but it seems also limited in what it can tell you.

Westes: There are still many aspects of this I do not understand well. One key idea that all of these calculations seem to just miss is what part of the water is the adhesion of water to particles and what part of the water is held inside large particles? Theoretically, you could have a soil mix with small particle sizes and very high WHC, and we would probably criticize that mix because most of the water is stealing the air space and drowning roots. But what if that water was held tightly inside of particles and had the same WHC? Wouldn't that soil potentially be leaving plenty of air between particles? Yet we seem to have no way to measure this. Or at least I never see the texts talking about it.

This is a bit confusing to me. Can you clarify what exactly you want and how will it help? There are two kinds of pores: internal to soil particles and external between soil particles. Both can be large or small. Internal pores may not be accessible such as in perlite which has closed cell structure. For one component type say turface, both kind of pores will behave the same way with respect to water and is determined by the size of the pores. If particles are small then the external pores are also small and thus likely to be saturated. I do not see how your scenarios will happen in real situations. May be you can further clarify.

Ditto with Tropic of Cancer. Similarly, interlocking of soil particles (flocculation) would also have to be considered, and that's not taken into account.

There are so many variables I don't think one could ever really get a good handle on any of this and, even if one did, one's soil is going to change character over time, and even as it saturates and dries out. Simple re-mixing of it will change the drainage paths and patterns. It always does.

It's definitely a simplification. No question the interactions and details are complex. The idea is to have a simple quantitative comparison and cost breakdown for soil components.
Even if I had used a more comprehensive formula would anybody actually use it for their home growing? If it's all to confusing I can just delete my thread. =)

@ewwmayo I am just proposing some approach that brings consistency to how air and water content of the soil mix are reported. Do not delete threads. Your data is very very valuable!

@tropicofcancer the simple statement would be that the water holding capacity measurements do not seem to distinguish internal pores within a particle and the adhesion and cohesion of water between different particles. It seems like common sense that you would want to know how much of the held water is inside particles, and how much is between particles?

Maybe the bottom line is that we should really care about the air space, because if the air space is low then we know for sure that the space between particles is flooded? If air space is good, then my question becomes more important. How much of the water in that soil is being held inside particles?

@westes Zone 9b California SF Bay and @tropicofcancer (6b SW-PA) - Took a little time to go over the measurements and think about it. It seems like the intent of Air Space % (Drained / Fill) is to gauge how much air remains in the soil mix after watering.

Your suggestion for Air Space % (Drained/Volume) does make sense because that would tell you how much air is in the mix when it is totally dry. Knowing adhesion vs cohesion would be interesting but in terms of water being available for the plant roots I'm not sure how much it matters.

I did the simplest implementation I could think of - simply adding a row with the different calculated value.

@ewwmayo Perfect, thank you!

@ewwmayo So with the new data, it is worth pointing out that something is off with the Air Space % (drained/volume) for Turface. Tapla has said good soil mix should have at least 30% Air Space %. Your Turface measurement has the single lowest Air Space % of any substrate - at 17% - including sand. That cannot be right, can it? Turface cannot be leaving half the amount of air that small 2-3 mm particles of sand leave?

I am about to start my own experiments with three different versions of Turface, so we will see what kind of numbers I get as well.

@ewwmayo when you did your experiments, how did you determine that the water line was at the top of the soil? The way I am doing it is to put just enough water that I can barely detect freestanding water in the lowest sections of the top soil. This leaves a few particles on top of the soil not covered by water.

This is an area that could introduce a lot of experimental differences across different users. Some people might cover the soil with a thin layer of water such that no particle rises above the water line. Others might go the other direction and eye the water line on the side of the container, leaving many particles at the top not in water.

@westes Zone 9b California SF Bay I did it to the top of the pot line such as it was to the maximum but not overflowing with water bulging over the top by surface tension. Usually there were some particles sticking up and out.

I was not able to look directly across the pot for the water line (as I would have done with a glass beaker in chemistry) because the tub I used was taller than the pot and opaque. Instead, my view was at a slight downward angle because of my scale and countertop.