I understand the points you make here..I haven't made gritty mix but for several years I've used bark based media in all of my pots..filled completely from top to bottom..nothing else..no perlite..no ballast (rocks, broken terra cotta pieces, bottles etc)..I first used MiracleGro Organic Choice..contains no perlite..looked like a fine mulch..it's no longer available so now I use a raised bed mix..looks almost identical..my plants are doing well..I grow tropicals and annuals in pots..no succulents..I think the media is prettier without perlite Lol..I water thoroughly when I water but I don't water as often as my neighbors..I see them watering almost everyday..would you agree that for my moisture loving summer plants I don't need perlite?..

I can agree easily enough that you don't really need perlite; however, based on what you described (I think you're using a raised bed mix) I believe you could provide greater opportunity for your plants to realize a larger fraction of their potential if you took steps to eliminate or mitigate the limitations imposed by perched water.

I'm really surprised the lack of full sun had such a limiting effect on the castor beans. I'm wondering if the plants are also having trouble competing with the tree roots for their fare share of moisture and/or nutrients?

Al

I just wish someone would simplify things for me and make something commercially available. In Colorado, we have access to a lot of different commercial soil blends. But I can never figure out which one to use. I bought some Promix BRK last fall and it was a disaster. It held way too much water. It's so weird because it is advertised as low water retention, but this was not at all my experience. And I live in a dry climate!

interesting!..I didn't consider that tree roots could be inhibiting my castor beans..I thought perched water tables were a problem in a pot that had a layer of "filler" at the bottom..where people are attempting to save money on soil or make the pot lighter or thinking they're getting better drainage..so I used only media..I didn't think that I have perched water tables but maybe I do??..I was pleased with how my plants look and assumed I have good drainage..I overwintered some of the tubers in the pots and they didn't rot..darn..I thought I figured out a system..what should I change?..

Pop.... - there are ways to make usable what would normally be considered a soil so water-retentive it was unusable. For instance, if you have a very poor soil that supports 6" of perched water, and use a mix of equal parts or peastone and the soil in the bottom 6" of the pot, you've halved your water retention problem; or, eliminate it entirely by using ballast. If the pot is small enough, do the up and down thing over the sink to remove ALL perched water from any manageable pot with a drain hole. Use a wick or make a pot-in-pot set-up.

Virtually all growers run up against poor soil as an obstacle at some early point in time on the journey to a green thumb. Not all, by a large measure, get beyond it, even though the fixes are easy and inexpensive. In fact, most are free and require no effort on the part of the grower once the planting is established.

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N - Most off the shelf media support 3-6" of perched water. You can easily determine the ht of the PWT any soil supports. You need a clear plastic cup, a pair of pliers, a nail or screw, a piece of tape, and a cup full of soil. Melt a hole in the bottom of the cup with a hot nail (that's what the pliers are for - holding the hot nail/screw). Cover the hole with the tape and make sure it is sealed. Fill the cup to about 1/2" from the top, then fill with water and wait until you're sure the soil is fully saturated. Remove the tape and allow the water to drain until it stops. If you look through the wall of the cup, you'll see the lower part of the cup is full of water. As your eye moves upward, at some point you'll see air pockets in the soil. The point of transition between air and no air is the ht of the PWT. If it's taller than an inch, anything you do to reduce its ht will provide greater opportunity for your plant to realize more of its genetic potential, within the limiting effects of other less than ideal cultural conditions, of course.

Al

I saw a similar experiment years ago on a gardening show..2 clear plastic cups with drainage holes..one cup with "filler" at the bottom and soil..and one with soil only..they watered both..the soil above the "filler" layer was holding more water than the soil in the cup with soil only..slightly more water drained from the cup with soil only..they were telling viewers not to use anything in their pots but soil..I don't think there was any discussion about types of soil..so I've used soil only since then..so does every pot have a perched water table?.. some having just a little and others a lot?..I had 2 tall pots without drainage holes that came with "drainage trays" in the bottom..I tried them and they were awful..if I hadn't taken the plants out they would've rotted..my husband drilled holes in them and they're so much better..I feel as if I have a free draining bark based soil..the water flows out when I water..I'm reluctant to change anything unless my plants start declining..

...... so does every pot have a perched water table?.. some having just a little and others a lot? Not all soils support perched water. E.g., the gritty mix is designed specifically so it doesn't hold any PW if it's made correctly (screened to appropriate size particles). The ht of the PWT is primarily a function of particle size. The smaller the size of the particles that make up the soil, the taller the PWT will be. Water molecules are attracted to other water molecules (cohesion) and attracted to other soil particles (adhesion). If you wet a sponge in a tub, when you lift it from the tub, all the water would exit the sponge without adhesive/cohesive attraction. Without cohesion, the molecules in a drip from the faucet would simply fly off into space and disappear. Without adhesion, water could not exist on a nonporous surface that wasn't perfectly horizontal - step out of the shower and the only wet spot would be the floor and the top of your head. With regard to water in soils, adhesive and cohesive attraction combined can be strong enough to resist the force of gravity, which means the water stays in the pot, resisting gravity and occupying space that would better be occupied by the air roots need to function.

A jar full of marbles has far less surface area than a jar full of BBs, and a jar full of play sand has far more surface area than either of the former. Water will flow right through a jar of marbles or BBs because the surface area water has to adhere to is so small the sum of adhesive + cohesive attraction cannot resist the force of gravity. Play sand, however, has significantly more surface area for water to stick to, so the sum of adhesive/cohesive attraction of water in play sand is stronger than gravity, which is why water perches in that material.

That you are happy with your results is all that matters. I would still encourage you to do some experimenting. My experience has shown over and over and over that because a plant looks healthy is not a good indication that it is realizing anywhere near it's genetic potential. I'm thinking of a Ficus burtt-davyi I repotted this summer in June. It looked perfectly fine, but hadn't been repotted in 3 years. It was in a bonsai pot, so it was badly rootbound, but as noted still looked perfectly fine. I pruned and repotted it, and within a week it had back-budded profusely and put on more growth than it had in the entire preceding growth period. Like root congestion, poor soils that are water-retentive are sneaky and rob plants of an immense amount of their potential, often w/o notice. Soil saturation kills fine roots first, the plant's work horses. After fine roots die, plant central gets chemical messengers telling it to stop top growth and focus energy output on replacing the roots lost to soil saturation. As the plant dries down and the lost roots replaced, along comes the grower to water again. The cyclical death and regeneration of roots is very expensive in terms of energy outlay and lost potential; this, because the energy devoted to replacing lost roots would have, in healthier plants, been dedicated to new growth, more fruit or blooms, etc.

Al

I honestly don't know if I have perched water tables (probably do)..or if I do what size they might be..growing non native plants in pots is fun and I'm thrilled just to keep them alive!..I understand what you're saying that although a plant has a nice appearance if compared to one growing in it's native environment it's not reaching it's full potential..there's always more to know..I'll keep reading..I do think that using a bark based mix instead of an ordinary potting mix was a good change..I tried a "moisture control" mix once and hated it..thanks Al..

P.S. to be honest..I have about 70 pots and quite a few are large..reworking all of the soil sounds overwhelming..and as popmama said it's not simple..I'm not sure of exactly what to do..I'll keep thinking about it..

No one should think they're doing something wrong if they decide the extra effort isn't worth it. I completely understand that everyone's priorities are ordered differently. As long as you're pleased with the results you're seeing, that's all that's important. I tend to look at things from the plant's perspective, and share things that can help others get more from the growing experience, but it's not a judgemental thing. I care if you do or don't put into practice the suggestions I offer, but only to the degree I would like to see everyone get more from their growing experience. The information is there for those who care to use it, but I'm fine if people ignore it, too.

Almost all my contributions are more for for the collective than the individual. IOW, if Joe has a problem, when I share how to resolve it, I'm also explaining to to the forum how to remedy Joe's issue if ever they encounter it. Houzz taking over GW and giving members the opportunity to 'like' a particular post was eye-opening for me. I had no idea how many people came across very old posts, more than 10 years old is very common, and found them to be of value until I started getting notifications of the 'likes' they received. Because of that, most of what I contribute is with the idea others will likely be reading those posts many years from now.

So, if you have questions, the fora are always here, and I'm not too difficult to find if you want input from me.

Best of everything to you.

Al

It is a basic physical principal that finer the material the more water and the higher the water column. Conseqently the coarser material holds less water. Here in humid Puget Sound I was killing cactus with soggy 50/50 mix. I am now on straight pumice and things are looking up.

Hi Al,

I really enjoy reading your explanations of the consequences of different media.

I was wondering - are there cases where particular types of plants grow better in media with fine particles? For example, the literature on mesembs often suggests that these plants need fine particles (even clay) in their media. Is there any evidence that plants with very fine roots don't fare as well in gritty mix and would grow better using a fine mix combined with the ballast/wick/etc approach for dealing with the PWT. Thanks for any insight.

It would be great if someone started making and selling the 5:1:1 and the gritty mix. Presuming they are as wonderful for plants as they are supposed to be, the venture would be a roaring success, what with having houseplants becoming a thing thanks to the instagram influencers, there are so many more potted plant lovers out there in need of good potting mixes. It could even be one of those crowd fund things that is funded by advance orders from interested buyers. I and people of my ilk (too lazy to find the right materials, screen and perfect these mixes) would become captive clients. I have about 100 pots during summers and easily consume 3-4 large bags of assorted potting materials.....There are plenty out there who buy the gunk sold by MG, stay green et al.

I've seen 5-1-1 mixes for sale on Etsy but that's small potatoes really. And some of the special bonsai mixes come pretty close as well. But yes, I would really like it was available in easier to purchase format.

More than a few have tried to make a buck selling the mixes, and there are a number of entities that advertise 'gritty mix' for sale on the net. Those that continue to advertise some variation of the name "gritty mix" more often than not leave out the most important step, which is the screening. If you don't screen the gritty mix, there is little to separate it from less expensive mixes, so there is little sense in using it. It's hard for a commercial operation to see 2/5 of a bag of Turface go to waste after screening. For the most part, that would approximate their profit margin.

As far as the 5:1:1 mix, once you're familiar with it you'll see it's hard to rely on a rigid recipe if you're trying to get the most out of it. Every bag/pallet of bark is different in size and the volume of fines, which necessarily affects the ratio of the major ingredients.

Tip: Stay away from Hoffman's bonsai soil. It's EXTREMELY water retentive and poorly suited to bonsai or succulents.

Al

:(

That is such a shame. Perhaps economies of scale could help and the finer materials could be sold for alternative uses...

Surely, gardeners would be willing to pay a little premium over MG prices to get a better product.

I think bonsai jack seems to be a reputable source for gritty like mixes. You can build your own custom mix too: https://www.bonsaijack.com/category/premixed-bonsai-soil/build-your-own-bonsai-soil/

It is pricey. Heard good things about them but I have not bought myself.

Al, thanks for starting another great topic.

A few weeks back I had give a talk on tropicals at our local bonsai club and talked a bit about PWT. So I experimented with a sponge to illustrate it. Here are the pics.

First a dry sponge:

That is super expensive.

Making your own will be cheaper but then you will have to source the ingredients, sift, rinse, dry and mix. Add the time, labor and shipping the stated price is not too bad for a gritty mix.

Al, is there any Fafard Mixes, now Sun-Gro that suit what you are talking about for a good mix, almost equall to the 5.1.1 mix? They use to have some heavy almost all bark mixes that were awesome. They seem to have gotten more water retentive since Fafard was bought out. Can you think of any?

Thanks

Thanks for the discussion, Al. I like your recommendation of overturned pots at the bottom soils with perched water. I do have some strong concerns though about many of your facts. Some key points that are incorrect:

1. "Soils based on high % of peat or other fine ingredients cannot be made to drain well" This is incorrect. Check any of the high-quality mixes at your local hydroponics store. Follow the money. The most research, the highest quality, and the most money for best ingredients has always been from the hydroponics growers because of the illicit, but most profitable, crops associated with that method. These high-drainage peat-based mixes are made to let water freely drain out the bottom, but still not have to water at unreasonably short intervals, like the mix mentioned above.

2. "nor does adding perlite increase aeration appreciably." Again this is false, and proven by the real experts who spend huge money that says this is incorrect (most of the wholesale container growers in this country rely on perlite for aeration). It's also been specifically proven in university studies. I can go dig one up if you want, but this is really a silly claim.

3. "What happened to the large air spaces between the perlite? They're completely gone" You completely misunderstand how perlite aerates. Crush a piece between your fingers. It's all air. This is why the studies show aeration increases very directly in relation to perlite increase. Again, follow the money. Perlite's aeration & benefits are overwhelmingly and massively proven by all the commercial growers who rely on it and all the authors who recommend it for that reason.

4. "adding perlite or any other coarse ingredient makes things worse before they start to get better" Here you are obviously thinking of the research in regards to adding sand, and it would certainly be true for sand, but you're trying to stretch it over to cover perlite. Sand is very different in every respect than perlite and this research does not apply. This statement is false.

5. "For any given soil, the ht of a PWT (at container capacity) remains the same, regardless of the size or shape of the container." Ouch. This is the biggest problem of all. You've taken a real problem with your type of ingredients, bark, and tried to incorrectly apply it to peat-based mixes. Peat, unlike bark, can move water UP and throughout the mix to eliminate perched water. In fact it moves water more than twice as well as any type of bark. So peat-based mixes will evaporate water from the surface more than twice as fast as a bark mix. This water movement helps balance water throughout the mix and eliminates perched water tables. Be care careful of container soil studies out of Oregon universities, like this one I know you have relied on. They can sometimes be vague about whether their research applies only to bark mixes because of the their strong preference for bark there.

AL-- amazing as always!

TomH: Like Al, I like to follow the science and logic instead.

Commercial nurseries are looking after their bottom line and many compromises are made. Peat based mixes are popular and sort of works for short term plantings including the "weed" world. Any reputable nursery interested in longer term viability of their plants will rely on more structurally stable mixes. Bark, sand, perlite usually used in this regard. Even then it is deliberately made more water retentive so as to survive sitting in the stores and to reduce the need for frequent watering. Commercially another big concern is weight to keep shipping/handling costs down. So you see following the money leads me to a different path.

Before attempting to dissuade you from your beliefs I would like to say one thing first: You just joined the forum with very few comments so far and yet to establish some credibility. Al has been meticulously researching, experimenting and writing here for probably more than 20 yrs. In short he has far more credibility than you can imagine.

I will start with some material properties to illustrate why you are very much off-base in your responses.

Perlite: It is produced by heating up volcanic glass to 2000F so that they expand rapidly and pop like a popcorn. In the process it forms thousands of very tiny closed cells trapping air. No liquid or air can penetrate these cells. Except the near the surface where some of the cells are ruptured. This is a very important property along with the fact it has a very low bulk density.

Bulk of perlite produced is used as a substitute for gravel/sand in construction industry to produce very lightweight concrete. If perlite absorbs any substantial amount of water it will make the concrete very weak. The same property of enclosed air space makes it an excellent high/low temperature insulator.

A small fraction (15%) is used in horticulture. It holds a small amount of water at its surface. It does not provide any aeration by itself (as stated above). In the industry, its primary purpose in soil mixes is to take up space and prevent soil compaction. It is by preventing soil compaction that it helps in keeping a aerated soil from compacting and losing its aeration property. The air inside perlite is unavailable to the plant since it cannot escape.

If perlite could absorb water then perlite would sink in water eventually. I challenge anyone to make that happen. Even bark will eventually sink once it has absorbed water especially if it is slightly aged. The pores in bark are open allowing air and water to move through them.

Peat: Peat is fibrous and can absorb 3-4 times its weight of water. Although dry peat will float but saturated peat will sink in water. Lot like shredded paper. Like paper it is extremely prone to compaction once wet. Once compacted there is no air left in the mix. Perlite takes up space and prevents compaction. One would think grit, turface, sand, pumice, etc could be used instead but their bulk density is so high that it actually will aid in compaction. So soil mixes containing these denser material inherently need structurally stable components. Bark is the only organic matter that fits the bill.

Perlite, on the other hand, has the reverse problem - it tends to float up leaving the media. Our neighbors buy many annual plants in such mixes. By the end of the season it is very easy to observe that the media has collapsed by an inch or two and the surface is white with perlite that have floated up. At that point there is nothing much you can do but to discard the soil - a fact that works in favor of the sellers.

Finally, your "Ouch" at concept of perched water made me chuckle. This is not the first time someone completely misunderstood the concept. Peat is no different and like other media it is subjected to same science. Being finer in structure leads to a higher perched water. Perched water is the area in the bottom where the soil will remain saturated. The article you link is absolutely correct and focusing on bark does not preclude that it does not happen for peat. Peat is not something a magical substance that science does not apply to it.

"Peat, unlike bark, can move water UP and throughout the mix to eliminate perched water" - That sentence was dead giveaway of not understanding science. It is not peat or bark that draws water up. It is water that moves up through pores within and outside of the medium. And it cannot happen indefinitely.

It is a fight between cohesive forces (holding water molecules together) and adhesive forces that makes water molecules stick to another substance. If watch you closely in a glass of water you can see at edge water surface curves up a bit along the glass walls. The adhesive force between glass and water makes it climb up but only a little since cohesive forces within water tries to drag it back to the water. If instead you use a very thin glass tube (called capillary tube) or even a straw you will see water climbs up the glass wall even more. This is because the volume of water between the walls of the tube is smaller and thus has net lower cohesive force. Relatively there is more glass wall and thus the net adhesive force increases. This differential in forces is what draws water up. If the tube is narrow enough then water will seemingly defy gravity and climb up the tube. At some point gravity will win and water will not be able to climb up on its own. The narrower the tube (think space between particles) the higher is the force. The height to which the water will climb up depends how narrow is the tube and the kind of material (peat, perlite, grit, bark, etc, etc). The material dictates the adhesive forces. Sometimes wetting agents are used to reduce the cohesive forces of water when the adhesive force is not sufficiently high. Dry peat has very low adhesive force and that makes it hydrophobic. Once wet the adhesive forces increases dramatically.

Perched water is the height of water in a medium where the soil is saturated. This is the height where gravity balances out with capillary forces. The lower the space between particles, higher is the force and higher is the perched water height. That does not mean there cannot be any water in the mix above this height. All the water above this height is held within and at the surface of the particles of the media and not between in the air spaces. Means not saturated with water.

When roots encounter this saturated region they will avoid it. Some plants with aggressive root systems may have less of a problem if they can absorb up the water quickly reducing the saturation. But for these plants it may also mean that the top will become bone dry quickly. Prompting owners to immediately water again - means the bottom will never have chance to dry out. Often this will lead to plants circling the pot aggressively (the only place where there is some air) in a futile attempt to find a better medium. Result is a steady decline in plant health.

Lastly evaporation of water from surface of soil does not draw water up from below unless it is already saturated. Evaporation is governed by different process and does not create any force differential to aid in moving water up.

Hello, tropicofcancer, it’s nice to meet you. Is that what you want to be called? I loved your excellent photo representation of perched water!

So, I tend to believe the facts should speak for themselves, and not rely on “credibility” gained by repeating one’s opinions longer & louder & with more big words than anyone else. So I’ll try to show you science for everything I say. Resumes don’t matter to me, but since they do to you: I'm a rare plant collector, avid plant trader, custom-soil builder, & lazy weeder. I was formerly the ISHS International Registrar for all cultivars in Brugmansia & Datura, a director of the American Brugmansia & Datura Society, and a moderator for the University of British Columbia plant forum. Now I'm just an old gardener. I’ve been posting here since the early days of GardenWeb. I’ve updated my profile page with some pics of the plants I grow now.

Let’s start with perlite. Your ideas about perlite not adding aeration sound really good, but they don’t seem to agree with the facts. Here is the science to show how much aeration it adds to soils, especially peat. Note from p 20, table 3 that only a 20% addition of perlite to peat nearly doubled the air porosity of a fully wetted, then drained, mix. And it went up from there. Perhaps this suggests that most commercial growers, perlite users, may actually know something about aeration.

So it’s more complex than simply reducing all potting materials to their particle size as you both are doing, as if bark & perlite were both marbles with the exact same characteristics (you used that exact analogy last year). They have much different rates of the amount of air they will hold in a mix, regardless of particle size, and Perlite adds the highest air content of all the commonly used ingredients for container soils (here is the science Sec. 7.2.2.1 ¶ 2).

I can agree with you that excess perlite in a mix floats up, and so that ratio you mentioned of 15% is a good one. I use 17% in my basic tropical mix, but I need twice that much high-drainage material, so I cut the perlite half-and-half with pumice (33% total). It’s a little heavier but gives a reasonably high aeration, so overall it eliminates the floating problem and gets me to my desired amount of air space without having to use bark and suffering its nitrogen-sucking nutrient instability (here is the science), and its miserable ability to move & balance water inside a pot (here is the science p 484 ¶ 2, peat is 2.5 times better)

For peat moss, I’m not sure where you got the “3-4 times its weight of water”, but it's way off. Could that be for sedge peat? In the US, mostly Canadian peat moss is used. This can absorb up to 8 times it’s weight in oil (here is the science), and up to 20 times or more it’s weight in water (here is the science), and blonde peat, which I love, even higher. That is important to correct, because you really need to understand, out of all the ingredients commonly used for container soils in the whole world, which two ingredients hold the most air and water in combination? One holds more water, one more air, but both are at the top of the pile in “total porosity” (air & water combined), and they make a pretty decent soil mix between them. I challenge you to find those 2 worldwide top ingredients (hint: you’ve been disparaging both).

You “chuckled” at my sad ignorance to believe peat was different from bark and could move water UP in container soils at a higher rate. Water evaporates out of the top of a pot of peat-based soil at 2.5 times the rate of pine bark (here is the science p 484 ¶ 2; more science p 125 #3). The method it moves is irrelevant, as long as it is leaving the pot. Another way to say the same thing is that water festers down in bark’s perched water table for days longer than in peat’s (you & I both know why Al talked about overturned pots up above). This movement of water makes perched water tables not a serious problem for peat. So peat actually does have a very unique structure with multiple kinds of porosity & water transfer mechanisms, to which the laws of physics do seem to apply differently. It is unique & complex, here is the science. It’s not just smaller particles.

About peat you said it is “extremely prone to compaction once wet. Once compacted there is no air left in the mix.” Again this doesn’t match the facts (here is the science “They have pores in their walls through which water can enter readily, and they are usually reinforced on the inside by ringlike or spiral thickenings (fig. 2) which prevent the cells from collapsing even when subjected to high pressure.”) (more science “The lignified cell walls… gives Sphagnum peats their unique properties. Much of a Sphagnum moss leaf is made up of large cells about 0.1 to 0.2 mm long. These cells are internally ‘buttressed’ so that they resist pressures that would otherwise collapse them”). Fortunately, you also said directly that adding perlite eliminates compaction. I will simply agree and wonder then why you even claimed there was a compaction “problem”. Do you see the Alocasia on the left in this photo? It’s been 21 years in that same pot of peat-perlite-pumice-compost (including 4 years dormant!) It is still uncompacted and drains freely. The soil has lost about 1.5” of height in that time. Very acceptable and not at all a problem.

I’m really not here to knock Al’s mix. I just can’t sit by any more years as some great and valuable mixes that are widely used by top expert growers are wrongly criticized with false claims.

In a potted plant, my preferred amendment (because I'm largely a cacsuccer) is pumice. However, that's not easily obtainable here so I use a mix of Turface MVP, #2 poultry grit, and some sieved top soil,

In the ground and needing porous-giving qualities? Anything - old potting mix, sand, gravel, bark - it doesn't really matter, because we're on the side of a hill and things do drain naturally.

Tom,

Bravo, sir.

And there is presently a pumice raft of approximately 150 sq. km in size in the South Pacific. Oh to be able to harvest a railroad hopper-car-full.

There are several piles (40 x 300 feet) along I5 at the Toutle River, I doubt anyone with bother you if you took a railroad hopper full.

The next time we go to the Oregon Coast I'll bring some garbage bags and get it started. Though from the sounds of it I'll have to mill it to the size I want (3/8").

Al: Apologies again for having diverted from the main topic.

TomH: Nice to meet you too. From your profile it is not evident to me that you have been around in GW for a long time. The first comment in your profile is only a couple of months back. But I will take your word for it. Perhaps you created a new ID.

I have also read many papers and thanks for the links to various pubs that you posted. We all read and interpret in various ways. It took me a while to go through the articles. I like to understand the whole context of an article when I read them. Mostly it was informative for me and I hope my analysis below is useful to some.

It is long but could not help that.

But I would like to start with couple of observations I have.

A. All the research is done under lab or greenhouse conditions. In greenhouse like conditions every aspect of growing plants can be controlled. Outdoors plants are subjected to vagaries of nature outside our control.

B. Published properties of soil mixes are always initial conditions. None talk about long term properties under real life conditions where the soil is subjected to the elements, undergo degradation and growing plants. All organic material will undergo degradation including peat and bark. As they degrade, the soil properties become worse with time.

Let me state first that both peat and perlite have some nice properties just like other components have certain good properties too. Similarly they all have downsides too. In the hands of an expert, many types of mixes can be made to work, especially in well controlled environments. My reluctance to use peat or any other component is solely due to its particle/pore size. If peat came in little balls of 2-6 mm range I would have no trouble using it as the organic component of my mix.

I will go through some of your linked publications and provide my thoughts on how I interpreted those.

Article 1: https://www.researchgate.net/publication/327992532_A_RESEARCH_ON_THE_INFLUENCE_OF_POROSITY_ON_PERLITE_SUBSTRATE_AND_ITS_INTERACTION_ON_POROSITY_OF_TWO_TYPES_OF_SOIL_AND_PEAT_SUBSTRATE

The authors refer to 1:1 to 2:3 air:water porosity needed for a good mix. From the table that would require perlite fraction to be 50-70%. So it would take that much of perlite to make peat a good soil.

Using bulk density to calculate total porosity is a bad method. That is why the authors have a very high total porosity of 90% of pure perlite. To me the results are very suspect. In fact there are many articles out there that say that total porosity of perlite is around 75%.

Eg https://journals.ashs.org/horttech/view/journals/horttech/17/3/article-p312.xml
They state total porosity of perlite is 74%. If you look at Table 1 you will see that the authors state that 60% perlite with peat air porosity is a meager 13% and water porosity is a whopping 59%. No where close to 1:1 or 2:3 ratio the previous paper suggested. Which one do I believe? The authors in the second paper cited air porosity needed is 10-20% again very different from the previous one.

Bark+Peat: https://oregonstate.edu/dept/nursery-weeds/presentations/nw_ag_phys.pdf
Before anyone says bark will rob nitrogen - easy to solve - apply more N.
On pages 50-53 the author compares bark+peat mixes. Shows that bark by itself has better water holding and air porosity than perlite. By adding small amount of peat to bark one can alter these characteristics. One can achieve much better properties than by adding perlite to peat.

Article #2: https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/perlite.
You used this reference to bolster your support of perlite based on this sentence I presume: "Perlite is probably the medium ingredient having the highest air-filled porosity." Probably it is. Is the air that is trapped inside perlite available? Likely not since bulk of it is a closed cell structure where the air is permanently trapped. Eg: http://www.perlite.com.eg/Download/Agricultural_Perlite.pdf. Quote: "Horticultural Perlite particles have a closed cell structure with multiplicity of tiny crevices on their surface. These crevices trap water and hold it against drainage and evaporation, yet making it available to plant roots on demand" Perlite's contribution to holding air and water is entirely dependent on the surface properties. That is why using bulk density to calculate total porosity (as Article #1) did is such a bad idea for perlite. Size matters too. Large perlite particles have relatively small surface area compared to the volume. So these mostly occupy space. Smaller it gets there is more surface relative to volume and becomes more useful for plants. Too small (less than 2mm or so) perched water will become a problem.

"So it’s more complex than simply reducing all potting materials to their particle size as you both are doing, as if bark & perlite were both marbles with the exact same characteristics (you used that exact analogy last year)." An analogy by definition means similar and not necessarily exact but significant. In science, analogies are frequently used to convey complex ideas in a more accessible form. It would be foolish to assume that they are exactly the same thing. I use them in my engineering classes just like the sponge analogy earlier to illustrate what perched water is. You want to delegate particle size considerations as a lesser consideration but your own references delves on that topic as I will point out later. In fact, all decent publications talk about particle sizes in a neutral fashion. Some better ones resort to visuals and analogies.

Article #3: https://journals.ashs.org/hortsci/view/journals/hortsci/44/3/article-p781.xml on nitrogen immobilization

Really?? The article is about comparison of woody substrates against peat and pine-bark. They clearly mention that it is a routine procedure to use extra initial charge of nitrogen to counter the effect in pine bark. That is a common practice. Apparently, according to the article, that does not work so well for woody substrates like sawdust and others. They use PB for an intermediate data point like all good scientists should. Otherwise comparison of sawdust against peat will be completely lopsided.

You see peat also consumes Nitrogen - so it also decomposes and degrades. Means the particles will become smaller. Bark decomposition will also have the same effect. But I can always start with larger bark as a portion of the mix.

Article #4: https://journals.ashs.org/horttech/abstract/journals/horttech/8/4/article-p481.xml on root medium properties and your contention that peat moves water 2.5 times better.

The above article, you referenced, actually talks about a number of topics and quite revealing. You based your conclusion based on the fact that peat "loses" water to evaporation at 2.5 times than Pine bark. Is that a good thing? Don't you want your medium to retain water as much as possible so that it is available to the roots?

You are making your own conclusion that evaporation decreases the impact of perched water (indirectly admitting that perched water is present and needs to be dealt with). Whereas the authors conclude it is a negative. Quoting them "Thus, the water holding capacity of peat compared to other is offset in part by evaporation ... ". Later, "Evapotranspiration can be reduced by placing a barrier over the surface ..."

In several places in the above article the authors talk about the impact of particle size. Page 482 in section: Particle size and Pore Space Distribution. Notice how the discussion does not mention peat or bark till the end. That is because the basic physics applies to any medium.

The section "Settling" on the same page talks about compaction issues in peat based media.

Next page section "Moisture release characteristics", the last para on the page. Quote: "Of the organic material peat held the most water ... but plants grown in peat took the shortest time to wilt. Pine bark held 30% less water ... pine bark went 80% longer to wilt ..."

In conclusion section pg 484. Quote "In transplant production media water is never a limiting factor ... Instead aeration is the primary concern" They go on to describe three methods to increase aeration. The very first method is to increase particle size but admit cannot be done since they are stuck with using medium with fine particles because it makes it easier to quickly and uniformly fill the pots. A business decision.

The second is to increase container height so that the saturated soil (perched water) is way below. Nurseries do not like that apparently. So they opt for the third method - saturate with water and then use a combination of heating, ventilation and air flow. Only possible in a greenhouse. Perhaps this where the rapid evaporation loss of peat is useful to greenhouse operators.

The authors did an excellent job of convincing me that I should be ultra careful about using peat in my containers. Thanks for the link.

Article 5: https://books.google.com/books?id=d1v5pAmhYXEC&lpg=PA53&pg=PA125#v=onepage&q&f=false

I am astonished how you want us to believe evaporation is a good thing and then link an article which says exactly the opposite. Quote: "High peat mixes rapidly lose water ... A mulch of bark ... greatly reduces the loss. These last few points indicate that high peat mixes are generally inferior in physical properties to mixes based on best composted pine bark. The best way to use peat is to use in small proportions to increase water holding capacity."

The above quote basically provides more justification for Al's 511 mix of bark, perlite and peat.

"In the US, mostly Canadian peat moss is used. This can absorb up to 8 times it’s weight in oil (here is the science), and up to 20 times or more it’s weight in water (here is the science), and blonde peat, which I love, even higher." I stand corrected. But isn't very high water retention a problem also in soil mixes? Article 4 above thinks so and greenhouses have to employ specific water management procedures to counter that.

"Water evaporates out of the top of a pot of peat-based soil at 2.5 times the rate of pine bark (here is the science p 484 ¶ 2; more science p 125 #3). The method it moves is irrelevant, as long as it is leaving the pot. Another way to say the same thing is that water festers down in bark’s perched water table for days longer than in peat’s (you & I both know why Al talked about overturned pots up above). This movement of water makes perched water tables not a serious problem for peat. So peat actually does have a very unique structure with multiple kinds of porosity & water transfer mechanisms, to which the laws of physics do seem to apply differently." Do not know where to start but here it goes.

Laws of physics (or more generally science) always apply the same way - that is why they are called laws. Every material has a complex and unique structure. You want to depend up evaporation loses as a mechanism to control perched water? How is that going to work out outdoors. Hot dry days will dry out the soil fast. Cool rainy days will keep the container saturated. Properly mixed mixes has very little perched water - so nothing to control. I do not have to worry about vagaries of nature saturating my containers. On the other hand I can always water when needed because that is completely under my control. Even if it means watering daily during hot dry spells. Ballasts (overturned pots) are a great way to deal with saturated soil but why bother when I can make mix that does not have saturated water zone or is extremely small.

First you tell me that peat holds a great deal of water and convince me that is a good thing. Then quickly followed it by how fast it loses water and that is a good thing too !! Complex indeed. I would rather have a mix that has consistent moisture levels throughout the soil instead of dry zone at top and wet zone at the bottom.

Article 6: https://www.sciencedirect.com/science/article/pii/S0009254116301243: Nice paper on peat. But I do not see how it is relevant except to state that it is complex. Plus it will take me a while to understand most of the paper. If you have a better analysis I will love to hear.

Article 7: http://www.dnr.wa.gov/Publications/ger_b44_peat_reasources_wa_1.pdf to support that peat has unique cell walls that resist compaction under high pressure. I wonder how the industry compresses and bags them in those 3 cu ft bales? I would have thought, instead, it would be a good argument to state that peat will spring back to its original shape easily when allowed to - because of its cell structure.

Article 8: https://books.google.com/books?id=d1v5pAmhYXEC&lpg=PA53&pg=PA123#v=onepage&q&f=false The fact that peat has unique properties is already established. It is a tenuous argument that peat does not compact when your own referenced article (4) above says it needs to be avoided. In fact, if you look up nursery manuals they will precisely detail how to fill pots with peat based mix to minimize compaction related effects.

"Fortunately, you also said directly that adding perlite eliminates compaction." I said it prevents and does not mean it will not compact.

"Do you see the Alocasia on the left in this photo? It’s been 21 years in that same pot of peat-perlite-pumice-compost (including 4 years dormant!) It is still uncompacted and drains freely. The soil has lost about 1.5” of height in that time. Very acceptable and not at all a problem." Nice plant. Guestimating that the pot is 10" high - so a 15% loss in volume for 1.5 inch drop in soil level. Pumice and perlite will not collapse. So let us say half of it is peat and compost - so it collapsed by 30% !!!

Peat does have two properties that are truly great. One is its superior ability to release most of the water it holds. Thus roots do not have to fight as much to extract the water from the media. Second is its superior ability to hold and release nutrients. By adding a small amount of peat to a bark-based mix allows one to take advantage of that.

No worries, ToC. I've been too busy getting my business sold to put together a reply as comprehensive as it would need to be (like yours, e.g.) to address the many points Tom made further upthread.
Practically speaking, when it comes to the fora we usually associate with growing in containers, the lion's share of the issues that bring people there seeking resolution are ultimately caused by poor root health, which has been caused by a chronic excess of water and the accompanying lack of air porosity in the medium. If the grower has been watering to beyond the point of soil saturation then waiting until just before drought stress becomes an issue before watering again, only to note the plant is failing due to root health, it's difficult to put a label of 'over-waterer' on the grower. The underlying problem is almost always associated with use of an off the shelf potting medium based on fine particulates, like peat, coir, compost, composted forest products, sand, topsoil, etc.. We see examples/proof of this statement on a daily bases across several fora. These media commonly support 4-6" of perched water, as they come from the package. If a pot is 4" deep, and the medium supports 4" of perched water, air porosity at container capacity, which is the critical factor, is 0%, i.e. 100% saturated. It doesn't matter how skilled the grower is, it's going to be extremely difficult if not impossible to coax as much genetic potential from a planting affected by a medium prone to excess water retention as one that's unaffected.

I'm sorry to say so, Tom, but you've taken a great deal of liberty in describing how water behaves in container media, and in interpreting what I've said. When I get some time to myself, I'll go through your original 5 points and highlight areas where your information is in error or you misinterpreted what I've said. I'll also read through the conversation you're having with ToC and see if there is anything additional I'd like to comment on.

I hope everyone has a good holiday weekend, and those with a mind to, will offer up a prayer for those in Dorian's path.

Al