Hi Emmers, I would also be curious to read a response to your questions, so hopefully someone with more expertise will chime in. I'm still new to the water movement, water retention, and gritty mix concepts myself, but I can take a stab at your questions and thought experiment (to get the conversation started).

I can relate to the confusion/ambiguity about "water retention" encompassing two things, as you wrote: "water held between particles (macro pores/perched water/undesirable) but also water held within or on particles (micro pores)." To clarify the issue for myself, I made up a distinction in my mind between "water retention" (bad perched water, etc.) and "moisture retention" (water within/on pores in the medium, etc.). Hydroponics aside, roots typically don't want to be wet; instead, they want to be moist. So the aim is to retain moisture (or wetness), not water.

Addressing your thought experiment about using 100% screened Turface as a growing medium: it is definitely possible from the perspective of particle size. It would also be possible to use 100% grit of the appropriate particle size, but this would be annoying for the grower (one would have to water frequently). Remember, Al has said that it's possible to grow a plant in a pot of marbles.

Not only should it be possible to grow a plant in 100% screened Turface, but I suspect that it could work out decently enough. In Germany, we don't have Turface; instead, there is a product called Seramis (clay granules) that is similar. Seramis is marketed and sold as a soil-less growing medium, which means that people exclusively using Seramis are growing their plants in something similar to 100% screened Turface. I don't have experience doing this, but it seems to work out just fine (and is much better than growing in 100% potting soil).

My guess (and this is where I would need some confirmation and further clarification from an expert–I don't want to pass along incorrect information) is that 100% screened Turface isn't undesirable, but it's not ideal. Al has suggested that the gritty mix's moisture-retention degree can be adjusted by increasing the percentage of Turface and correspondingly decreasing the percentage of grit (while maintaining 1/3 of the mix as pine bark). Extending this, I think that, if you wanted a (relatively) very moisture-retentive gritty mix, you could have 2:1 Turface and pine bark, completely leaving out the grit; however, I don't know whether this kind of mix would still be considered best practice or the circumstances under which you might decide to use this mix instead of the classic gritty mix recipe. In any case, I have the impression that a key principle of Al's gritty mix is that 1/3 should be organic (in the form of pine bark fines). I confess that I don't know/can't remember exactly why (lower pH in the medium? lower cost of the ingredient?).

I don't know the answer to this question, which is the crux of the matter: "If your particle size is sufficient to eliminate a perched water table and ensure root aeration, why then wouldn’t you want to use the substrate with the maximum water [i.e. moisture] retention?" I would love to hear an opinion on it. @tapla (mid-Michigan, USDA z5b-6a), if you have time, could you address this question? Perhaps it has already been addressed in one of the many threads on water movement and retention, but I have not come across it. And if you do, please feel free to correct any misunderstandings on my part in my response to Emmers, as I am still learning! :)

Both analyses are great, as are the questions, and in part the answer to the questions can probably be cleared up with an "if only" observation. If only all Turface particles were appropriately sized, the idea you could grow in Turface MVP only, without worrying about over-watering would be valid. The issue is, if we screened all Turface particles so they are larger than .100" (2.5mm), most of what's in the package would be waste. Essentially, a concession is being made to make it reasonable to use Turface because of its relatively high CEC and water/ air holding ability. That Turface represents (usually) only 1/3 of the medium by volume, and only a fraction of that 1/3 is smaller than the roughly .100" size that supports perched water, in the overall scheme of things the small volume of what would be considered to be 'fines' by gritty mix standards doesn't make a difference we would notice insofar as perched water is concerned. In a perfect world, the entire contents of a bag of Turface MVP would be prescreened such that all particles were in the 1/10-1/8" particle size.

Bit, when we consider using Turface along with it's fraction of 'too small' particles, or even the product with fines passing aluminum insect screen removed, there would be a water retention issue. The issue wouldn't likely be as serious as what we encounter when using a good % of the commercially prepared media based on almost all fine ingredients, but when you're making an extraordinary effort to eliminate as much excess water as possible, why consider all Turface when the grit to use as the control element (for water retention) is close at hand and less expensive than Turface?

So, the reason Seramis can be used straight from the package as a single ingredient grow medium and Turface MVP cannot, is particle size.

I do know a grower (Justaguy) who was well-respected here that started out as a hard 5:1:1/ gritty mix skeptic, but ended up growing in a mix of equal parts of pine bark and Turface MVP, both screened (over insect screen) to remove the dust.

Al

Hello! A dedicated student of Tapla, so to speak, and his methods of container growing, here! (Insert smiley emoji!)

I would heartily agree with the above premise: "To clarify the issue for myself, I made up a distinction in my mind between "water retention" (bad perched water, etc.) and "moisture retention" (water within/on pores in the medium, etc.). Hydroponics aside, roots typically don't want to be wet; instead, they want to be moist. So the aim is to retain moisture (or wetness), not water."

I think that's an excellent way to couch the issue in layman's terms. To whit, using the Gritty Mix, or even the 5:1:1, will require a more watchful eye and more frequent and thorough watering, but I find it all an easy tradeoff, considering the more than likely damage caused by using the average available commercial or retail bagged potting soils, most of which are not much more than bagged silt.

Roots definitely require oxygen, or more to the point, the exchange of oxygen and accumulated gases within the pot, under the soil surface. Whenever I take a plant out of its pot and I see roots circling the outside portion of the whole root-ball, medium and all, close to the pot's sides, I know that the medium toward the center of that pot didn't have the opportunity to dry out sufficiently, causing the roots to seek out a more hospitable environment so they could "breathe", and not drown.

My particular container plants of choice are any tender bulbs, mainly Hippeastrum (Amaryllis), that cannot be grown outdoors in my zone 5b garden. Since bulbs are rather susceptible to fungi and rotting if kept too wet, the Gritty Mix presents the perfect medium for healthy growth. And since I learned from Tapla what roots actually require and how to give them those requirements in a containerized environment, I have had much success and joy with my little indoor garden! No more soggy, rotting bulbs in silty soil with dead and dying roots! No more wasting money on the same variety of Amaryllis bulbs yearly to enjoy their blooms. I can keep the same bulbs healthy and ready to bloom again, often with more flower scapes as the bulb grows and ages, forming its own offspring in the form of bulblets!

I suppose I would be remiss if I didn't mention that the gardening industry as a whole is like any other industry; it exists to profit. Therefore, it is beyond difficult to find a medium that meets what plants would buy if they could search out their own medium to grow and thrive in! Some pre-mixed bonsai mediums are okay, but they are usually sold... as far as I can find locally... in amounts too small for any average sized pot of 8" to 10" or larger.

Steering back to the issue of moisture... I often rework ingredient ratios or certain substitutions as necessary for my own purposes, given my climate, environment and micro-environment, plant type, container size, where the pot will be placed, etc. But I do tend toward staying with the main few ingredients... pine bark, granite chips, coarse perlite... whatever additions or changes I may make. In essence, once I fully grasped the concept of using a more aerated, free-draining particulate as medium, and I saw the benefits of such use in my own home and mini-greenhouse... let's just say that you couldn't get me to buy any pre-bagged silt calling itself potting soil!

Here's to "moist", without being "wet"! (Insert wink/smiley emoji here!) Happy Gardening!

Hi JodiK, it's so nice to read about your success with your Amaryllis! :) Thanks for sharing! And I appreciate Al's response too.

Thank you, bluebelle, and it's my pleasure! Al has been instrumental in my gardening success, sharing good solid information that makes perfect sense! I've learned so much by reading his writings! I'm pretty sure I have more to learn, but in the meantime, I'm happy to keep sharing this information whenever possible, and how it has positively affected my own gardening experiences. If sharing helps one person and gives them what they need to be successful in growing plants, then it's all worth it!