A couple comments:

The temporary advantage of plants in pots: Nurseries raise cuttings in sterile media in sterile pots. There is no phytopthora present to cause a problem. Eventually it does come in to the hoop house. I have been at Rarefind in the spring picking up plants and saw plants here and there that had died from phytopthora. Almost all natural soil has phytopthora in it already and the plant survives until conditions become more favorable for the phytopthora than for the plant. A good B&B nursery can raise its plants in the field and they are strong enough to keep phytopthora at bay. Container plants are inherently weaker when they are eventually exposed to phytopthora that is in soil.

The disadvantage of inorganic media: Drainage is essential but it isn't everything. Yes all organic media will eventually become inorganic unless there is life in it producing more organic media. People that faithfully replenish the mulch in their rhododendron gardens find that often the roots stay in the interface between the organic rich soil and the mulch. Sometimes they can pick up a plant and the roots are all in the mulch. Rhododendrons prefer the organic media to the mineral soil. In the wild, leaf mold and tree litter naturally provide a mulch layer which becomes loam, but in most of our gardens we must fill this vital function and maintain the mulch layer ourselves by adding organic material.

Soil modifiers are just that: Products such as sand, vermiculite (expanded weathered mica), perlite (amorphous volcanic glass), permatill (expanded slate), and turface (fired calcined clay) are great to modify high clay soils to make them useful. They have no nutrients and can't support life in most plants. They are used to make sterile mixes when blended with sphagnum moss fines or pine bark fines. They are also used to modify soils that have nutrients in them. However, what is even better is a good soil that can be mulched regularly. After all that is what nature has been doing for millions of years and it works and has withstood the test of time.

No doubt Turface will help to make stiff soil more friable. Whether it's significantly better than other, less expensive additives is another question entirely. It's always dangerous to generalize because individual gardens and climates vary so much, but for what it's worth: My garden is steeply sloped with granite and granitc gneiss ledge close to the surface. The best mixture I've found consists of partially decomposed conifer bark and needles, locally available Nutrimulch - a bark and compost mixture - and at least 10% round pea gravel. Rhododendron species such as those in the Taliensia section which require extremely sharp drainage/areation have done better in this than in the areas in which I previously used Turface. This does not mean the Turface would not be superior in some other area. I also totally agree with rhodyman that maintaining a mulch layer is critical for long term success. In many cases I have sizable rhododendrons happily growing in 8 inches of the mixture described above with granite below. They are mulched each spring and again in the fall, if needed. In fact, I have come to believe that it's better to plant rhododendrons above the surface of just about any soil than to dig and then attempt to modify the existing soil unless it is already pretty suitable for rhododendrons as in a woodland.

As for the root rot question, temperature - a variable that's pretty much impossible to control - seems to me to be the most important factor. For example, Jenny Tabol and Capistrano grow easily here (knock on wood) with the same degree of drainage and aereation needed by almost all rhododendrons. They experienced no problems coping with the uncharacteristic hot spell in the summer of 2013 when I lost a number of small rhododendrons growing in pots.

I live half way up a 1,000 foot high ridge with a SE exposure. I have both artesian wells and good drainage. Speaking of planting on top of the ground, a couple years ago I planted a rhododendron bed about 75 feet long that had natural springs flowing across it after we have had more rain than normal and the springs are getting more active. I decided that this was the time to try planting on top of the soil. I didn't dig holes at all, I just put down a bag of peat humous for each plant and planted in the peat humous and mulched thoroughly with pine bark. In the area where the spring crossed the bed, I chose plants with R hyperythrum in their pedigree from the Southgate series of R. hyperythrum hybrids by Dr. John Thornton of Franklinton, LA. The first winter, some varmint tried to pull some of the plants out, but that crises is over and this bed is doing very well. I don't know why I didn't start doing this sooner. It makes so much sense, but the habit of planting in the ground is hard to break.

It's been more than 70 years since Edward Faulkner published Plowman's Folly and roughly the same number of years since Louis Bromfield began demonstrating no till agriculture at Malabar Farm. It's been 50 years since Ruth Stout advocated permanent mulch and no till gardens. All three dealt with agriculture rather than horticulture and were addressing different issues, but the lessons remain valuable. Rhododendrons and other ericaceous plants need highly organic "soil" which drains well while retaining moisture and is highly permeable to oxygen. The ericacea are also dependent to a greater or lesser degree on soil microbes for good growth. The needed microbes often do not find a suitable environment in ordinary soil.The best way to acieve the needed conditions is to mix a suitable planting medium and place it on top of what's there without doing any digging at all. It took me many years to understand this when even a casual inspection of how plants grow naturally -no tilling, permanent mulch of leaves, twigs, etc -.should have made it completely obvious The drawback is hauling, mixing and shoveling is a lot of work. Plus you don't get the atavistic satisfaction of sinking a spade into the ground.

Thanks for sharing your views. I agree one mustn't generalize, which is why I tried to add a lot of caveats.

"Rhododendrons prefer the organic media to the mineral soil."
Well that could be true of most of our species; but I'm fairly certain many though perhaps not most of the Asian species grow on gritty mountain soils that probably have only a very thin layer of organic duff; the plants surely have to do most of their anchoring in very abiotic mineral soils. In fact I seem to vaguely recall reading a paper pointing out that these screes have a unique mineral balance such that, even though they are not particularly acidic (suggesting again a paucity of organic material) the native Rhododendrons are uniquely adapted to their mix of cation traces and thrive.

"My garden is steeply sloped with granite and granitc gneiss ledge close to the surface." So, not to seem argumentative, but you're close to having ideal conditions for avoiding root rot...not to even mention the vastly cooler climate of Maine. That you would even have a bit of trouble with, well, anything, just underscores what a difficult genus it can be. The fastest death I ever saw was a 'Biskra' from RSF. Literally keeled over a week or so after the steam bath started in late May or June. I've grown scads of oddball plants (within reason, I certainly haven't tried any Meconopsis!) but you really have to search high and low for something that sensitive. The alpine proteas of South Africa can die that quickly; though I surprised myself by getting a Protea dracomontana through at least one summer a few years ago.

Anyhow...I think these products do give people in _certain_ climates a slight edge. For example I know a very serious conifer collection in NC who swears by permatil, and the NCSU arboretum used it extensively for some of their beds. But they are not a magic bullet. Correct planting and siting is the bigger part of the battle. As I probably posted elsewhere I rescued a 'My Jane' merely by moving it onto a mound...of my plain loamy clay. It was in an area I had not "improved" yet. As you said: "and place it on top of what's there without doing any digging at all." My approach is pretty close to that; most of the root mass above the level of the surrounding soil. Then mound up. However the "turfaced" bed preceded this realization and the plants in it, 'Leo', 'Captain Jack', 'Sun Dust', a few others I can't remember now...all have been doing very well for several years. And, again, it's hard for me to think things would be exactly the same if I'd (I now realize) too deep planted them in what was there before. I did give 'Leo' a little lift of 3" because I was worried it was getting a little too low.

Because my county has several rock quarries I can get pea gravel delivered rather cheaply. I might try that for another tilling/grading/mounding project one day. It will be cheaper than turface by far.

It's funny I'm even discussing this minutiae though...the real problem w/my rhododendron collection is I just have too many plants that I want to allow to grow unimpeded, and not enough room for them. Room with shade I should say. I've obtained a whole bunch of these Taxodium hybrids with the hope that they will create reliable, high shade faster than just about anything else out there. (cf: http://www.camforest.com/product_p/ts-tdisbanita.htm) I've been in "denial" about my high shade problem but at least have a plan of attack now. (I had not planned to take rhodies nearly so seriously until I visited Rarefind in spring of 2006...and then the addiction crept up on me without really pausing and thinking how to deal with it!)

Rhodyman:
"The temporary advantage of plants in pots: Nurseries raise cuttings in sterile media in sterile pots. There is no phytopthora present to cause a problem."

That is a perfectly reasonable thought. It's also simple so Occam's razor says it should be correct. However...
I know I had stuff in my potted plants area that had been in and out of other parts of the garden. An in fact at times of very heavy rain, washoff from my annoying neighbor's yard swelled into that area, a north facing concrete pad sloping down to a cellar entrance...and filled it with a thin layer of dirt. So, IMHO, the darn phytopthora spores had to be everywhere down there. Furthermore, even in full shade, black nursery pots get hot in the middle of summer.

So...you could be right but I still wonder if something else is going on. I've even wondered if, when you plant a typical container grown plant in a typical garden, you suddenly have a lot of lower animal life start moving through the pot's soil column. Worms do eat compost right? So maybe they pull the pathogens in from surrounding soil. Maybe it's even some growth factor in the plant that is affected by being depotted - could fast growing plants be more susceptible? Maybe I wasn't breaking the roots up enough, and...compared to being the pot they actually became more consistently wet, because of the dreaded "bathtub effect". At any rate I had...ohhhh...2 smaller R. rexes, a couple R. xanthocodons, a R. 'Cherry Brandy' species forgotten ATM, that lasted a year or two in pots, but died when planted out. Ohhh...a 'Paprika Spiced', too. Probably several others.

davidrt: Whatever works best in any gardner's specific climate and conditions can certainly vary widely and can only be determined by the kind of experimentation you've been doing.

Rhododendrons which naturally grow in scree/alpine conditions would need a higher proportion of grit or gravel in a mix for them. Whether this is because of improved drainage and aereation, resistance to pathogens or a combinationof these and unknown factors isn't clear.

The cool summer temperatures and (usually) moderate winters of coastal Maine do make for good rhododendron growing. The steep slope of my own garden is also a factor as it provides drainage for water in the summer and for cold air in the winter. I do find it necessary to provide temporary wind shelter for many of the borderline species. Things are not perfect, though, even in addition to too little space. I've failed several times with r. bathyphyllum even though r.talinese, r. clementinae, r. roxieanum, and r. bureavii do very well under what seem to be identical conditions. Btw; I grew meconopsis with moderate to good success for a number of years. It's main nemesis was excessive winter moisture.

Re pot growing: I noticed a while ago on the Van Veen website that they mention routinely using Actinovate or Rootshield II for cuttings, etc. This may be worth looking into.

It's typical for woody plants of all kinds in humid areas to produce a pancake of shallow roots, with a minimal amount of deep rooting. In my area when a tall native conifer goes over the bottom of the stump often looks like a bullet that has been shot against an impenetrable surface in a laboratory - the complete lack of rooting beneath a very specific level in the soil is extremely apparent and dramatic.

In E Asian mountainous regions many rhododendrons grow on limestone ranges. This is possible because the limestone is usually a Dolomite type AND the roots are mostly in the humus layer above the parent material, with the copious summer rainfall and spring snow-melt typical of the areas most kinds of these shrubs grow in washing through it all.

On steep slopes there rhododendrons of some size rather often get tipped over by moving snow, to continue growing on their sides.