Thank you, ToxCrusadr! Your reminder of the fact that DG are solid, unchanged particles rather than the porous end product of expanded shale gave me my "DUH/I'm a moron" moment. Regarding expanded shale research: I enjoyed a brief (grant-sponsored, part-time) employment at the Tex A&M Urban Solutions Center in Dallas a few recent years ago. I was very fortunate to spend a brief time involved in soil sciences. However, my experience with this discipline was limited to soil fertility. Having said that, I feel that some of their non-fertility amendment based research (corporation funded grants) and subsequent results are often an effort in ridiculous predictability. My own extreme and annoyingly sarcastic example would be "would an ice cube retain its crystalline structure in an oven or a freezer". Therefore, I hope that the general gardener would not decide to attempt a successful garden using expanded shale alone. My understanding of the affects of various amendments to soil structure is limited to classroom lecture, persistent questions and personal experimentation. I am truly grateful for the chance to get advice from many varied angles.

Well, one thing you need to do is get up there right after an irrigation session and see how much the root systems are actually getting. Just because you are running the irrigation doesn't mean the plant is actually getting watered. Sticking your fingers into the soil and seeing if the soil is moist or not in the spots where the plants have died (wear gloves if you have safety concerns).is the best way. Also be super careful on a wet slope. If it is not safe do not do it. What is your irrigation system? Spray heads or what? What type of spray head? Or do you have Drip? My slope experience is that once the plants start growing, some dry spots emerge with spray heads. So many landscape contractors put spray heads on slopes in OC and it's criminal. Then they plant shrubs that block the water to parts of the slope. Idiotic. Either areas don't get water, or much of the time the water blows away and does nothing for the plants, especially in the dry Santa Ana wind season. If the water is not getting to the soil with spray heads look at converting to a netafim system which will save $$ on water and be a permanent solution before you spend any money on plants. Or at least change the heads to MP Rotators and adjust the heights of the heads to the mature size of the plants so that everything gets watered. Which is going to get iffy since you have Bougies and they'll get huge unless you are cutting them back regularly. Looking at that slope eventually the Bougies could envelope the entire slope and you could shut off your irrigation at that point. Once matured they will live on rain alone. The trees up there would likely not survive, however. Also, tell us exactly what plants died and what has survived. That will help us to help you further. If everything is actually getting watered...looking at the soil would be somewhat helpful. MV is pretty sandy but there are pockets of adobe here and there. Also do the soil-in-a-jar-shake-and-settle test and tell us what you have. Been there and done that with slopes! Here is a link that might be useful: what type of soil do you have?

This is a Virginia document, but it looks like it speaks to much of the Appalachians. You could look up the zone limitation of anything you see that you think you'd like to try. http://www.dcr.virginia.gov/natural-heritage/natural-communities/nctivf

Wow, thanks a lot for all this advice everyone! It was a while in coming, I think everyone was busy with their gardens, so in the meantime I did pour a solution of fish fertilizer and liquid seaweed on it, with the fish at half the recommended concentration, and the seaweed @ 1/3. Nothing happened. My plants still look okay but are absolutely static. The only thing that changed was the G. acaulis, but that was because an expert grower in Belgium, ala Internet, advised me that they had to have manure. So I dug them up and put manure under them and watered them. I came out next morning and found they had doubled in size (which isn't saying much). Anyhow, long story short, by now I was figuring it was something like what the terrestrialman says: I must have put too much sand in the soil. As a matter of fact, he is quite correct that the mixture I made, which I had envisioned would be aerated, is not. Actually it is quite dense. So, having already resigned myself to digging all the plants up in autumn and changing the soil (not a deal, my little alpine experiment is so small), I am very grateful for this advice. As I understand it, then, I should return to the natural silty soil and mix the shale into it. The idea of placing bowls underneath sounds fascinating, but I had understood these alpines didn't want to have wet feet (?)