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.

I have been where shale is and saw nothing growing in that shale unless there was an accumulation of organic matter. What your soil needs is organic matter which feeds the earthworms and other members of the Soil Food Web.

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?

I can't be of any help here, but I too am looking for alternatives for peat and vermiculite. (What is "Coir", by the way, linderbean?) I can get both of these items here in Pittsburgh, but I am trying to purchase everything locally -- and I wondered if composted leaves would do for at least part of the perfect soil product -- or would they be too acid?