Why would the soil underneath be relatively drier?

There is also no valid reason to use what is essentially a soil-less mix intended specifically for containers in a raised bed with no solid bottom. That is not a container! Unnecessary and overly expensive.

My thought is the succulent should be alright. I suggest you start slow and check out whats happening.

@floral_uk I guess the thinking would be that the water table during the Spring and Summer here is at least 50 feet down. The clay is going to hold some water for a few days and then totally dry out. By contrast, even a mix with large particles like the gritty mix still has some perched water table in a container, and the plant's roots can continue to drink from that lower part of the pot for a few more days than in the situation with the in-ground planting.

It would be interesting to make actual measurements of the tension of the soil at root depth in both cases: gritty mix in a container versus gritty mix on top of clay soil. Not sure how that would play out over the course of a week after watering.... I am just observing that the gritty mix in this configuration appears to be drying out faster. Some plants in this soil have a very hard time getting rooted, although the plants that finally do root are thriving in it.

@gardengal48 (PNW Z8/9) Possible reasons to use a different soil mix above the ground:

* More air space for the roots

* More attractive. Particularly for a desert plant, seeing the surrounding soil in a grittier mix looks better than clay.

The issue here is you need to construct raised beds of things they are going to stay raised LOL. Using a gritty, mineral based mix as a raised or mounded bed is a valid approach and is used, for example, at the NCSU arboretum for their succulent bed. They brought in what must have been a couple truckloads and 10s of thousands of dollars of permatil, to allow them to grow plants that would not easily grow in the native clay soil. I myself have a strip of rubble, sand and grit along my house, where the eaves protect succulents that otherwise melt in my rainy winters. (enhanced compared to much of the east coast by having almost 200 miles of the bay waters to the south of me. Any synoptic weather system moving up from the south picks up additional moisture)

OTOH if you made a raised bed of a soilless mix, eventually, depending on climate, it's going to turn back into C02. And no longer exist. Ok being slightly facetious but point is, a raised bed should be made of SOIL of some kind, not a soilless mix. Now in some cases people have used a mix of organic material and soil, and they are constantly replenishing the organic part, so any loss is replaced. Down in Alexandria, VA, along the river there was a guy who had sandy soil so he made raised bed of soil + compost to grow his hedychium collection. The compost retained moisture since those are kind of water hogs, and it's even possible the fresh layer of "hot compost" he added each winter gave an insulating blanket effect because of its decomposition. I'll never forget standing near the community compost pile of Vienna, VA, on a fall day and feeling the heat emanating from it...as though it was a giant radiator!

Still no reason to use a container mix in a non-container setting :-) Just use some sort of raised bed soil, which is typically a mineral based garden or topsoil base with some additives - bark fines, grit, perhaps compost. Depends on the plants being grown. Rarely is drainage a further issue as by definition an elevated or raised planting area will drain faster than the same sort of soil mix at ground level

I rarely consider the aesthetic appearance or a soil or soil mix but instead focus on whether or not it fits the plant and the situation. You can always mulch or topdress with rocks, gravel or more grit if you prefer that look.

@gardengal48 What is are some brands of topsoil you like?

If you were planting succulents in a raised mound on top of clay loam soil, would you prefer just mounding the same clay loam to build the mound, or would you build the mound from topsoil?

@davidrt28 (zone 7) I buy what you say, and in building a raised mound would want no more than 10% organics versus 90% real mineral soil. I need to look at actual ingredients in what is being sold as "topsoil". If that is more than 30% organic material I worry about it collapsing over time.

The organics should be at the surface and compost seems to be working great for me.

"the water table during the Spring and Summer here is at least 50 feet
down. The clay is going to hold some water for a few days and then
totally dry out."

I don't think that's how it works. I'm not in a dry climate to be able to say how it is where you are, though. But soil with some organic materials and mulch will buffer some moisture, drip/targetted irrigation and watering would be more effective, and I'd be surprised if deeper clay soil really did dry out that quickly.

@armoured I would love to see a book that did actual measurements and science around that question. Or - rather - I would love to read a summary of that book.

I'm a total amateur with succulents in the ground.

When we moved into this house 4 years ago, we wanted to do something different than the lawn/boxwood that everyone in this fancy schmancy neighborhood has!

We had to be a bit careful as it is an historical neighborhood, so we started with our hellstrip.

We went to the local landscape materials place and told them we wanted to plant succulents.

They gave us a 1/2&1/2 mix of soil and sand. It's been great!

• We've had many compliments from the neighbors, so we added an island in the middle of the lawn using the same 1/2 71/2 mix (also had the lawn guy adjust the sprinklers so they don't hit that area)
• Many more compliments!
• We're hoping to eventually be sprinkler free and teach our lawn guy more about lawn free yards! And learn more ourselves as we are in yet another drought here in California!

@gardengal48 (PNW Z8/9) My favorite supplier here makes the point that topsoil is rarely available in our area. So how can we reasonably recreate it? If I bought a pure sandy loam, could I just mix that with 10% compost and call that reasonable topsoil? How would you amend a pure sandy loam?

And to make it a targeted question, how would you amend a pure sandy loam to create a topsoil targeted to:

1) succulents

2) acid-loving plants like rhododendrons?

@nancyjane_gardener I have also had some great success with heavy amendment of soil using sand. I do not have a formula yet, but I used something near 50% the times I used it.

Could you ask your supplier what is the exact composition of the "soil" that she is then amending with the sand? I am expecting the answer to be something like a "clay loam" or "heavy clay" soil with very little organics added. But confirm that if you could.

@gardengal48 (PNW Z8/9) Here is a product that is manufactured as "topsoil" in our area. The manufacturer is telling me that they make this from 55% compost and 45% sandy loam. Is that a normal composition ratio for topsoil?