The only way you can know how much organic matter is actually in your soil is to have it tested. But slow or poor drainage in clay soils is typically an indication of low OM levels....if it were higher, the soil would drain better and it would be loose and very friable and easy to work. So it is likely you have less OM than you think

By definition, a raised/elevated bed should have decent drainage. You do not need to get elaborate about soil mixes......this is NOT a container and you do not need to use container soils :-) Ideally the best raised bed soil is garden soil mixed with coarse organic matter......not rocks or sand or gravel. Just a mineral based soil enhanced with a decent percentage of textural OM.

If this was my bed, I'd just import in some bulk landscape "garden mix" soil and call it good. I would not use the existing clay (if that is for sure what you have) nor would I add any of the gravel or fill sand. FWIW, a mix as I described - a garden planting mix or a 3- or 5-way mix or just topsoil enriched with compost - is used by thousands of rasied bed gardeners with great success. There is really no need to reinvent the wheel here! And it shouldn't cost $100/ton - 3-way mix here sells for around $35 a cubic yard (approx 2000# dry).

"One third each of the native clay, 1/8" gravel, and fill sand (this would make the soil a little like a "loamy" soil, usually considered an ideal growing medium for many kinds of plants)"

Reread what the definition of loam is - 40% silt, 40% sand, 20% clay. And there will be a pecentage of OM involved as well, typically in small enough particles that it wil not be very recognizable or distinct from the rest of the soil ingredients.

i would visit various dirt stores ... and ask to inspect what kind of garden soil they have to offer ...

i would approach a pile.. and make a snowball out of the soil ... smell it .. and hold my hand open... and expect the ball to fall apart in my hand ... i want friable soil ... that smells good ... [too much clay wont fall apart ... too much sand wont hold together ... and stink is not good .... etc]

https://duckduckgo.com/?q=friable+soil&t=ffcm&ia=web

beyond that.. i dont know how you are going to order up.. a certain 'mix' of ingredients ... and based on the scope of your project.. personally.. i would not go the bag route ... but that is a budget issue for me ...

you might need to go out shopping after a rain .. as bone dry material is impossible to figure out ...

keep in mind ... your plants are going to grow down thru this soil .. into the native soil ... and all this stuff is for.. is the period from planting to becoming fully established .... so.. imo.. there is no need to get all fancy with it .. as its job will be done in a year or two ....

i do wish you could find anything other than the plant you insist on.. but.. well.. its your garden.. all the power to ya .... and good luck ...

ken

But GG most raised beds are used for vegetables. We have places around here that sell 'raised bed mix' and the presumption is that you are growing food crops. For woody plants I continue to believe you need more structure in the soil.

"But GG most raised beds are used for vegetables. "

Not at all!! Raised beds can be and are used for growing just about any kind of plant and as this discussion outlines, poor soil conditions (bad drainage, heavy clay soils, excessively sandy soils, improper soil pH, too many roots) are one of the primary reasons for using raised beds.

I use raised beds frequently in my garden designs.....seldom very structured or permanently sided raised beds but more bermed or mounded applications. But they are still raised beds!! And the type of soil mix I referred to is what is commonly used. It is not specifically intended for raised beds - it is a general planting mix. In fact, this is the type of soil that is bought in for pretty much ALL landscaping applications (the exception being needing a huge quantity for a specific need and then fill is used for the bulk with the better soil topping at a suggested planting depth). I have never known any woody plants that require more than this outside of a few very arid loving choices.

I didn't suggest a "raised bed mix".....I suggested a landscaping "garden mix". This can be used for any garden soil purpose - new lawns, planting beds, berms or mounds, raised beds, etc. Or with any new construction were the soil is scraped of existing topsoil. And feel free to disagree :-) All I can rely on is my understanding and background in soil science and my experience using this sort of soil product, both in my own gardens over the course of many years and with countless 100's of design projects. To put it as simply as I can, it works......and works very well!!

There may very well be situations when something in addition to this sort of soil is called for but those tend to be somewhat isolated and unique situations. And impossible to tell in this situation without seeing the OP's garden and evaluating the soil firsthand. FWIW, I have never encountered any reliable horticultural source or knowledgeable individual that recommended adding a quantity of sand and/or rocks or gravel to a soil to improve drainage....unless for something like a curtain or French drain. But that is a very different application.

No, no I know you didn't; my point was that to the landscape supply places, the term 'raised bed' connotes vegetables. If the suppliers would stop with these high organic matter soil mixes, I doubt that I'd have to add pebbles (I would never add sand; our native soil is clay). I feel like if anything, the soils available around here are getting more and more full of compost. It's practically a religion.

@gardengal48 What are the typical ingredients in a 3 way soil mix? Looking at this vendor's soil mixes, which ones strike you as close to the typical mix you use?

Regarding loam, yes it is 40% silt, 40% sand, and 20% clay. That isn't really that much different than 30% sand, 30% lava, 30% clay. The lava is also loaded with silt that washes off. What are the key differences you see there?

I am really confused how can loam be ideal growing soil for most plants, but you prefer to use an organic three way mix instead? I am not saying that 3-way does not work and I would not know. I am just perplexed why you do not want to try to emulate loam.

@Sara_Malone Okay, so let's just make your version of the soil explicit. Given I have clay, you would mix a soil that is 50% native clay and 50% lava rock? I don't know if you have tried to wash off that 5/16" horticultural lava that some rockeries sell, but it is LOADED with silt. If I want to emulate loam soil, maybe I leave that silt there and do not pre-wash.

@gardengal48 what would be your objection to just filling the raised bed with the same clay soil that the plants are already in? So soil quality would be worse, but raising the bed would at least prevent waterlogged soil during weeks of rain.

Clay is very water retentive (when wet) and very moisture impervious when dried into a brick/pottery. Not an ideal media.

westes, of that vendor's mixes, I would go with the landscape/nursery mix. It is similar to what is used here for that purpose. And there is no need to emulate loam - the mix IS loam primarily, just augmented with the organics for increased drainage and fertility.

Your 30% lava rock, 30% clay, 30% sand is not at all close to loam. The lava rock doesn't count at all (rock is not a component of soil until it is fully weathered and mineralized over many millions of years). And there is no silt in the mix. The lava rock may produce a coarse dust but silt is the sediment produced by running water so whatever is coating the lava rock, it is not silt.

You can of course go with any soil mix you want....it's your garden!! I am only trying to help based on many years of experience working with garden soils - many of which are clay based - and in an area that receives appreciable winter rainfall. And there really isn't the need to overthink this as much as I believe you are doing. Your situation is one that is encountered often and the method of importing a quantity of already prepared, well draining loamy soil with some added OM is the classic solution.

@gardengal48 Do you have a professional website with contact information? Maybe I could pay you to take a deeper look at what I have and give some additional opinions?

What's coating the lava pebbles ls fines. I get it without fines. And GG is correct - rock is not a component of soil per se. I grow many plants that are native to very rocky or poor soils - Australians, succulents and conifers primarily. Maybe the term term 'substrate' is better! In order to grow winter-dry plants here (waaayyyy out of their normal rain pattern) I must keep their roots as dry as possible, which means planting on mounds with a high rock/pebble component. Ruth Bancroft Garden in Walnut Creek is apparently redoing their beds with a significant component of rocky substance. I have found that the conifers do well in this mix, too. The Pittosporum is likely much less fussy and doesn't mind the wet feet as much. I have successfully been able to overwinter succulents( that rotted in past) because of the soil mix that I now use. FWIW, I now plant all of my container conifers (mostly those that are still too small to plant out - the jackrabbits are vicious) in cactus mix. I would imagine that woodland plants or any that prefer a richer soil would not like this, but the Japanese Maples do fine, as do the Ginkgo, oaks, etc etc.

Westes I think that GG is also correct that you are likely overthinking this. The Pitts should be ok with almost any of the choices.

Sorry, westes, but I do not. My business comes from word of mouth referrals, a couple of landscape contractors I work with frequently and from my local nursery. And to be honest, I would not feel comfortable providing any advice more specific than what I have provided here without seeing the situation firsthand. And a trip to NoCal is not in the cards at this time!!

Sara I can't believe you get more rain in winter than we do here in the PNW :-) And yet I grow all my containered dwarf conifers (and JM's, hydrangeas and various other containered trees and shrubs and virtually everything else I grow in containers) in the 5-1-1 mix. And have for the last 20+ years. I do use a cactus mix as well but only for the succulents and cacti I grow, indoors and out. I have never lost a plant to rot yet...........

@Sara_Malone Just in the interest of finishing off the soil suggestion you made, if you were doing this raised bed and trying to re-use some of the native clay, what is the exact formula you would use? What percentages of native clay, lava, and anything else?

Changing the topic, I have had a serious problem getting gritty mix to work with succulents over Winter. I rot out more plants than I save. The ones that establish in gritty mix and do not mind wet feet really thrive in it, but many plants simply rot out over the Winter. What soil mix are you using for succulents that you leave outside in containers? Anything dry enough for the Winter ends up being way too dry in the Summer and requiring constant water.

@gardengal48 What is the soil composition of your cactus mix? I am pretty shocked that you grow succulents outdoors in Washington and have never lost a plant to rot....

@gardengal48 I was going to take videos for you of the affected plants and then based on your further requests provide additional photos and videos. My main concern is do the plants have a lethal disease or not.

And I can't tell you that :-) Even with a video! You need a plant pathologist with a lab to do so.

If conditions were prefect I would mix 1/3 native soil, 1/3 lava and 1/3 amended loam. In this context i'd be ok with the loam having a nontrivial amount of organic material as it would be helpful to break up the clay. This is rarely possible - the abode spends the winter as glue and the summer as concrete and passes through friable for about 20 minutes. I have access to an unlimited supply of compost (horse ranch) so if I had more energy and the timing were better I might try harder to work with the adobe.

For succulents I've been pretty successful using about 1/2 loam and 1/2 lava pebbles. I have almost none in containers - just some huge agaves - and I haven't had many deaths. The ones that rotted this year were the Agave 'Sharkskin Shoes' and they have never done very well. I plant very high and sometimes even on top of a pile of rocks. AND I plant only in spring. They have to develop enough roots before the rainy season.

@Sara_Malone That's a great point about giving the plant time to root before exposing it to rain. I have the same opinion based on many experiments.

Your succulent soil is almost all minerals and that might be the right strategy for succulents. Traditional gritty mix has 30% bark, and I feel that bark component is what is responsible for rotting out my succulents in the rain. Does the bark encourage more fungal growth? I have been tempted to try a succulent soil with equal parts of peat, lava, and turface.

Regarding the soil for the pittosporum, do you see value in adding some organic soil into that mix?

I am not sure about the bark; it's just one more thing and my garden is so large that I try to keep things as simple as possible. I get a lot of my succulents (many large Aloe and Agave) from a local specialty nursery and the owner and I have compared notes for years. He sits lower and colder than I do so what survives at his place is almost certainly ok here. And he has moved to grittier soil.

I don't think that the Pitts are fussy. I think that GG's point about basic garden mixes is correct. Conifers (especially many of the ones that I plant) really need good drainage, as do the Australians. Both do fine in 'poor' soil. Again, keeping things simple, I have just moved to using more or less the same mix everywhere. And since I plant on mounds, soil structure is important. Think of the rocky mountainous soil - that is more or less what I am trying to replicate.

@Sara_Malone have you experimented with Turface, and is there a reason you avoid it in your succulent mixes? Too much water retention?