Hi! I'm a relatively new gardener and I added a few raised beds to my garden this year. Two of them are over concrete. I made a ton of compost to partially fill them with, but I know it's not good to grow in pure compost, so I'm planning to mix my compost with mineral soil. I live in Alabama so I have lots of clay soil around me, and that's what I plan on mixing with my compost (about 50:50). The only problem is that I would like to devote one bed (the long, skinny one) to herbs and root crops, meaning it should be well-draining and probably sandy. A 50:50 sand and compost mixture sounds horrible though, and I've heard that mixing clay with sand makes a whole big mess.

My question is: what should I supplement my compost with to fill this bed? Would that much sand work, or would it be way too much? Could some clay make up the needed volume, or would the combo of clay and sand ruin things?

Thank you so much for your time.

P.S. The black stuff is landscape fabric, and since taking the picture I have added a layer of twigs in the bottom for drainage.

No answer, but starting the discussion--
i'm not sure how much to worry about draining when you're above concrete and above surroundings level.
The compost is a huge help, no matter what. The organic part really helps any soil structure. That's a LOT of compost.
The sticks are good, not because they will help drainage, but because they will gradually rot -see 'hugelkultur'
https://www.permaculture.co.uk...
which doesn't seem to worry at all about your native soil.
sand -clay problems I think mainly concerns those who have a lot of clay and think they can fix the garden bed with some sand-failing to realize that the amount they plan to add is way too small in proportion, and to take into account that they are still growing on level with the surrounding clay.

sallyg said:No answer, but starting the discussion--
i'm not sure how much to worry about draining when you're above concrete and above surroundings level.
The compost is a huge help, no matter what. The organic part really helps any soil structure. That's a LOT of compost.
The sticks are good, not because they will help drainage, but because they will gradually rot -see 'hugelkultur'
which doesn't seem to worry at all about your native soil.
sand -clay problems I think mainly concerns those who have a lot of clay and think they can fix the garden bed with some sand-failing to realize that the amount they plan to add is way too small in proportion, and to take into account that they are still growing on level with the surrounding clay.

Ok! Your comment prompted me to look more into guidelines around adding sand to clay and vice versa. I think I may use a similar amount of native soil in this bed and just supplement with sand, but I need to finish reading the best practices to make sure I don't screw something up.

You are on concrete, unless you have hole so it can drain onto the cement that water is going nowhere.
The fabric you have in there will make sure it goes nowhere.
If you get heavy rain it will water log regardless of the soil type.
With that set-up soil type means nothing; the wate will go as far as the concrete and stop.
The finer the soil, the quicker it will hit the concrete but so what?

Are those two by twelve or two by sixteen?
Unless you fill them to the brim you will only have eight to ten inches of soil and the bottom will always be water logged unless you have put drains in and then you will have wet concrete.
If you want root crops you will need two by twenty side boards minimum and then it will still water log if the water does run out.

A women on one of these type sites put shiny cardboard under her raised bed; it acted like concrete.
Only she was fortunate the water drained out rather than saturating and killing things but at the same time, she was constantly watering because the water ran out.

My sig. other put plants in wood barrels over two feet deep.
Her plants were dying because the water did not run out.
I drilled hole as low as possible and the water ran out; the water stank like a wet slough.

Looking again, might be best if you use potting mix. they are big 'containers' and won't behave as they would being on the ground but raised.

RpR said:You are on concrete, unless you have hole so it can drain onto the cement that water is going nowhere.
The fabric you have in there will make sure it goes nowhere.
If you get heavy rain it will water log regardless of the soil type.
With that set-up soil type means nothing; the wate will go as far as the concrete and stop.
The finer the soil, the quicker it will hit the concrete but so what?

Are those two by twelve or two by sixteen?
Unless you fill them to the brim you will only have eight to ten inches of soil and the bottom will always be water logged unless you have put drains in and then you will have wet concrete.
If you want root crops you will need two by twenty side boards minimum and then it will still water log if the water does run out.

A women on one of these type sites put shiny cardboard under her raised bed; it acted like concrete.
Only she was fortunate the water drained out rather than saturating and killing things but at the same time, she was constantly watering because the water ran out.

My sig. other put plants in wood barrels over two feet deep.
Her plants were dying because the water did not run out.
I drilled hole as low as possible and the water ran out; the water stank like a wet slough.

Thanks for your input! Luckily the spot both beds are in is a bit above the rest of the area, and water flows pretty well underneath and away from them.

I actually have had that 4x8 bed there for 2 years, and filled it with bagged potting/garden soil. I then removed that soil used it to fill other beds that are over the ground, where I thought the organic matter would be useful but less nutrition was needed. I planned to fill this one with the compost I made, and I know that I also need some mineral soil in there to hold on to nutrients and stuff.

They're both 12 inches high, but I'm not worried about the depth. I've grown tomatoes, watermelon, and zucchini there in past years and had a lot of success. I do think more depth would be better, so this year I'll actually be growing my tomatoes in-ground and saving this shallower bed for eggplant.

All I'm wondering about is what I should use for the mineral soil. I guess I should have been more specific about the drainage situation when I wrote my original post.

Save yourself alot of trouble and hard work and buy a few m³ of topsoil. A soil based medium is best for long term raised beds and compost can always be added later on an annual basis

If you have good drainage you will get away with it but what root crops are you thinking of or have grown before.
Compost sinks, often a LOT.
Lee-Roy has the best idea.

RpR said:If you have good drainage you will get away with it but what root crops are you thinking of or have grown before.
Compost sinks, often a LOT.
Lee-Roy has the best idea.

I've grown carrots and radishes before, and I'm planning to try my hand at beets too.

I didn't think about that. Yeah, I bet topsoil is my best option.