Safe to Grow Vegetables Where Concrete Used to Be?

[catchthewind]

Hi there,

I'm new to this forum. I've been spending a lot of time trying to find an answer and no luck, so hopefully someone here can help! It seems like a good place so far. We just removed a concrete slab from my backyard. It was an 8x8 slab, 4-5" thick, and was in the sunniest part of the yard, and likely the best spot for vegetables I have. I would really like to plant something there this year, but I'm not sure how safe the dirt underneath is for planting. I am planning on raising the bed, but I'm wondering if I should put some landscape fabric down first or if the soil that was under the slab will be okay? I know concrete has lime in it, so I'm going to test the pH. Other than that though, is there anything I should know about the components in concrete? It seems the soil underneath is a mixture of very fine sand, and large rocks. I'm going to do my best to pull the rocks out before I add more soil on top. The friend who actually took off the slab told me there is silica dust in the concrete and there was a lot of dust while he was working, so I'm sure there is some left in the soil.

If I should put landscape fabric down, how deep would I have to make it? I was hoping to only have to add another 6" of soil or so. That plot is where I want to plant peppers, broccoli, cauliflower, raspberries, tomatoes, zucchini, cucumber, maybe a melon if I can get one to grow here.

Thanks!

[The Helpful Gardener]

I'm sure the soil is fine; most concrete is earth products anyway, lime and gravel and sand. They do mix fly ash in sometimes, and that can have bad stuff, but that is mostly gone I suspec,t as it does leach quickly with water. [url=https://www.concretewashout.com/pages/industry_problems/concrete_washwater/]Wet cement can be nasty stuff[/url]...


and there does seem to be a lot of [url=https://www.atsdr.cdc.gov/tfacts5.html]heavy metals[/url] in concrete (from the clinkers most likely), [url=https://www.osha.gov/OshDoc/data_General_Facts/hexavalent_chromium.pdf]not the best stuff for you[/url]...

But again, in older concrete that stuff has likely migrated already. Still the smart thing to do (and this is ALWAYS the smart thing to do, no matter where you are gardening) is get a soil test. In this case I would spring for a complete assay (a chromato-spectography) of all the elements (tell them you are concerned about heavy metals). Better safe than sorry, and six inches of soil will not work for most veggies...

You could phytoremediate the soil this year and grow in containers; phytoremediation is using plants to uptake any soil impurities. Garlic mustard in particular is noted for it's ability to uptake toxic metals. They [url=https://www3.interscience.wiley.com/journal/113512159/abstract]are even using plants to suck up TNT[/url]; probably using habaneros

Grow out the phytocrop, weed it and dispose of it in your garbage (or turn it in on your toxic waste disposal day; that'll be a story for the guys).

HG

[Royal Rebel]

[url=https://img14.imageshack.us/my.php?image=blkgar.jpg][img]https://img14.imageshack.us/img14/1371/blkgar.th.jpg[/img][/url] don't know if this would be the same but it works o.k. so far...all I had was clay and sand ..added a block border in the picture , then filled with dirt compost and the like nothing really special, and have been using it now for 6 years, I forget the size but I think it is 8x16 ? goodluck happy gardening,

[jal_ut]

I would not be afraid to garden there.

The soil or sand under that slab will be pretty sterile since it has been locked away from the air for years. It will also be lacking in organic matter which is food for the micro community that lives in soil.

What I am saying is that you will need to add something to get the fertility of the soil up. If you are going to put some topsoil on it that would do it. Add some compost too.

Don't worry about silica dust, as long as you don't breath it. Silica is what sand and rock is made of.

[catchthewind]

Thank you all! I feel better about gardening there now. There's another sunny spot in the garden where I was going to plant a small butterfly garden, so I think I will do the butterfly garden where the cement slab was this year and do the veggies where the flowers were going to go, just to be on the safe side. I am going to look into photoremediation and plant some mustard and other plants in the flower garden as well. I'm going to look into getting the soil tested too, but we're going away at the end of this week and I wanted to get stuff planted before we leave (I have a friend watching the garden), so I think that's the best solution for us right now.

Thanks again!

[farmers tan]

Your garden spot will be fine. Till the organic material in and do the soil test just to make sure but I planted a garden where I had a pad a couple of years ago and things grew great. I had a lot of rocks to deal will but everything grew well.