Knowledgebase
Toxicity of concrete/cinder blocks used as a raised bed material in gardens?? #287745
Asked October 29, 2015, 1:21 PM EDT
I have been planning and spearheading a school garden at my children's elementary school in Golden Co for the last year and a half. As a Learning Garden committee we decided that using concrete blocks as a raised bed material would be more sustainable than using any treated wood which would possibly expose our soil to chemicals.
We received an amazing grant from our local Lowe's and they are building our raised beds for us. After the first weekend of work a teacher came into school and saw that the raised beds were made of cinder blocks and said that they are potentially dangerous as well!! Oy!! We had never heard of this and after some internet research and speaking with an expert we have learned the problem is when these bricks are made with Fly Ash (which ours are - an 8% concentration) which is a by-product of coal - they can leach things such as radium and arsenic. Seems like this may not happen unless the bricks are actually fractured but since the potential is there and we have caught it early we want to do something about it.
Does anyone out there know anything about this? The solutions seem to be sealing the bricks - which would be using some sort of chemical product which we would prefer to avoid. Or lining the sides of the beds with a heavy plastic to avoid the soil contacting the bricks.
I would love any insight or tips that anyone has.....I'm sort of freaking out as we hope to fill our beds with soil this weekend.
Many many thanks in advance!
Liz Bravman
Boulder County Colorado
Expert Response
In the research that I did for your question the main thing that I found was that the blocks will (over time) leach lime which alters the soil's pH. Because of the fly ash there is the possibility of some leaching of other compounds, however I did not find any conclusive research that deemed them dangerous to use for your application. There is a lot of anecdotal information online from various sources, but again, I didn't find any science based research. In fact most of the Extension sources I found from universities around the country still recommend using them for raised bed gardens. There are studies that show that using treated wood is not okay.
Since I'm not able to provide conclusive evidence, if you feel more comfortable, you can always line the blocks with a thick (6 mil) plastic. Don't line the bottom however, because you want the soil to be able to drain rather than create a pond. Or, you can use another material like untreated wood planks.
Good luck. I hope your garden is a success!
Thank you again for your assistance!
Best,
Liz
