Where I live the county cleans out the topsoil that washes into the ditches on the sides of the dirt roads every year and people like to use that for grading around their house. A few years ago, my parents had them dump some topsoil in their lawn to flatten it out a bit (not what I would have done, but wasn’t my call.) Unfortunately, the soil they brought came from the sides of the asphalt country roads, not the dirt roads, and it was pretty sandy, mixed with bits of gravel and small pieces of asphalt. Given that they had a mountain of soil on their lawn and didn’t want to deal with finding someone to pick it up and haul it away, they used it for the grading anyway. Just recently, I built some raised beds for them where they had the grading done. However I am now realizing that the soil under the raised beds has more asphalt than I thought and I am concerned about the toxins being in the soil and it getting into their annual veggies. The raised beds are 12” high. As I was building them, I also dug a shovels depth lower to loosen the soil and give more room for the roots to grow. Now, I’m second guessing myself, thinking maybe I should sacrifice root space, put down a root barrier, and just build up above the asphalt-laden soil. However, lots of people tear up asphalt and plant underneath, and I found this thread saying asphalt isn’t water soluble. https://permies.com/t/13571/Safety-planting-asphalt Do you think I should be concerned about the asphalt pieces below the raised beds? I am open to doing soil tests, but I dont know what to be looking for at this point.
Hi Ryan. I have no experience with asphalt gardening or the chemical testing thereof, but I would suggest that the hydrocarbons bonding together the aggregate in asphalt will continue to break down at a slow rate. While these oil like compounds are not soluble in water at high levels, they can be carried by water as clathrates....water molecules forming hydrogen bonded cages around the hydrophobic molecule. I'm not sure if this a problem or not, but I suspect that fungal dominated soil might consume these oil like molecules. This makes me wonder if using this soil for a hugelkultur bed might be advantageous. Generally ramping up the soil microbial hordes seems to help a lot of things.
Biochar maker/enthusiast whose mind wants to dance, but whose body is a really awkward white guy.
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Location: Northern pennsylvania, zone 5b
posted 4 months ago
Greg, that makes a lot of sense. I didn’t explain this in the original post for the sake of keeping at simple as possible, but I am doing modified hugelculture, burying about 6-8” of wood as I turn the soil a shovels depth below the raised beds.
Does anyone know if people who rip out asphalt and plant gardens underneath worry about toxins from the asphalt?
I tried to find if there is significant risk of leaching contamination, and most articles and studies seemed to indicate the leaching of toxins is below dangerous levels. That doesn't mean no toxins are leached, and "acceptable levels" also may depend on how much other exposure you get to other toxic hydrocarbons. Personally I don't like it in our environment - we "inherited" the neighbors' old asphalt driveway when they made it into a dam in the creek which blew out, spreading asphalt chunks down the entire length of our creek (!)
If I had asphalt chunks to garden on, I would want to create an environment high in fungal activity. Not edible mushrooms, but fungi which will tie up the toxins and possibly render them inert. Buried wood beds, hugelkultur, etc, will create these conditions.
I skimmed this study titled "Leaching Characteristics of Asphalt Road Waste" and found out that the leaching hydrocarbons are actually less of a concern than Lead, although lead wasn't very common either. Older roads that were used while lead was still a gasoline additive were shown to have higher levels of Lead. This means the heavy metals came from the vehicles and the older the road, the more likely it is to contain heavy metals.
Given that I know the road where this asphalt came from is a rural, quiet country road with very little traffic, I am not very concerned about it's lead content. I still dont like that this is the soil at the bottom of the raised beds, but I will continue to use it as planned. I'll sift out the larger chunks as I cover the wood in the bottom of the raised beds and then top it with another 10" of nice topsoil mixed with composted manure.