I have a plot in an organic community garden, and the city had the bad idea of paving an area with asphalt to pile up the compost we receive. Plots are around that area. With some other gardeners I'm worried about toxicity as we grow food there.Do Hydrocarbons and other components leak in the soil with the rain, and are absorbed by plants? Could you tell me something about that? And possibly add some documentation? (articles, etc.) Thank you!

1 Answer 1


Unlike a highway, the asphalt under the compost won't be collecting vehicle byproduct pollutants.

But compost has been demonstrated to be an effective biofiltration method. So leave a layer at the bottom (don't scrape down to the asphalt) and it will deal with the byproducts of the fresh asphalt. You can also put a bed of plant filters a foot or two wide around 3 sides of the patch, (presumably one side will need to allow access to the truck bringing the compost.)

By next year any free surface hydrocarbons should have been processed and you can scrape down to the asphalt. Put the bottom layer of compost on flowerbeds or the filter strip area if you prefer not to use it on the food plots, though there should be little concern by that point (the hydrocarbons are processed by the bacteria, not merely stored.)

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    Just to point out something to be aware of: heat buildup especially on a black asphalt surface in the sun, or in plastic bags/under a tarpaulin in the sun, may often be sufficiently high to sterilize the composting microorganisms present. To my mind not desirable for plant feeding, and also removing the biofiltration benefit, at least in the bottom layer. Although under good conditions, the "dead" organic material should be colonized again from "live" compost. I don't have figures, but maybe consider some measures to ameliorate this effect (shading, moisturizing, mixing etc.)
    – frIT
    Commented May 16 at 15:24
  • It is possible the asphalt was recycled from a source that did collect heavy metals- this study suggests some (relatively laborious) methods for testing that. Personally, I would do as you suggest, and send the un-scraped compost for a heavy metal test before I used it. sciencedirect.com/science/article/abs/pii/….
    – MackM
    Commented May 16 at 16:00
  • @frIT that's a good point, the bacterial viability of the bottom layer of compost depends on the asphalt not being too hot when the compost is initially placed on it. However, once a few layers are on, the compost itself will prevent the surface from heating up in that way unless it's a very small pile. Compost heats itself up in other ways, though. And the edges might still be hot from the sun on the asphalt. Commented May 16 at 18:10
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    Even if the asphalt was very hot in the sun, the retained heat when it had a load of compost dumped on it (immediately shading it from further sun exposure) could hardly sterilize much depth of compost. Indeed, the UGA page on "how hot pavement gets in the sun" barely reaches an acceptable composting temperature of 140F site.extension.uga.edu/climate/2022/05/…
    – Ecnerwal
    Commented May 16 at 19:04

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