Sterilizing soil on asphalt?

Hey everybody,

When sterilizing forest topsoil to kill the dormant seeds, or to make seed starting/fern spore potting soil, it's recommended that you put the soil out in a sunny place and cover with clear plastic sheets (which heats the soil even better than black plastic). Does anyone know to what extent this process is sped up by setting the soil out on the driveway? Asphalt in the hot summer sun could really improve this system methinks, but I'd love to hear from anyone else who has tried this before!

Asphalt has some nasty toxic stuff in it that would probably off-gas into your potting soil.

John Polk wrote:Asphalt has some nasty toxic stuff in it that would probably off-gas into your potting soil.

Good to know, thanks!

The asphalt won't "off-gas". Asphalt is mainly made up of heavier petroleum products that aren't especially volatile which is why as previous threads have stated that asphalt has a low leaching potential. The asphalt would be a great surface to sterilize soil and you could always lay down some poly (plastic) sheeting beneath the soil if you are still concerned.

Asphalt will off-gas if heated. Have you ever driven passed a paving crew on a hot summer day?

John...you are correct...when asphalt is being laid it is off-gassing...but the asphalt comes from the plant at ~ 300 deg F and the crews are compacting it and adding water (that's where the steam and off-gassing comes from) to get the final product. From a quick internet search I found that the surface temperature of asphalt is usually only 1.1 x hotter than air temp in direct sunlight...so even on a 120 deg F day with no wind the asphalt shouldn't get hotter than ~ 130 deg F...and the SVOCs/VOCs in the asphalt won't volatilize until at least > 200 deg F.

Patrick...if the concrete is newer and gets soft and tarry in hot weather I would probably not put soils directly on, would lay down plastic sheeting. Actually I would lay down plastci sheeting anyways just to keep it cleaner.

> asphalt is usually only 1.1 x hotter than air temp in direct sunlight...

Uh... That doesn't square with my barefoot-across-the-parking-lot test.


Rufus