Q: We bought a house recently and have been looking forward to planting the backyard which is mostly grass that isn’t doing so well. I discovered that the previous owners had 3 junk cars parked back there before cleaning it up to sell so I’m wondering if the soil is toxic (oil, gas, antifreeze?) and whether I should remove it or what?
A: This is certainly a difficult situation without a straightforward solution. In a perfect scenario, I would suggest removing any soil that appears oily or smells of petroleum. In my experience, oil contamination is usually not very deep. I would try to remove it to a depth of at least six inches. The problem with this is what to do with the removed soil? Proper and legal disposal will be costly. Check with the EPA and local toxic waste disposal firms.
Alternatively, you can amend the soil attempting to both dilute the contamination and to allow nature to work on healing the soil. Irrigate the area thoroughly, checking that the moisture has penetrated deeply. Start by applying a natural detergent spray. Orange-based detergents work well. Apply activated carbon at a rate of 20 pounds per 1000 square feet. It works best if you mix it with water first. Carbon is useful in absorbing toxins but not necessarily petroleum. Next, spread a couple of inches of organic compost over the area and use a rototiller to incorporate the materials to a depth of at least six inches. The healthy microbes in the compost can help to gradually break down the toxic materials. If you execute this process in the springtime, sow a cover-crop on the surface (such as annual rye grass) and irrigate regularly. Spray the area biweekly with diluted worm tea or homemade compost tea. The teas will energize the healthy microbes. Keep the treated area constantly moist. Monitor the area for spots where the annual rye grass is dying or growing poorly. Apply extra doses of tea to these areas. When the cover-crop appears to grow healthily (this may take more than one season) you can consider planting with permanent plants.