If that was asphalt you need to keep in mind that the base material for asphalt is petroleum. Often oil saturated soil will harden and appear like asphalt although it can be a bit more easily tilled up. Because of the need to recycle old asphalt there are some studies that have found that when crushed and mixed with soil asphalt can help stabilize that soil, but I have not yet seen anything about soil contamination. This may be something like recycling tires, the precautionary principle should apply and until it is proven safe it would not be a good idea.
Since your soil already has this material mixed in it and seperating whatever this was out may be too difficult, I'd not plant a vegetable garden there.

Asphalt does contain a host of heavy hydrocarbons (how's that for some asphalt alliteration?) some of which are toxic. Most notably the family of polycyclic aromatic hydrocarbons or PAHs. These also occur in soot, diesel exhaust, and airborne deposition from coal burning in decades past. They are ubiquitous but no less toxic for that. A 3" thick layer is likely to contain a significant amount of these. I myself would think twice or three times before growing food there, even though I am not certain what the uptake rates are for PAHs into plants. At a minimum I would not plant root crops, to decrease the exposure to trace amounts of actual dirt sticking to the vegetables. That will certainly help.

Are you in a heavily urbanized area or more recently build suburbs or what? Just curious about other exposure rates.

No clue but, apparently, there are people out there building raised beds on top of asphalt.

Here is a link that might be useful: Asphalt Gardens

In some few instances, like when the soil under the asphalt is contaminated, it may be preferable to spend large sums of money hauling in soil to garden on top of asphalt, but far better is to not contaminate the soil in the first place. However, this question is about asphalt, potentially, mixed into the soil and even though asphalt is 95 percent aggregate, stone, sand, etc., and only 5 percent binders, which are bitumen, petroleum products, stuff left over from crude oil refining, that 5 percent can be extremely harmful if ingested by human beings.
So the OP needs to determine if this was asphalt or some other petroleum product that bound that soil into a hard, cohesive mass, and whether they really want to grow food in this soil.

Just contact your local county AG extension and ask for a soil test that includes testing for contaminants. Shouldn't be more than $10-15. Then you will know.

Personally, I wouldn't worry about it given it's age and that is was buried. Most of the possible contaminants would have leached out by now into the deeper soil. If worst comes to worst, you can either move the bed or raise the bed sides and add several more inches of good quality compost to it.

JMO

Dave

I would guess that somebody dumped oil there many years ago and buried it. I found something similar in my own back yard, and a previous owner had an illegal car repair business going out of his (now my) garage. It is much as you describe, a thick layer of hard but somewhat flexible/flaky/brittle black stuff with a somewhat oily smell. I don't grow edibles back there so I'm not too concerned, but I try to dig it out as I find it as it is nearly impenetrable to both water and plant roots.

I thought I had replied to this but I've been having browser issues this past week.

The compost you already applied is the best thing you could have done. In addition to the special soil test suggested by tox above, I would get a soil test from The Texas Plant and Soil Lab and tell them what you suspect. They will give you many ideas as to how to correct any issues they find. TPSL just revamped their pricing. Get the tests that add up to $35 - those tests used to be their basic ($35) test.

I should point out that spraying the soil with diesel fuel is a common way to rid the fence line of weeds in Texas. They sometimes will follow up with molasses spray to help kick start the microbes to restore the soil they just sprayed. I don't know how well that works but the people at TPSL should know.

I wonder if it could have been a rock driveway that was sprayed with oil. In the old days they did that to cut down on the ruts and dust. Many people didn't want red clay gravel drives and couldn't afford pavement so they spread rock and oiled it. Enough years of spraying oil and it would have stuck together a bit and looked like asphalt.

Note, used oil is much lower in PAHs than asphalt but they are present due to the heat and combustion blowby... and the other components - the oil - DO break down with microbial action much better than asphalt. Smaller more chewable molecules.

The way to tell which is maybe to note whether there is gravel aggregate or not?

My but soil tests are really expensive in your neck of the woods toxcrusader! We can get all sorts of toxic material tests done down here @ 10-$15 each - the OP doesn't need the entire range of potential toxins tested after all.

Be that as it may, it is way too easy to go to extremes when discussing issues such as this. We are growing in dirt after all and not a sterile environment. Few have the access to the detailed history of their soil so one makes basic assumptions as to its quality/safety, adds compost to compensate for any questionable elements, trusts to a degree the natural filtering abilities of plants, has it tested, and then moves on.

OR, if one lives in fear of their dirt, they have the option of using containers with sterile potting mix or creating an enclosed bed filled with sterile potting mix and then enclose it so no possible contaminants can gain access to it.

Life is full of risks. At my age and after going on 50 years of gardening now I can attest that your garden soil is one of the lesser ones. ;)

Dave

I love Gardenweb--such a great assortment of answers!

I found some papers about the uptake of PAH's into plants, and they suggested that a)it's low, and b) most uptake occurs via surface contact of leaves or roots to soil. Avoiding the parts that have been in direct contact with the soil (or by covering the soil with sand) seemed a good solution.

That said, I think I'll get the soil tested. I'll repost those results for the curious. I'm not sure of the history of this property, but the car repair in backyard scenario jives with some spotty tales of yore I've been told. Seattle as a whole is a young city, and this neighborhood is on the nicer end of urban housing.

Thanks!