I'd dig it into your soil, and then plant. Just don't plant edibles there since you don't know what was burned.

That brings up a question I've had for a while now. What might be in the ground that can be taken up by edible foliage or fruit? I've always assumed there's nothing that survives a burn except metals and their oxides and salts. I try very hard to avoid man made surprises in the soil. What am I missing?

I had something similar and just planted grass.

I think the advice here is good, don't grow food in it just for safety. If the amount is so much that you have to remove it to get down to soil, you could include a few scoops every week in your trash and send it to the landfill. That's what I'd do, but mainly because I'm a neatnik.

I assume they were burning household trash and not just yard waste. To answer grubby's question about what may be in that kind of ash: uncontrolled burning (i.e. not in a specially designed incinerator) will produce a number of organic compounds from incomplete combustion. Polycyclic aromatic hydrocarbons (PAH) are one, they are found in soot and are carcinogens in high doses. Traces of other stuff can be found in ash such as polychlorinated dioxins and dibenzofurans. As far as plant uptake, it's very complicated due to the variety of chemicals, soil conditions and plants. So there are a million combinations to research and it's not getting done very fast. Hence the advice just to play it safe and not grow food in it. More of a precaution than a known risk. We do know more about common contaminants like lead, which is not taken up very well by plants. The advice there is to avoid root crops and wash produce well to remove traces of soil clinging to the outside.