Can Sheet Mulch work on previously chemically treated land?

It looks like a previous owner has added chemical fertilisers and possibly bio-cides to a patch of land a couple of years ago. The soil is weak and the area filled with pioneer plants. I am planning on using a comprehensive sheet mulch mixture inspired by Toby Hemenway's "Ultimate, Bombproof Sheet Mulch" to rest on top of the existing soil and plant vegetables there. I am pretty confident that this method will restore the quality of the soil, but I have concerns as to whether it will negate the chemicals in the soil and compromise the organic qualities of the vegetables.

Should I be concerned about any residual chemicals still in the soil underneath 30 cm (1 foot) deep sheet mulch?
Would it be safe to grow vegetables on this land? If not, then what can be done to remove the blight?

Jason Nicoll wrote:

Should I be concerned about any residual chemicals still in the soil underneath 30 cm (1 foot) deep sheet mulch?
Would it be safe to grow vegetables on this land? If not, then what can be done to remove the blight?

Even as we sit here, wondering what to do, soil fungi are degrading the chemicals that were applied. As long as the sheet mulch is well aerated and the fungi can breathe, they can metabolize all that was put down there. I wouldn't start out with root vegetables right away, fruits on long vining plants are the least likely to pick up a residual molecule from a foot deep. Start out with tomatoes or cucumbers, then move to the leafy green vegetables, and after you are satisfied that they are fine, then you can try some carrots and turnips.

And in the meantime, if you get flushes of mushrooms popping out of your mulch after a heavy rain, that's a good indicator that chemicals have been broken down.

I saw this question a bit earlier and knew the answer, having read a dozen versions of it before. Then I thought, I wonder how long until John sees that.

John, I think at some point it might make sense to cut and paste some of the answers to regularly asked questions onto a thread in the section for beginners. Or, it could be a Q and A page that is posted to a few different locations.

Thanks for the responses, especially about the vining fruits. I'd second the suggestion about a FAQ, perhaps in the form of a sticky at the top of each forum?

Hopefully the sheet mulch will provide the aerated, damp environment, rich in decaying bio-matter suitable for fungi. Is there any advice on how to increase or accelerate their growth beyond this?

Jason Nicoll wrote: Is there any advice on how to increase or accelerate their growth beyond this?

One thing to look for and one thing to smell for: (1) if you can dig into the mulch and see lots of white stringy hyphae, or white fuzz all over pieces, that's good and (2) it should smell earthy, like you stuck your nose into a package of mushrooms. If it smells at all sulfurous, like a sewer, it's not getting enough air and you need to dig it up and fluff up the mulch a bit.

Oh, and any mushrooms you find while you are walking around, pick them and toss them on your mulch.

I'll take the suggestion for a FAQ under advisement.