My neighbours used to burn their rubbish in the back here, but further into the back, close to the centre, where I have the lawn now. I didn't see anything when I cleared this patch for veggies.. Except bones lol Goodness, we found enough of those around the back to start a small cemetery. My landlord said he knows nothing about it or where it came from.Maybe it's over an old burn pile? Tomatoes like ash.