A day of devastation

I don't think weedkiller would work that fast. How much potassium bicarbonate did you use? I usually stick with a 0.5% solution, so 5 grams per litre of water. Did you also use soap?

1 tablespoon to one gallon, ½ teaspoon soap (a bit of hand wash which might be the culprit) and a  ½ teaspoon of veg oil.  Whether its the soap or weedkiller residue I'm still stuffed.

So if 1 tbsp is about 15 g then that's about 3 g per litre, so that doesn't seem like the issue. That amount of soap doesn't seem like it should cause so much damage either, unless it contained some strong essential oils. Vegetable oil wouldn't do it either.