The soil is an extremely complicated community of fungi and bacteria and tiny animals, and it only just occurred to me tonight that the tap water has chloramide (and probably some fluorine compound as well). Quick googling says yes, this can sometimes cause problems (and sometimes it doesn't make any difference) and getting the chloramide out isn't altogether trivial. People aren't terribly clear about the detailed causes of the problems when they do happen, but nobody understands soil ecology very well so that's not surprising.
I tried rotting out a tree stump using the "drill a bunch of 1" holes, with side vent holes and pour in potassium nitrate and water" method. (Other sources suggested setting the stump on fire after that mix had a chance to percolate, but I'm not interested in melting the underground cables that lace our little berm.) Yes, we finally got some mushrooms growing, but a year late, and the stump still has most of its structural integrity. Maybe the tap water I poured in killed the fungi, and the potassium nitrate didn't have much to fertilize?
Maybe I need to reroute the downspouts into rain barrels. Or call UW Extension. That sounds easier.