Can anyone think of a good reason NOT to use diluted household
ammonia, to raise pH of compost (presently pH 3 to 4) up to a
desirable level?

I would expect usage would result in an immediate pH change,
compared to waiting on slower lime products. And ammonia
should provide nitrogen, as well.

If i remember correctly, farmers once used anhydrous ammonia
applied directly to the soil.


As a quick and dirty soltion to low pH it should work ok. It is quickly
neutralized so it has little lasting effect if the souce of acidity continies
to be present. It is a minor source of nitrogen but has to undergo chemical
reaction before it is usable by plants. You did say dilute, so you may or may
not kill the organisms that break down your compost.