Foul water odor after wash

There is always a littlee water left in the sump of a washer after it is emptied. If the washer sits unused for as little as 2 or 3 days it can have that odor. One thing that I do is add about 1/4 cup of ammonia after the washer is full of water. DO NOT USE AMMONIA IF YOU USE BLEACH IN THE SAME WASH!!! IT WILL CAUSE CLORINE GAS TO BE RELEASED!! If bleach was used in the previous cycle it will be ok.