When outhouses begin filling up the owner of the outhouse would add some lime to help the smell and break down some waste.
However when the outhouse got near completely full the outhouse owner would remove the waste by scooping it out which was a very unpleasant experience for the one doing it.
Then they would use the waste as fertilizer for fields or gardens.
Some outhouses might have used some containers underneath to catch the waste which was then removed and then either burned or spread out onto fields or gardens for fertilizer.
Just like a septic tank that gets full it then needs pumped out and then the waste from the septic tank is used as fertilizer.