I would maybe add something like : you need to have slaves to work in the field when you have more than 10 crops, otherwise it can virtually be abused if someone just plants thousands of crops.
See this is where you could add some interesting RP aspects, as would it be moral to work slaves in the first place, would others think less of you or would you be subject to some kind of attack from wasteland police types.
If there was a way to distinguish the player as good, bad or neutral, while he was building the farm (whether he uses slaves and underpays people, or whether he invests happily into his community and hires new workers), then the "end result" of the farm could be effected by this choice.
Say if the player has basically become a heartless slaver to get his farm up and running, then there is a chance his slaves will die instead of yielding result, or that they will revolt, or that some "good guy" NPC faction will attack the farm to try and free the slaves. Conversely if the player has been good to all of his community and made them all happy (and moderately wealthy too), then the farm will be a good target for bandit npc faction attacks, and the player himself might not be as rich as the slaver type.