The game is very slow btw. and I was so happy to finally have 500 cash and invested it to expand my farm...only to realize: I still have 4 plots and restarted the game wiht that... Kind of disappointing.
Farm plots expand based on the experience you have when you purchase the new farm. Experience gain is boosted by the amount of knowledge you have when you gain it. There's a decent number of 'hidden' mechanics that I've been trying to find a good way to display to the user without cluttering up the UI.