You have all explained in How to play. And you can see all info about it if you click on farm. Groeth, mortality rate, bonuses...
Click on farm: "Decrease population losses by 5% per level from the previous one given the bases 5400." What population losse r mentioned?
Look in HowTo: "Farm construction decreases the population losses due to shortage of food by 5% per level from the previous one, given the basis 7200 people." (and some words about increasing happiness after 5lvl)
But it is still NOT clear: What is the formula of those "losses".