If you've ever seen Archenemy and Hero you'll know what I'm talking about, unless of course you are a farmer who is familiar with the subject.
Basically crop rotation involves :
"Growing the same crop in the same place for many years in a row disproportionately depletes the soil of certain nutrients. With rotation, a crop that leaches the soil of one kind of nutrient is followed during the next growing season by a dissimilar crop that returns that nutrient to the soil or draws a different ratio of nutrients. In addition, crop rotation mitigates the buildup of pathogens and pests that often occurs when one species is continuously cropped, and can also improve soil structure and fertility by increasing biomass from varied root structures."
So if we were to say, grow potatoes in a given farm, then next year grow wheat instead, followed by grass, then potatoes again, the next time potatoes grows there it will have a boosted growth rate. If you were to grow potatoes over and over again in the exact same area, it would take a hit to its growth rate and you wouldn't get as much.
I did see that there was one other post on the subject, but it was over a month ago and only received one reply. I liked the manure idea though. If we fertilized the fields on top of that we could make a farm that would only grow say, 1000 potatoes on a 10x20 farm in a single year repetitively, grow double the amount just by switching the farms it was growing on. A simple mechanic that wouldn't need much to do and would add a little further interaction.