You could also have a Farming Events Table, similar to how that evens can happen in NUTS.
*Whenever the sum of you REP roll = 7, an event has occurred on one of two tables: The Growing Table and the Harvest Table.
From March to September = Growing Table, October =Harvest Table
The Growing Table could have things happen like good weather, bad weather, blight, insects, critters, etc. That increase or decrease the Farm Rating, or add or subtract from your Harvest Table toll.
The Harvest Table could have things happen like Good harvest, Bad Harvest, Marauders (generate an Encounter) try and steal your crops, critters, etc. that add or subtract Food generated or result in an Encounter.
Example, I have a Farm with a Rating of 2, and I have at least one person working the field so I roll 3d6 total. I roll 3d6 to see how well my Growing Season goes. My roll is 2, 2, 3 -- I pass three, so I have a good season and increase my Farm Rating to 3. But since the sum of my roll = 7 (2+2+3), I have an even. I roll 2d6 on the Growing Table and get Good Weather, so I can add +1REP to my Harvest roll.
Then I roll for Harvest. I get to roll 2d6 at +1Rep, and roll a 3, 4. I pass 2d6 so I get 1 Food per Farm rating, which is a "3" now, so I get 3 food. However, this also calls for a roll on the Harvest Event table. I end up rolling a Defense Encounter as some marauders try and Raid my crops! Guess they want to farm the Farmers!
I'd also consider saying that you need to have more people assigned to the farm than just 1 person. Farming is pretty labor intensive, particularly around planting time and harvest. Love to see what you come up with, I like the idea of building a community as the basis for a campaign.