While this includes knowledge of farming, it's also a more macro level skill of lords organizing the agriculture in the lands under their control. One warlord from Thrax famously said only peasants need to know anything about it. The resulting famine was blamed on foreigners.
As a scale:
0: Can throw things in dirt and hope for the best. Kudzu performs okay.
1: Likely can maintain a small garden, with a bit of luck.
2: Could adequately farm and support themselves through hard times.
3: Is the person to call in when there are hard times to not starve.
4: Probably could prepare a domain to ride out a famine.
5: Probably has prevented a famine.
6: "...And thanks to her, that's why there's still apple trees in Arvum."