Okay using the figures in the article I'd work a arvest DC using the following
Expected Yield / Labour = Harvest DC where Expected Yeild is Crop/season x Area
So Wheat has a Crop of 8 per Acre
Beef a Crop of 2 per Acre
Grass Crop of 50
With your 12 acres planted in Wheat we get an Expected Yeild of 96/Season
to be Harvested by our 7 agrarian heroes
96/7 = 14 (13.7)
So Farmer Joe and the boys rolls Prof:Farmer check with DC 14 and successfully brings in a good harvest to be divided amongst themselves.
Add whatever modifiers for weather, rivers, irrigation systems, crop rotation techniques, disease, insects, or raids you like
However I'd probably not go do to the level of specific crops though and probably go for something like an Average 'Fertility' modified for Terrain and Climate
(eg say Temperate Hills = Fertility 3/acre for Food (regardless of if its Wheat, Fish or Purple worm jerky) but Desert Hills has Fertility of 0.25/acre (ie it takes 4 acres to produce 1 food unit)
So 20 acres in Temperate hills -> expected yield 60 food
& 20 acres of Desert Hills -> expected yeild 5 food