This means that a single farm tile can produce 8 x 5 = 40 plants per year. More if you bother to fertilize it, but most stacks of plants top out of 5.
As each dwarf only needs 8 units of food per year, 1 single tile of farm land can feed 5 dwarves.
This would be like feeding 5 adult humans from a piece of land the same size as a dining room table.
Exactly. And they'd only have to plant it once a year.
The only thing I can think of that's like that is zucchini (ever grown zucchini? 1 vine feeds 100 people for a month)
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You'd still need to tend the farm year around. Remember, that value is depending on getting 8 harvests of plump helmets in, which is doable if you have enough farmers.
But its still an absurd crop yield per unit of land. A 10x10 plump helmet farm will feed 500 dwarves! Forever! Also drinks too from booze.
Probably the best solution is to drastically increase growing time. Making dwarves eat more often would be bad as it would cut significantly into productivity, as they would spend all of their time in the dining room. Making them eat more at once could be doable though.
Perhaps add in rations? Make it a global option.
Meager = 1 food per meal = unhappy thought
Normal = 2 food per meal = neutral thought
Extra = 3 food per meal = happy thought
Double = 4 food per meal = extremely happy thought
Or it might be easier to just increase growing times, which will reduce the yield of each unit of land, requiring much more land to sustain a large population.