Farms become area buildings that are not on a 1:1 equivalency. Dwarf chooses a random square and works. Farming subjobs (not in the sense of skills necessarily, just assigned tasks) Add farming "furniture" buildings so that each farm has a place where workers can store supplies and/or produce to let players keep their skilled farmers farming and not hauling.
Farm has 4 possible statuses: Planted, Fallow, Ready, Not Ready. Every so often each farm does a check and increments variables for weeds, pests, blights, moisture, fertility and yield.
Weeds: Builds slowly, and slowly reduces yield. Yield increased as it is fixed.
Pests: Random, but once it is present builds quickly and reduces yield quickly.
Blight: Builds slowly and reduces yield. Yield is also reduced by fixing it, but at least it's gone.
Moisture: Increases yield slowly over time if farm is kept irrigated/watered. If moisture goes below the safe zone for the crop, it dies quickly.
Fertility: Once a crop is done growing the resulting amount of food is factored by the fertility value somehow. Fertility goes down as crops are grown, but increases over time when the farm's status is "Fallow" and by fixed amounts when fertilized or tilled. (Can only till until the farm is "ready" again)
If Toady wanted to get really in depth, he could create an arbitrary number of equally arbitrary nutrient variables and have some plants add to some and take away from others so players could set farming schedules for their dwarves.
This would open the door for some more sophistication in the Growing skill/knowledge area at least. Irrigation would need engineers or water hauling, plant caretaking would keep farmers engaged during the seasons they grew crops in and might even give a good reason to let farmers have quarters near their farms, namely to give them more chances to check on the fields to make certain nothing bad was happening. Of course, berry bushes/fruit trees could work with this too but their fields would of course not be regularly tilled.