If the system is in place that determines the viability of growing plants per tile (tile fertility) is in place, perhaps a new use could be added for spoiled food to slightly increase each tile for a single year, or in whatever time increments it is calculated in.
This could be done in one of two ways, the less time intensive way would be to allow characters to drop spoiled foods onto a tile before it is prepared, and then the spoiled food item would be consumed during the burning process, adding it's value to the tiles fertility rating.
The other, more realistic way that would require more actual character time (but be simpler than managing multiple drops) would be the addition of a "Bury fertilizer" command in the agriculture skill menu, letting the character bury spoiled food (or even food that isn't spoiled, if they so choose) while standing on a tile with a shovel in their inventory. Roughly 3-5 minutes per lb of material buried would probably be accurate. (much like people burying carp in their vegetable gardens to improve the yield)
I suppose, in an effort to minimize the number of stray items cluttering a zone, another method could be used, in the form of building a compost pile. Unwanted organic items could be added into a pile once built (10 boards, 20 stones perhaps?)
Food would be irretrievable once placed into a pile, and be converted to a generic state (organic matter still composting) tracked only by weight. For the composting process, a simple random variable could be used to determine the rate, either once per day, a random amount of organic material could convert to half it's weight in compost , 2-10 pounds of organic matter ends up turning into 1-5 pounds of compost(0 could also be a result, no progress in a single day). A recommended addition to this would be the ability to craft buckets, weight of 2 lbs, able to hold 10 or 12 lbs of material, cost a block of wood and a slender trunk to craft.
The resulting compost (carried only by buckets) could be buried into the ground much like the food items earlier, but the confirmation screen for what to bury would simply ask how many lbs of compost to bury at the tile.
I hope this makes sense, of course, this entire suggestion will be rendered as useless if the soil fertility system doesn't exist yet. However, from the wording of the Unreal World in-game help, I believe that such a system either does exist or is planned for the future.
This sounds interesting, but I say we add a chance of attracting predators if it's spoiled meat that's being used as fertilizer rather than vegetable matter. I would also be interested in being able to use draught animals for agriculture to speed up the process/increase yields/allow for cultivation of larger areas, with the balancing factor being the need to feed the animals and the fact that your cow won't make as much milk if it's being forced to work or possibly won't give you as much fat/meat when you slaughter it. Just as there are wooden and metal shovels, we could have homemade wooden plows or expensive metal plows that must be traded for.
Also, maybe add an option to periodically tend your fields (mostly weeding, I guess) over the summer to increase yields.