Why not have farms cost lumber to first build (for the plow?) and they will be like they are now, give off a bit of food and disappear. But also give them the ability to upgrade them, costing food(seeds) and more lumber(more plows?) have them "grow" faster. the farm will no longer disappear once the crops run out but will "grow" more crops over time, and maybe being able to upgrade them more than once but each costing quite a bit more than the last. Or you can go the AoE way and have replenish farms on the mill(which i would always forget to top up)
