Here is a mod that could be useful to you if you only work on small fields because it allows to increase the crop yield.
Details:
Wheat: yield x10
Barley: yield x8
Oats: yield x6
Cotton: yield x2
Rapeseed: yield x6
Sunflower: yield x4
Soybeans: yield x10
Corn: yield x6
Potato: yield x2
Beets: yield x2
oilseed radish: yield x2
poplar: yield x2
Sugar cane: yield x2
Grass: yield x6
Yields for fertilizers are also increased.
If you want to change the yield value, just change it in the mod's .xml!
