Crops go through stages before they can be harvested. If left too long at the harvest stage, they will wither and die (it is still possible to harvest withered crops but with a big penalty in yield). Applying any type of fertilizer available in the game will double your crop's yield.
Note that in Farming Simulator 2013, crop yield isn't impacted by when you fertilize a field. You will receive a double yield even if fertilizing is done at the harvest stage of your crop.