Do I need to fertilize a ripe field ? (Farming Simulator 17)

Strange and stupid question, isn't it ?
But this is only at first glance. Buying a new field to the already Mature crop, note whether it is fertilized ! If not - before the harvest, make fertilizers ! You will get additional yield from 80 to 100 % !!!
Weird and unlikely, but it's a fact !
( It should be in FS 15 the same test )