After harvesting cotton, I notitced, that one of my fields upkeep value went from 50% to 600%, instead of 55%. I used the very same seeds and fertilizers on the field next to it, and that's working completely fine.
I don't know wether it really is a bug, or some planned feature to make me spend 30 humus on tilling a single field, but just in case, I list it here.
EDIT:
I put all 30 pieces of humus into that field, but I still can't plant it, probably because the necessary seed ammount is sky high too, more than 100.