600% upkeep

Hi, I have just harvested my first crop of cabbages. I followed the wiki in preparing for a decent cotton crop on 3rd rotation. The stats on the wiki are outdated and i have been making my own table for what effect the fertilizers do. I used the same fertilizers on 9 tilled fields. all fields except 2 have ended with 95% upkeep. The other 2 have 600% upkeep and I cant figure out what I have done wrong. 4 x 6 humus to prep a field is not good for me and I need to avoid it in the future. Any help appreciated.