Have you been incorporating cover crops for nutrient needs??? How are the fields doing in late winter...Jan/Feb? The one thing that I don't know if I'll ever be able to do without is a winter application of nitrogen. When soil temps drop below a certain point then the soil activity shuts down....and so does any nutrient processing. You might notice that your field may go N deficient late in the winter but then turn green again once warmer spring weather comes.....I believe that's the soil warming up and your nutrient cycling factory getting cranked back up. That organic N doesn't seem to be getting the job done once the soil temps fall below a certain point though.