For the first planting of a field use 10 wood choppings to get the upkeep down to 50. Following that I usually dump as much coal as I can spare to speed up the process.
For subsequent plantings I fertilize with three coal, then one wood chopping, then five more coal. This gives 204% speed and maintains 50% upkeep.
Following this I spam a fertilizer depending on what I'm doing. If I am farming for product I dump 25 lime to hit around 300% plenty. If I'm after influence I dump humus.
Just remember: the more fields the better

Don't worry if you end up with fields that are fallow from time to time. It's better to under-utilize your fields than to under-utilize your playing time.