at 0 fertilizer used 1 wood chop can do about -10. they drop as low as -1 point. typically you would plant cereal first as it grows fastest and unless you have a town with seeds to start.. is typically cheaper to buy from npc stall or players.
after that is harvested you only need about 5 wood chops to reset the 5 points you gain replanting.. assuming you do not use choppings first thing, as that would lower the better fertilizers you would want to use on the 2nd planting of the field. also having three fields i think improves the amount they lower by. as iv had -1% actually take off 2% some how

unless it was 1.5% and i had 52% so it just rounded up to -2%...
but ya brand new field you do
1) 5 humus
2) seeds (typically 50 cereal)
3) 10 or so wood choppings to drop to 50% upkeep
4) harvest.
then replant and use 3 humus and 5 choppings or so (if you use choppings last after you fertilize what you want to increase "plenty/influence/speed" they are less effective but its better to do it last)