All our farms are gps mapped and fert is applies on an as needed basis. We had a very poor crop last year. I plugged in a calculation that used a P and K removal rate per bushel harvested. When I subtracted that from what we put down we had a lot of fert that is theoretically still out there. Some fields it looks like I dont even need any this year? That feels kinda funny. What say?
As for P it wont go anywhere. It dosen't leach or volatilize, so if ur calculations say you dont need any its probably pretty close.
This is a nice calculator that I ran across this year.
Do you have geo-referenced yield data? If so, you should be able to cross-reference that with your soil test data to find out what you'll be needing (or don't need, for that matter.)
Another consideration is where your soil test levels are at. An emerging philosophy in soil fertililty is looking at how much of a particular nutrient is needed per day, per bushel of yield, during the period of peak uptake. With nutrients such as potassium in corn, it's a staggering amount of fertility per day during that peak uptake period. This is where your soil test levels come into play. Soils that have higher soil test levels are far more capable of supplying the proper fertility to the crop than those that are "optimum" or "adequate". Make sure they're where they need to be before you make the decision to not apply fertilizer.
Soil fertility is one of those subjects in which there are a lot of varying philosophies; some of it depends on your point of view.
Im starting to see a need to overlay yield data and the the soil test maps. I dont have the yield data, but can put my GPS gear on the custom cutters yield monitor this fall. Then I can do replacement from what I pulled out plus build up were needed basis soil maps. Trying to use just soil maps and field level removal isnt working very well. I wish I had more time to get really good at this stuff.