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Rolla, ND | Well Roger,
I think in general yield maps are a great way to manage the nonmobile nutrients like P and K.
I would say that the best time to use the corn yield map in my opinion is the next time you grow corn on the field, but I'm biased in that generally our soil test levels here are in the low category for P and we don't use much K because of high soil test levels. In addition, most of our P is applied at seeding, not broadcast. So I would use it as a basic application map for the next field of corn.
The main reason for using it the next time with corn would be to help have enough P in a band where the better corn production is.
I suspect you have a higher level so that your primary concern is maintaining phosphorus levels, so putting it back soon after you take it off makes a lot of sense.
If you are at relatively high levels, I don't think you really need to be concerned about the wheat and soys and coming up with a number that you should apply. If you are going to manage using crop removal because levels are generally pretty good, then just move to using crop removal.
I'm not sure how your P an K recs are made, I'm assuming some sort of yield goal with higher yield meaning higher application, but I think mostly you are creating a headache by trying to combine two things in your head. The numbers you would come up with for the yield of the wheat and the yield of the beans are unlikely to be accurate when moving down to a grid size unit. Instead of trying to anticipate what the yield is going to be, simply move to using the yield that was and maintaining levels.
Ideally, if I was doing it here, the P would be applied through the drill in a band for the wheat and the soys would be allowed to scavenge. But again that's due to our lower P levels. Wheat really responds to a band, soys don't.
Crediting the application for the amount of P that was applied and not removed is a round about way of setting up a yield goal map for the wheat and soys. Effectively you would be saying that the lower yielding corn areas will get less P and K for the wheat and soys. The effect is just like if you followed the yield map for the corn and set yield goals for the wheat and soys based on that and then varied your application. That's why I asked the one question, you actually get to somewhat the same place as far as how much fertilizer you would apply but the mental path is different. Whether you credit for P not recovered or you use low corn yield is probably low wheat and soy yield, the effect is similar.
Now one adjustment I would make would be made on soil test levels. If you have areas that test low (category) for P or K, I would probably apply something like 125% of removal, medium category, apply 115%, high 105%, and very high 100% or depending on the critical level for P solubility for your soil maybe less than crop removal. I haven't thought about those numbers a lot but the idea is to build a zone if it is low, maintain it if it is high, reduce it if it is excessive.
At that point, I don't see why you would use soil tests to generate recs unless some guy from the government who's there to help you requires you to do so. Soil tests would basically be a monitoring tool. With directed sampling when something seems out of ordinary. Or just a check every 3 to 5 years to see how things are generally doing.
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Something for Everyone to Shoot Holes IN !! Moved From Crop Talk.
