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West-Central Illinois | We've been successfully using VRT NH3 for 6 years using the approach that you are referring to. We are capturing the N value from VRT application of DAP, 18-46-0. I follow the tried and true University of Illinois formula of 1.2 x proven yield, - all nitrogen credits. I allow 40 units of N if corn is following soybean. Don't forget to allow N credits for any starter fertilizers, herbicide carriers or manures.
I've been exploring using soil types for creating VRT N-P-K prescriptions and like what I see so far. If you have fields with large variability in crop productivity ratings this may make sense. I typically compare the average yield by soil type for a field with the grower's proven yield for that field and establish a management factor to correct for real life vs. book value. Quite often a management factor of 1.2 x published yield comes very close to the proven yield on a field-wide basis.
If you overlay a yield map with the crop productivity ratings for a field you may learn that you are producing more on the "poorer" soil types and not enough on the "better" soil types. I suspect that this is due to the field being historically fertilized and farmed for the field average and not for the soil mapping unit yield potential. I don't feel that I've not compared enough yield data with the soil productivity ratings to draw a firm conclusion on this theory.
VRT NH3 to take credit for the N in VRT phosphate fertilizers is definitely a sound decision. Tread cautiously with VRT N (P or K) based on soil productivity ratings, but I encourage you to experiment with it on a limited basis.
A final note on nitrogen recommendations for corn: There are many different formulas out there and many states have their own preferred method. Be sure to follow your State's recommended procedure for calculating N rates, especially if your farm is enrolled in any government cost-sharing programs that require a written Nutrient Management Plan. Do not "mix and match" nitrogen recommendation formulas from different states to try to establish your own; there's actually quite a bit of research behind all of these formulas.
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VRT NH3
