By Peter C. Scharf, University of Missouri, Division of Plant Sciences
Yes, of course wheat needs N in fall. Timely planting and decent fall tillering are very important to wheat yield potential. And adequate N supports that tillering.
But soil has N, regardless of whether you put any there. Is it enough, or not enough, to maximize wheat yield potential?
In 8 experiments spread across Missouri, only 1 gave higher yield with 1/3 of the N in the fall and 2/3 in the spring, relative to applying all of the N in spring and none in the fall. This indicates that the amount of N already in the soil in fall is usually high enough to support all the fall growth that's needed. Averaged over all 8 experiments (most likely to reflect what would happen on your farm) and two N rates (80 and 120), there was a 1-bushel difference between having fall N or not.
Another way to look at these experiments is to compare yields with 80 lb N applied in the spring, and either 0 or 40 applied in the fall. On average, wheat yield was 2.5 bushels higher with 40 applied in the fall, which is not nearly enough to pay for the extra N and the trip.
Given how hard it is to make money farming these days, cutting out fall N and potentially a trip across the field may be more interesting this year than in most years.
However, you automatically get N when you apply P. Do you need P? We have not had much research to answer that question in Missouri. The two experiments I know of with and without fall P did not show any yield benefit. Again, wheat needs P, but in these cases the soil was supplying enough for full yield.
The classic answer is that wheat needs higher soil P levels than corn or soybean. This is because of the importance of cool-weather growth for wheat-soil P chemistry slows down at these lower temperatures. Soil test P targets have generally been set to be adequate for wheat (and alfalfa) and above what is needed for corn and soybean.
If your soil test calls for P, I'd go ahead and apply it before planting wheat. But I don't see any need to add extra N to the P. If you apply enough P to replace what is taken off in an 80 bushel wheat crop, you're getting 10 (MAP) to 20 (DAP) lb N/acre with your P fertilizer. Our research suggests that should be plenty to maximize wheat yield potential.
Similar to what I'm reporting here, Missouri research generally shows that in spring, later N application (just before jointing) is better than early N (at greenup).