johnny skeptical - 1/15/2021 19:34

Probably location dependent.......but here is what we experienced.

for as long as I can remember our local coop and most of the other fertilizer retailers carried DAP, so early in my career it was always said that the N in dap was the most stable, cheapest form of “N” ,okay...just about all of it is fall applied .

And then about 15-20 years ago the common mantra was that the “N” in dap wasn’t actually that stable, so supplemental “N” rates were raised to compensate for this.

 Then starting about 2011 “N” supplemental rates were pared back because of the high fertilizer costs that the runup in prices that the 2011 to 2016 era caused. So the fine line between enough and deficit “N” was explored with the idea being that the “N” from dap was indeed somewhat stable and available.
  
So with that being posted, about 4-5 years ago most of the local fert. retailers and coops started carrying MAP. At the typical phosphate application rates for our area (apps once every 2 years) the difference in “N” from MAP to DAP is about 22-30 units.

With supplemental “N” rates pared to the bone, we started to see some nitrogen deficiency issues in corn when we didn’t account for the difference between map and dap.

so in closing my opinion is that if the “N” in MAP or DAP wasn’t there in the spring then we shouldn’t be seeing any difference between MAP or DAP fall applications.