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The Complexity of Nitrogen (08/16/18)

My project with Abbey Wick includes a cover crop site NE of Gardner.

The Complexity of Nitrogen

My project with Abbey Wick includes a cover crop site NE of Gardner. Within it is corn after cover crop or not, soybean after cover crop or not, and spring wheat with cover crop or not that we harvested just last Thursday. The spring wheat followed soybean, so one would expect and I think we received a nitrogen credit. Also, the residual nitrate-N this spring was just under 60 lb/acre. Granted, the wheat variety was a lower protein spring wheat, but even so, the 200 lb/acre N rate treatment made 13.8% protein. The yields were in the mid-70 bu/acre range. It took over 300 lb N/acre for the variety to make 13.8% protein! What?(!)

The ND spring wheat N calculator rate for a 2-yr-into no-till field in eastern ND with high yield potential and residual N of about 50 lb/acre after soybean is about 155 lb N/acre. In our study, the yield of 155 lb N/acre would again be over 70 bu/acre, and the protein for the 160 lb N/acre treatment was about 13.5%.

I received a call a couple days ago from a farmer in eastern ND and my calculator scenario above was almost identical to his results. He asked me what he should have done to get greater protein. I asked him if he was really willing to apply another 100 lb N/acre (cost about $40/acre more roughly) and he told me of course not. If it had rained (which it didn’t- pretty dry in NE ND all season), the wheat would have been on the ground. Exactly!

When it’s dry as its been this season in most of the state (but with decent early season rain, thankfully), the efficiency of any N applied and the release of N from the soil is extremely low. If the season had supported a higher yield, the efficiency of N available would have been much higher and yield would have been higher, and maybe even more protein if there had been July rain. This year it took far more N per bushel (and per protein gain) than in a ‘normal’ year, whatever that is. That’s why N rate and yield are not linked.


Dave Franzen

Extension Soil Specialist


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