By Dennis Franzen, Extension Soil Specialist
The N rate calculators for wheat (2010), corn (2014) and sunflower (2016) are not yield-based formulas. I continually am asked how much N is needed for a bushel of ‘X’. The answer is: ‘It’s not important’.
A colleague, Bill Raun, at Oklahoma State first came out with this radical idea when researching the use of the GreenSeeker in various crops and most people thought he was a little touched.
However, the Return to N models used by most Extension Soil Specialists in the North Central Region are yield response and economic based, but not yield goal based. The people responsible for the background data within the North Central models don’t talk about it specifically, but the model implies that specific yield and maximum return to N rates are not related.
In my wheat and corn work, I thought I saw the same thing, but it wasn’t until Al Cattanach at last winter’s Soil and Soil Water Workshop noted that our originally graphed sunflower yield response relationships were weak that I investigated it more closely.
I knew that the relationship we found was solid, but graphing all the sites with raw data on one graph looked like a skewed shotgun blast. Each individual site had a strong quadratic curve of N rate and yield. I wondered what would happen if I normalized each site before we put them together.
The result was a very high relationship between relative yield and N rate. This means that the N rate that produced the highest economically beneficial rate in a low-yielding environment was the N rate that produced the highest economically beneficial rate in a high-yielding environment.
We found that N rate within a site is important, but N rate is not related to yield over sites. You can’t force a crop to yield twice the amount that environment will allow by increasing N rates.
I think the reason for the lack of relationship over sites is that in a low-yielding environment, N use is limited by too dry conditions, too wet conditions, limited root exploration, or limited movement of N to the roots, and limited N mineralization.
In a high-yielding environment, N mineralization rate is high, crop access to N is maximized and N movement to the roots is maximized. The result is that a similar rate of N is required for maximum returns in all environments.
So don’t ask how much N is required to apply per bushel, because there is no relationship.