By Edwin Ritchey, Lloyd Murdock and Josh McGrath, Extension Soil Specialists
Many growers have completed or almost completed corn harvest and are getting ready to start drilling wheat. One question that keeps coming up is the need for a fall nitrogen (N) application.
In most years there is adequate residual N following corn. However, most areas in Kentucky in 2015 had very good corn yields and a considerable amount of rainfall earlier in the season. Good corn yields coupled with high rainfall probably means there is very little residual N remaining in the soil profile.
Numerous studies at University of Kentucky have not shown a consistent yield advantage to fall N applications for wheat. However, fall N does stimulate growth and establishment of the stand. Too much fall N could reduce yields by causing excessive growth and more winterkill.
What does this mean for fall wheat growth and what should be done? Some states (e.g. Maryland) will not allow an N application prior to March 1 unless a soil nitrate tests indicate very low levels of soil nitrate (less than 10 ppm) in the soil profile.
The University of Kentucky does not make N recommendations based on soil nitrate for wheat, but typical baseline soil nitrate values are around 8-10 ppm nitrate collected to a depth of 12 inches. If you are at or below this range, 30 pounds N per acre will probably not cause too much fall growth that would lead to management issues later in the season. The later wheat is planted, the more beneficial an N application will be to stimulate fall growth.
Growers that decide to make a fall N application can easily achieve this if DAP (18-46-0) is being applied at or near wheat planting. A strict N source like urea or UAN can also be used with little potential for volatilization losses, but make sure to not exceed 40 pounds N per acre in the fall. For producers that are not applying DAP fertilizer in the fall, a good option is to wait to observe fall wheat growth and make N adjustments as needed. If yellowing or slower than normal growth is observed, then topdressing with 30 pounds N per acre is an option.
If you decide to not apply fall N, but are still curious as to whether it would have provided a benefit, you can conduct your own research trial. Simply put three or more strips with 20-40 pounds N per acre in random locations in your field this fall. Mark these “treated” locations by GPS and/or with flags. Follow the same spring N program for the entire field and then check your yield monitor data at harvest to see if there was a benefit. For more information or help conducting you own on-farm research contact your county extension office.