Improving Nitrogen Management in Manure Amended Soils with Real-Time Optical Sensing.

            The in-season application rate of inorganic nitrogen (N) fertilizer on manure-amended fields is difficult to determine. Goals of this study were to determine N response of winter wheat on manure-amended soil, evaluate N availability calculations recommended by Kansas State University (KSU), and examine application of optical sensors for making in-season N recommendations in manure-amended fields. Split-plot field experiments were conducted at three sites (Blaine, Manhattan, and Hays) during the 2008-2009 winter wheat growing season. Whole plot treatments were pre-plant N source (manure or fertilizer) and sub-plot treatments were top-dress N rates with 28-0-0 fertilizer (0 to 90 kg/ha N) applied in season and a reference treatment of 136 kg/ha N were applied at planting. A GreenSeeker RT 200 (Ntech Industries, Inc. Ukiah, CA) was used to measure normalized difference vegetation index (NDVI) in winter wheat on whole-plot treatments. Winter wheat yields at the Blaine and Hays sites responded to top-dress N application. At the Manhattan site yields were greater in manure over commercial fertilizer treatments; however, this was not a result of N availability. At all three sites there was no interaction between pre-plant N source and top-dress N rate. This indicates that both manure and fertilizer treatments responded similarly to top-dress N application at the respective sites. Results of the evaluation of in-season N recommendation tools showed that the KSU recommendation performed well at the Blaine site and the Greenseeker performed well at the Manhattan site. Neither recommendation tool performed well at the Hays site. This indicates that both the KSU method and GreenSeeker method have the potential to produce appropriate recommendations on manure amended fields, however further research is need to clearly evaluate these tools.