Soybean yield and protein concentration are limited by nitrogen (N) supply, especially in high-yield environments. However, yield response to N fertilizer is inconsistent and usually not cost-effective. Here we plan to develop a method that allows in-season diagnosis of plant nitrogen status to better inform soybean producers on when and how much N fertilization can be applied to soybean to achieve a consistent and profitable yield response. To develop and validate this method, we propose a 3-year project to conduct N fertilization trials in producer fields in Nebraska. Each experiment will include three treatments: (i) a ‘zero- N’ that receives no N fertilizer; (i) a ‘full N’ in which the crop will grow under ample N supply, and (iii) an ‘optimal N’ in which we will apply N fertilizer during flowering with the goal of delivering an economically profitable yield response to N. We plan to conduct replicated trials at various NE farm sites. For N determination, soil and plant samples will be collected during the growing season, with seed yield and other crop parameters measured at crop maturity. Ultimately, this project expects to generate a decision tree tool that can guide producers on their decisions about N fertilizer application in soybean.