Identification of Nitrogen-Deficient Soybeans Facilitates Yield Rescue

Production of a healthy crop of protein- and oil-rich soybean seeds requires a significant amount of nitrogen. Under ideal conditions, most of this nitrogen comes from the root nodules, where a symbiotic relationship with Bradyrhizobium japonicum fixes nitrogen from the atmosphere. Thus, growers generally think of soybeans as not needing nitrogen fertilization. However, a lack of B. japonicum or other unfavorable field conditions can produce inefficient nodulation, which leads to reduced vigor and yield under conditions of low soil nitrogen availability. Thus, additional resources for identifying nitrogen deficiency in soybean and strategies for the rescue application of nitrogen are needed. To generate a gradient of nitrogen deficiency, we applied different amounts of nitrogen (in the form of urea) to nodulation-deficient soybeans grown in sandy fields in South Carolina. Comparison of the ground truth data and aerial imagery suggest that leaf nitrogen deficiency can be effectively identified in the field based on plant height and color. Side dressing nitrogen fertilizer in the form of urea to nitrogen-deficient plots at the R1 stage was shown to rescue plant growth and increase crop yield. We conclude that identification and fertilization of patches of soybeans with low nitrogen content is a feasible strategy to increase soybean productivity. Although additional studies are needed to expand these results to other soil conditions, we recommend that nitrogen-deficient soybeans be treated with 40–50 pounds per acre (45–56 kg per hectare) of nitrogen at or before flowering to restore yield potential.