The negative effect of soil nitrate (NO3-N) on soybean (Glycine max L.) nodulation is well documented (Gibson and Harper, 1985; Lim et al., 2014; Xia et al., 2017; McCoy et al., 2018). When soybean nodulation is sub-optimal, the plant’s ability to fix nitrogen (N) is decreased, especially fields without a history of soybean production or Bradyrhizobium japonicum inoculation (Heard et al., 2013; Franzen et al., 2019). Although soybeans do require some N early in the season, prior to nodule development, only around 50 lb N ac-1 is needed with 150-200 lb N ac-1 produced by the crop to satisfy its demands the remainder of the growing season (Heard et al., 2013; Manitoba Agriculture, 2024). Cases where decreases in soybean yield will occur is when soil NO3-N is high enough to decrease nodule formation but too low to satisfy all of the crop demands for N throughout the growing season. In addition to potential yield impacts, increased levels of soil NO3-N is an aggravating factor for iron deficiency chlorosis (IDC) in fields with high pH, which may further impact yield potential (Bloom et al., 2011; Franzen et al., 2019).
According to recommendations from Manitoba Agriculture, (2024) growing soybeans on fields with NO3-N levels exceeding 60-70 lb N ac-1 should be avoided, with this threshold level potentially decreasing as rhizobium levels increase with continued soybean cultivation. Since soybeans do not have high demands for N throughout the growing season, fields with high residual NO3-N levels may be best suited for the cultivation of higher N-demanding crops. Not only would avoiding planting soybeans on high NO3-N field prevent potential yield loss, but also it would allow residual soil N to be utilized more efficiently by N-demanding crops increasing overall farm profitability. The purpose of this research is to determine the impact of residual NO3-N on soybean yield specifically in North Dakota cropping systems and determine a threshold range of soil NO3-N under which soybean yield is not expected to be impacted.
Overview and Site Descriptions
The proposed research will take place over two growing seasons across the state of North Dakota. During the 2025 growing season, corn will be grown and fertilized with N rates ranging from 0-300 lb ac-1 at trial locations in Cass, Barnes, Morton, Adams, Stark, and Ward Counties, with the goal of determining how different rates and sources of nitrogen fertilizer impact corn yield. The rates of N fertilizer used in the corn study will likely result in residual NO3-N levels in excess of 200 lb N ac-1. Following corn cultivation, soybeans will be planted during the 2026 crop year and the impact of the varying NO3-N levels on yield, IDC prevalence, and nodulation will be quantified.
Experimental Design
During the 2025 growing season, corn will be fertilized with both granular urea (46-0-0) and SUPER-U (urea-based fertilizer treated with a nitrification and urease inhibitor) (Koch Agronomic Services LLC, 2019). Rates for both of the fertilizer products will range from 0 to 300 lb N ac-1 in 50 lb increments. The experiment will be arranged in a randomized complete block design with a split-plot arraignment; N fertilizer source will be the whole-plot treatment and rate as the sub-plot treatment. Blocks will be replicated four times, producing 56 experimental units at each site measuring 10 ft wide by 30 ft long. The following growing season, soybeans will be grown on the same sites following the corn in the same experimental arrangement.
Data Collection and Analysis
Following corn harvest in the fall of 2025, soil samples will be collected to a depth of 24 inches and analyzed for soil NO3-N to determine which sites have the highest and greatest range of residual soil NO3-N and thus will be used for the second (soybean) year of the study. The following spring, sampling and analysis for soil NO3-N will be repeated to quantify potential N losses over the winter and spring. Mid-season, soybean plants and roots will be collected and average nodule counts will be determined. Additionally, at sites where IDC is expected, ratings will be carried out in accordance with the rating system used by the NDSU soybean variety testing program (Endres et al., 2023). At maturity, soybeans will be harvested and grain analyzed to determine yield.