North Dakota has atypical P needs, due to generally low P levels across the state compared to other regions (Franzen, 2019). In addition to low P levels, soybean cultivation has also spread across the state into areas that previously saw few soybean acres. In order to support more careful P management in soybean, a coordinated study assessing P-needs in soybean across the state is needed. Much attention has been paid to fertility concerns regarding Nitrogen (N), but P-management may help improve yields and ROI for soybean production. However, more research is needed to improve P-management, which may be more complex than previously thought (Sims, 2023). North Dakota’s unique P-needs underline the importance of conducting P-rate trials across the region. Finally, P-management will also be under increasing scrutiny in coming years, due to global supply concerns as well as environmental consequences of overapplication (Zou et al., 2022). This information will aid soybean farmers by providing them with more accurate recommendations, reducing the likelihood of both potential yield losses or unnecessary spending on fertilizer. One year of this study was conducted in 2023, and repeating it for a second year will aid in producing more broadly relevant P-recommendations.
This coordinated project involves participation from NDSU’s main campus and three Research and Extension Centers (RECs): North Central, Dickinson, and Carrington. Two P-rate trials were deployed at each of these four locations, one on very-low P soils and one on low/moderate P soils (Figure 1).
The trials were located primarily on producer-managed acres, to maximize the relevance of the data to ND soybean farmers. Treatments include 5 P-rates: 0, 23, 46, 69, and 92 lbs/ac P2O5, and were replicated 4 times at each site in a randomized complete block design (RCBD). The preferred P-source was triple super phosphate (0-46-0) in order to minimize interaction of other nutrients, and will be applied in spring either preplant or soon after planting. Measurements will include soybean yield at the end of the season, as well as soil test phosphorus.
This study was conducted in 2023, and we would like to collect a second year of data, for a total of 16 site-years. We expect to see an increase in soybean yield with P-additions on very-low P soils, and an increase in soil test P when P is supplied in greater need than plant uptake. Analysis of all 16 site years will determine recommended rates for each region.
Results from these trials may also be leveraged to assess soil test phosphorus methods in the future, as well as assess the interaction with arbuscular mycorrhizal fungi at select sites. The proposed work does not include any funding request for these future areas of interest, but we wanted to underline the value of these trial sites beyond the near-term of improving P-rate recommendations based on current testing protocols.