Whether or not you need micronutrients in your starter should come down to last years soil or tissue tests. Over the last two years, our research projects have not revealed a deficiency in many micronutrients, but we still understand it is out there. In 2018, we conducted a study at the Carvel Research and Education Center (Georgetown, DE) with two rates of (Mn), zinc (Zn), and boron (B) in the starter. We observed no effect on yield, which was expected as these soils were adequate in Mn and Zn based on UD recommendations. Although starter B had no effect on yield, B did have a positive correlation with yield in the starter study. This implies that with increase tissue B concentrations, yield also increased. Correlations imply relationships, but not necessarily why this occurred. An environmental variable may have influenced both B uptake and yield in this case, such as saturated soils leaching B while reducing yield.
In a study sponsored by the Maryland Grain Producers we saw the same relationship between higher tissue B and greater yield, across a range in soil types. In both 2018 and 2019, very few corn ear leaf samples reached the critical threshold for boron, which would indicate we are having a difficult time maintaining B in our soils. Due to this result, we performed a study with split applications of B in Georgetown, but also observed no effect on yield.
While we have observed that B is tied to greater yield in corn tissue samples, the method of application or uptake has not been as straightforward. As an anion, soil tests for B may not be accurate. Levels of boron from a fall soil test may be leached from the upper soil by the time planting occurs. Mid-season soil samples from a soybean study have had a stronger correlation between soil B and whole plant B. The combination of mid-season tissue and soil samples may be a better way to manage B, but further research is needed. It remains possible that starter B applications may also be leached from the soil surface prior to plant uptake, so that tissue samples will be necessary following higher rainfall.
Very few of our tissue samples across Maryland and Delaware have been lacking in Zn, while all samples from the Maryland Grain Producers Study were above critical thresholds for Cu, Mn, and Fe in both 2018 and 2019. A major variable controlling micronutrient availability is pH, and we have observed greater uptake of Zn in soybeans with lower pH, even in soils not lacking in Zn. It is imperative that soil pH effects on micronutrients are considered when making lime applications, as this may have greater contributions to uptake and availability than starter applications.
Research in this study was supported by Maryland Grain Producers and the Delaware Soybean Board.
