Updated March 25, 2021:
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In 2020, the earliest planting was the first week of May, which missed some of the cooler weather that may have caused disease or damage to growth. The drought conditions in the early summer may have also had an effect across all three plantings, although this field was irrigated. Regardless, there were no observable differences in yield by planting from early May to early June in 2020.
There were differences in nutrient uptake, with interesting patterns in macro and micronutrients based on planting timing. This had no effect on yield in this study, but does raise the question about nutrient uptake where there is low or excessive concentrations. The optimum levels in this study may not have affected yield as they may in a field with wider variability in nutrient contents. That Al uptake, a toxic non-essential nutrient, dropped off in uptake with later plantings, is an interesting path to explore. It was obviously not high enough in the earlier planting to effect yield though.
Drone imagery found differences in growth within all three planting dates. Early and Mid-may were similar for leaf area (NDVI) throughout the growing season, while the June planting was always behind. In a year with freeze damage or drought in May, we may see a separation in early and mid-May plantings. This supports multiple years to study a question on soybean growth and improving yields. Plant heights had similar patterns, but the mid-May plantings were higher in August, which may be important, or an artifact of the 2020 study. Future replications of this study across the mid-Atlantic will improve our understanding of planting timing, soybean growth, and nutrient uptake.