The start of indeterminate soybean reproductive stages depends on or the detection of the length of night. As nights become longer, soybeans are triggered to begin the reproductive or “R” stages of maturity. Due to this, later planted beans do not have as much time to develop biomass, or leafy growth. Additional photosynthesis from leaves and nodes for pod production can mean additional yield with more time to grow.

However, issues with earlier planting have occurred where cooler, wetter soils slow germination. This may cause seeds to rot in the ground. Additionally, sudden death syndrome (SDS) infects soybean roots of earlier planted varieties, but won’t be noticed until later in the season. With newer varieties, it is necessary to evaluate the limits of planting earlier in Delaware, and determine if additional yield is outweighed by other biotic and abiotic factors. For this year’s project, research will also be replicated by the University of Maryland.

1) Plant full season soybeans on three different dates (early, mid, and late).
2) Evaluate soybean plots for deficiencies and disorders.
3) Compare tissue and soil contents to yield at the end of the season.

Soybeans will be planted for a third year at the Carvel Research and Education Center in Georgetown, DE on three different dates. This includes mid-April, early May, and late May. The same variety (mid group IV) will be planted on all three dates. Following emergence, plots will be assessed for early season seedling diseases and will be scouted for disease throughout the season. Tissue and soil samples from each plot will be sampled at R1-R2 to observe any differences in nutrient uptake. Yield will be collected at the end of the growing season using a plot combine. Tissue and soil samples will be analyzed by the University of Delaware Soil Testing Lab. Tissue nutrient content will be correlated to yield, soil nutrient levels, and soil type. Trends in yield related to planting date, nutrient content, and soil nutrient status will be compared.

Additionally, this project is also being submitted to the MD Soybean board by the University of Maryland to add additional sites across the Mid-Atlantic. This will allow for the robust examination of climate and soil type of soybean planting dates.

Updated July 23, 2022:
Soybeans were planted at three timings (mid April, late April, mid May) at the Carvel research and education center in the spring of 2020, following the burndown of a rye cover crop two weeks prior. Fields were monitored for growth stage by walking the field and through bi-weekly drone flights, recording growth stages and any emergence issues. Herbicide applications were made as needed to suppress weed growth through backpack and tractor sprayers. At the R2 growth stage for each planting date, upper leaf tissue was taken from each plot. These samplings were not on the same date, but did occur within a week of each other.

Updated December 26, 2022:
All tissue and soil samples were submitted to the University of Delaware Soil Testing Lab for analyses and should be completed early in 2023. Plots were harvested in late October 2022 and yield data was checked for accuracy. Statistical analyses were performed on yield data and shared with collaborators at the University of Maryland.

Updated March 19, 2023:

View uploaded report

Untreated group IV soybean seeds were planted at the Carvel Research and Education Center on April 13th, 26th, and May 11th. Despite freeze damage to the April 13th planting, yields, yields were similar across all treatments, ranging from 67 to 68 bushels per acre.

Tissue and soil samples were taken at the R2 growth stage, and almost all nutrients had some differences based on planting timing. However, all nutrients were within within sufficiency ranges, despite having different concentrations based on planting timing. Although each treatment was planted two weeks apart, tissue sampling occurred within one week. This is because most studies reached R1/R2 at similar Julian dates, since the 1st planting would have slower early growth with cooler weather.

Only Ca was borderline deficient in the tissue and was also lowest for the May 11th planting date. While S was not deficient, it was on the lower end of sufficiency ranges. Sulfur dropped in the soil with later planting but was higher in the tissue. Since most soil samples were taken at similar time points, this may represent plant uptake more than leaching from the profile. The nutrients P, K, Mg, Zn, and Fe were all higher with earlier planting dates, but B (like Ca and S) was highest in the tissue from the last planting date.

Like the 2020 and 2021 study of planting timings, Al concentrations were lower in the tissue with later plantings, with a pH relationship only occurring during the earliest planting. The uptake of Al within soybeans can be variety or pH related, but in all three years has certainly been planting date related.

So, while planting date can influence tissue concentrations of nutrients, it may only matter when fertility is lacking, or an antagonistic relationship occurs. This could include drought conditions, which were not evaluated in this study. Currently, we would not recommend any variation in soil fertility based on planting date, but antagonistic relationships with Al uptake should be studied.