Cover crops have been rapidly adopted in Delaware, with cereal rye being a popular option for soybean production. The benefits of a rye mulch is weed suppression and soil moisture conservation, but may also cause increased pest presence and disrupt the release of nitrogen (N) to cash crops. While soybeans may not be as affected by the N cycle as
corn, the mineralization and release of N in rye may also provide supplemental N to the plant mid-season. These fields may also include corn fodder from the previous cropping year, which will continue to breakdown through the soybean growing season, providing some carbon the soil surface. What is not currently known is now soybean populations and row spacing may affect the decomposition of residues on the soil surface. Earlier canopy may preserve soil moisture, allowing for increased residue decomposition, or may increase evapotranspiration reducing overall soil moisture. This study will take the first steps in measuring decomposition of residues under soybean planting densities.

1) Plant full season soybeans into a rye cover crop at five populations and two row spacings.
2) Use decomposition bags to measure breakdown of corn fodder and rye biomass under different soybean management.
3) Track soil moisture and temperature under the soybean canopy

Soybeans will be planted at the Carvel Research and Education Center in Georgetown, DE into a rye cover crop. The cover crop will be terminated two weeks prior to planting, like most field practices. Soybeans will be planted at five densities (80, 100, 120, 140, 160 thousand seeds per acre) and two row spacings (15 and 30 inch) and irrigated throughout the season. Ten
subsamples across a gradient will be compared to drone imagery to estimate total field rye biomass. Terminated rye biomass will be collected from outside the plot boundaries and corn fodder will be collected from fields at the research center. Biomass will be separated into decomposition bags for each plot (30 rye and 30 corn fodder), weighed, and placed back into the
planted plots in the center of a row. Three subsamples of each will be dried and saved to determine the initial carbon (C), N, and moisture content of the biomass. At the end of the season decomposition bags will be collected from the plots, dried, weighed, and analyzed for C, N, and the biomass loss. In three population plots (80, 120, 160), logging sensors will be placed in the 15 and 30” rows to measure EC, moisture, and temperature throughout the season. Yields will be collected with a plot combine in the late fall.
Data will be analyzed in SAS as a randomized complete block design structured by a factorial including biomass loss, changes in C and N, as well as yield. Yield will also be correlated to various predictors from the study.

Updated August 8, 2024:
Soybeans were planted between 80 to 180k seeds per acre at both 15 and 30 inch rows in May 2024. This was performed into a rye cover crop which had been terminated ten days prior. Rye resides were collected from the field and placed into decomposition bags, along with previously sampled corn fodder. After soybean emergence, bags were placed into the plots in the center two rows where good stands and coverage would occur. Moisture sensors were placed into plots ranging from 80 to 180k seeding rates to measure changes throughout the season as plots canopied. Irrigation has been performed as necessary to keep stress to to a minimum and drone flights have been performed every 10 days. While there is moderate weed pressure, the deer have largely left the field along, potentially deterred by the standing rye cover crop. A concurrent rain-fed field was placed nearby with the same populations, decomposition bags, and moisture sensors.

Updated January 3, 2025:
All soybean plots were harvested in October 2024 using a University of Delaware plot combine. Yields were adjusted to 13% moisture for analyses and included in an overview of yields based on population, row spacing, and irrigation in the Delaware Agronomy Blog. This includes additional plots established in a rainfed field to examine non-irrigated conditions for population and row spacing. Decomposition bags were removed from both fields, dried, and weighed to determine overall loss. These bags were then submitted to the UD Soil testing lab for analyses.

Updated April 4, 2025:

View uploaded report

This study aimed to examine how row spacing, population density, and residue type affect soybean yield, biomass loss, and carbon-to-nitrogen (C:N) ratios in both irrigated and rainfed conditions. With a focus on rye and corn residues, it also sought to understand how these factors influence nutrient cycling and residue decomposition, key processes for maintaining soil health and optimizing future crop production.

The research was conducted at the Carvel Research and Education Center in Georgetown, DE. Irrigated soybean were planted in experimental plots with five seeding rates (80,000–160,000 seeds per acre) and two row spacings (15-inch and 30-inch). An additional rainfed field was planted off-site. Residue from a terminated rye cover crop and corn fodder was collected and placed in decomposition bags to track changes in biomass, C:N ratios, and decomposition over the season.

Soybean yields were not significantly influenced by either row spacing or population density in both conditions, marking a shift from previous years where narrower row spacing had provided a yield advantage.

For biomass loss, rye and corn residues showed different decomposition patterns. Rye residue decomposition was more influenced by population under irrigated conditions, with denser canopy leading to greater breakdown. Corn residue decomposition was impacted by row spacing, with 30” rows under rainfed conditions causing greater loss, but mostly at higher populations.

The C:N ratios of both rye and corn residues were generally unaffected by population density. However, irrigated row spacing had a significant impact on rye C:N ratios in irrigated fields, with narrower rows resulting in a lower ratio, suggesting faster decomposition. While rye residues, with their higher initial C:N ratio, decomposed more quickly, corn residues maintained a higher ratio throughout, indicating slower breakdown. These results highlight the complex interactions between environmental conditions, row spacing, and population density in influencing residue decomposition and nutrient cycling.

Provides both updated information on population and row spacing effects on yield while overlaying with potential carbon sequestration.