Planting Depth Effects on Small and Large Soybean Seed Size

• Soybean seed size has little to no effect on overall emergence and yield potential.
• To maximize soybean emergence, focus on consistent seeding depth and planting into favorable soil conditions.

Purchasing soybeans by seed count is still a relatively new concept within the seed industry. Historically seed was priced per 50-lb bag independent of seed size. Small differences in seed size influenced the number of actual acres that could be planted per unit. Purchasing smaller seeds could in some cases reduce the dollar per acre cost since there were more seeds per bag. Purchasing seed by count may take away the opportunity to sometimes get more seed per bag but does allow growers to more accurately determine the number of seeds needed to plant a given area. With either method, small-seed soybeans will continue to be sold. In most situations, seed size has not influenced germination, seedling vigor or yield potential. Although, since small seed has less stored reserves, maintaining a consistent planting depth may be more important. The Agronomy in Action Research team took a closer look at the importance of soybean seed size and planting depth at 3 sites in 2022.

What Influences Seed Size

Environmental conditions during the seed filling period play an important role in determining the number and size of soybean seeds.¹ Growing conditions during the R5 and R6 growth stages have a large influence on the size of seed that a plant produces. Weather events that stress plant growth during this time can impact soybean yield potential greatly. Available moisture is the most important factor, as dry conditions can reduce the size of seeds and the number of seeds per pod. Extreme moisture stress can lead to the plant aborting flowers and small pods before they have a chance to produce seed.

Soil Conditions and Planting Depth

Soil conditions at planting and prior to emergence will typically have a larger influence on emergence than seed size. As a soybean germinates, the hypocotyl elongates and pulls the cotyledons through the soil to the surface. This process requires a great deal of energy by the seed. Crusted or compacted soil, which further stresses emergence, can sometimes create slight advantages to large seeds that have more stored energy reserves. Planting soybean seed too deep or in undesirable soil moisture conditions can create additional stress, leading to further stand and yield loss.

2022 Seed Size by Planting Depth Trials

To investigate further, trials were established in IA and MN to understand the effect that seed size and planting depth have on soybean yield. Large and small seed sizes of the same soybean variety were chosen and planted at 1- and 2-inch depths. A single variety was chosen for each location except for Slater, IA, which included a second variety of differing relative maturity (RM). Each combination of seed size and planting depth was replicated 4 times at a location. Plots were harvested at maturity with a research combine to collect grain yield and moisture. 

Discussion

There was no difference in stand establishment or yield at Blue Earth, MN, which had good conditions for emergence shortly after planting. Heavy rainfall and soil crusting were common in and around both the Sac City and Slater, IA, trialing locations in the spring of 2022. Stressful emergence conditions resulted in improved emergence and yields when planting both seeds sizes shallower (1-inch) at these locations (Graph 1). Within the 1-inch planting depth, there were insignificant yield differences between small and large seed sizes, which most of the time favored smaller seed sizes. Although the 2-inch seeding depths were statistically lower yielding at 2 of 4 trials (Sac City and Slater), there was no advantage to planting larger seed at that depth (Graph 1). 

Results from this trial suggest that seed size does not influence overall yield. Inconsistent planting depth and challenging emergence conditions after planting can have far larger impacts on yield potential than seed size. To maximize soybean emergence, focus on seed depth and soil conditions shortly before, during and shortly after planting. Excess soil moisture can lead to crusting and soil compaction issues, which can affect soybean emergence and reduce stands, ultimately lowering yields. Selecting varieties based on yield potential and needed pest resistance traits are far more important considerations than seed size when determining profitability. 

References:

¹Hoeft, R.; Nafziger, E.; Johnson, R.; Aldrich, S. 2000. Modern Corn and Soybean Production; pages 31-39.