Soybean seed size, does it matter?

Soybean seed size ranges from about a ¼ to ½ inch in diameter, and on average there are 2,100 to 2,900 seeds per pound. However, seed size can vary considerably.

What impacts soybean seed size?¹

Growing conditions impact seed size the most:

• Seed filling period: Growing conditions during seed fill are a major determinant of final seed size. Favorable rainfall during grain-fill can produce larger seeds, but drought, high heat, or stressful conditions can result in smaller seeds.
• Nutrient availability: A deficiency of key nutrients can influence seed size.
• Light and temperature: Sunlight intensity, day length, and temperature can impact seed size.
• Genotype interaction with the environment: Seed size is determined by the interaction of the genetics of the genotype and environment in which the plant has been grown.

Are there concerns with seed size and planting method?²

In recent years, soybean seed purchasing has changed from buying on weight (50 lb bag) to buying by seed count. The standard seed count/bag is now 140,000 seeds. For example, if seed weight is 2,800 seeds/lb, a seed bag weighing 50 pounds would contain 140,000 seeds. However, seed size can vary among soybean products and is largely determined by growing conditions during seed-fill. Assuming 2,800 seeds/lb is a “normal” seed size for a product, and seed size is slightly smaller than normal at 3,100 seeds/lb, a 50-pound bag would contain 155,000 seeds or a 140,000 seed count bag weight would be 45.2 pounds. If the seed product had 4,200 seeds/lb, a 140,000 seed count bag would weigh 33.3 pounds.

Farmers know the value of accurately calibrating planters to help achieve a desired seeding rate/acre rather than planting based on seed weight/acre. The manufacture’s operating manual should be referenced to help determine and set the planter or drill for each seed size. Knowing the seeding rate and purchasing seed by count helps growers to more accurately determine the number of seeds required for a field, regardless of seed size. Accuracy in buying the desired seed count helps save money.

Purchasing seed by count shifts the focus from seed size to more important soybean product characteristics. Agronomic traits, yield potential, and disease resistance are far more important than seed size in determining the potential profit from soybean production.

What is the impact of soybean seed size on plant vigor and emergence?²

In most situations, seed size does not affect emergence percentage, seedling vigor, or yield potential. Smaller seeds have less stored reserves which may influence planting depth for the seed. Small seeds resulting from premature maturity caused by extreme late-season stress may be less vigorous than normal seed. This is because physiological systems required for germination did not develop properly. Except for these extremes, seed size should not be a factor in variety selection or seed purchases.

While small seeds may have a lower emergence percentage, the major factor in determining yield potential is final plant stand and not seed size. Furthermore, small seeds may have an advantage over larger seeds when soil moisture is limiting as small seeds require less water to germinate.

Premature plant death from an early frost or other factors may result in small seeds that are less vigorous. However, required germination and vigor tests should identify any issues that would require the seed to be tagged as lower germination. With the identified germination percentage, seeding rates can be adjusted accordingly to help achieve the desired stand count.

What is the effect of seed size on seedling emergence and vigor?³

A study composed of 10 field experiments of over 6 years which compared 3 or 4 soybean seed sizes, and 3 to 8 soybean genotypes helps address this question.³ The results were that seed size had little to no effect on initial plant stand, plant mortality, lodging, and mean size of harvested seed. Average seed yield from large seed planted alone exceeded the yield from small seed planted alone by 5.4% in 10 experiments. Average seed yield from small seed equaled the yield from large-small in eight experiments. When large and small seed were planted in the same row, progeny from the large seed produced more yield than those from small seed. Since there was no genotype by seed size interaction effects on yield, the authors concluded that seed size effects on performance apparently were a response to relative seed size within a population and not a response to seed size alone.

The North Carolina Soybean Production Guide states “Soybean seed size can vary widely but generally does not affect yield potential. Seed size is influenced more by environmental conditions at the seed production site the previous year. Under typical planting conditions, seed size would also not influence emergence or seedling vigor.”⁴

For additional information regarding seed size and soybean germination issues, please visit Soybean Seeding Depth to Help Minimize Early Season Stress, Drought Effect on Pod Fill in Soybeans, and Handling and Planting Lower Germination Soybean Seed.

Sources

¹ Zhang, Y., Akhter Bhat, J., Zhang, Y., Yang, S. 2024. Understanding the molecular regulatory networks of seed size in soybean. Int J Mol Sci. https://pmc.ncbi.nlm.nih.gov/articles/PMC10855573/#:~:text=The%20seed%20size%20of%20soybeans,practical%20soybean%20breeding%20%5B12%5D.

²Wiebold, W.J. 2008. Soybean seed size does not affect yield performance. University of Missouri. https://ipm.missouri.edu/cropPest/2008/11/Soybean-Seed-Size-Does-Not-Affect-Yield-Performance/index.cfm

³Smith, T.J., and Camper Jr., H. M. 1975. Effects of Seed Size on Soybean Performance. Agronomy Journal. 67:681-684.

⁴Chandler, J., Choudoir, M., and Stowe, K.D., et al. 2025. North Carolina Soybean Production Guide. North Carolina State University. https://content.ces.ncsu.edu/north-carolina-soybean-production-guide

Web sources verified 11/18/25. 1216_703301