The number of seeds in a bag of soybeans is not a fixed figure. This count is highly variable and depends on the method used for packaging and sale. Commercial soybean seed is distributed in two primary formats: by a guaranteed weight (e.g., a 50-pound bag) or by a guaranteed seed count, which is the modern industry standard. The shift toward count-based sales acknowledges the significant natural variation in the size of individual soybean seeds.
Standard Soybean Bag Sizes and Units
Historically, soybean seed was commonly sold by weight, typically in 50-pound bags. This method meant the actual number of seeds a farmer received fluctuated, even for the same variety. A bag of a specific weight could contain a widely different number of seeds, complicating planting rate calculations. The weight-based system is still used, but it is becoming less common for planting seed.
The industry has increasingly adopted a count-based packaging system to provide consistency for the buyer. The most common standard is a bag guaranteed to contain 140,000 seeds. This shift allows growers to purchase the specific number of seeds required to plant a certain area, regardless of the seed’s physical size. While the seed count is fixed, the actual weight of a 140,000-seed bag changes depending on the size of the seeds inside.
Understanding Seed Count Variability
The main factor driving the variability in seed count within a weight-based bag is the natural variation in seed size. This size is quantified using the concept of Thousand Seed Weight (TSW), which is the weight in grams of 1,000 soybean seeds. A related measurement is Seeds Per Pound (SPP), which is the inverse of TSW and indicates how many seeds are in a single pound of seed.
Environmental factors during the growing season directly impact the TSW of the harvested seed. Favorable growing conditions, such as adequate rainfall during the seed-filling period, often result in larger, heavier seeds and a higher TSW. Conversely, drought or high heat stress can result in smaller, lighter seeds, meaning more seeds are required to make up a pound.
A typical range for soybean seed size is between 2,500 and 3,000 seeds per pound, though this varies depending on the variety and growing conditions. For example, if a 50-pound bag contains small seeds (3,100 seeds per pound), the total count is 155,000 seeds. If the seeds are large (2,600 seeds per pound), the count drops to 130,000 seeds in the same 50-pound bag. This illustrates why purchasing by a guaranteed count, such as 140,000 seeds, is preferred for precision planting.
Calculating Desired Planting Population
Determining the appropriate planting rate integrates the variable seed count information. The ultimate goal is to purchase enough seed to achieve a desired final plant stand, or population, in the field. This calculation must account for the seeding rate (seeds planted per acre), which is always higher than the desired final stand.
The calculation requires factoring in the seeds per bag, the desired final plant population (e.g., 140,000 plants per acre), the seed’s percent germination, and the estimated field emergence. Seed companies provide the germination rate on the seed tag, typically around 90%. Field emergence is usually lower, perhaps 80% to 90% of the viable seed, due to factors like soil conditions, pests, and disease.
A simplified calculation for the required seeding rate involves dividing the desired final plant population by the product of the germination rate and the estimated field emergence percentage. For example, if a grower wants 120,000 final plants per acre, with 90% germination and 85% expected emergence, the seeding rate is 120,000 divided by (0.90 multiplied by 0.85). This results in a required seeding rate of approximately 157,000 seeds per acre. The grower then uses this rate to determine the number of 140,000-seed bags needed for their total acreage.