The optimal soybean seeding rate per acre is dynamic, changing based on numerous factors specific to each field and growing season. Maximizing yield requires careful calculation to find the ideal balance between ensuring a healthy plant population and avoiding over-seeding. The correct seed rate is a precise number derived from agronomic targets, environmental conditions, seed quality, and equipment limitations. Determining the volume of seed requires establishing the final goal, adjusting for survival, and converting the count into a measurable weight.
Optimal Stand Count Targets
The goal of planting is to achieve an optimal final stand count: the number of healthy, harvestable plants per acre. This target is distinct from the initial seeding rate and is generally recommended to be in the range of 100,000 to 135,000 plants per acre for maximum yield potential in most environments. This population density helps the plants rapidly form a closed canopy, which is important for intercepting sunlight efficiently and suppressing weed growth by shading the soil surface.
Soybeans are highly flexible plants, possessing a unique ability to compensate for lower populations by increasing branching and producing more pods per plant. Research has shown that even final stands as low as 50,000 to 75,000 plants per acre can still produce yields that are near the maximum, often with only a minimal loss. However, relying on this compensatory ability increases the risk of lodging (plants falling over) later in the season.
The target stand count should be adjusted based on the expected yield environment. In lower-productivity fields, where factors like limited water or poor soil quality restrict individual plant growth and branching, a higher final stand count is often recommended. Conversely, in high-yield environments with optimal irrigation and fertility, a lower final stand within the recommended range can be sufficient because each plant can maximize its potential.
Variables Determining Final Seeding Rate
Once the desired final stand count is established, the initial seeding rate must be calculated by accounting for expected losses. These losses, which can reduce the final stand by 10% to 20%, are influenced by several agronomic and mechanical variables. The planting date is a significant factor, as late planting (after mid-May) reduces the time available for vegetative growth. Higher seeding rates are needed in these cases to ensure rapid canopy closure and maximize yield potential before the season ends.
Row spacing affects the required density. Narrow rows (15 inches or less) allow for earlier canopy closure than wide rows (30 inches), which can slightly reduce the overall seeding rate. The physical condition of the field at planting time introduces another layer of adjustment. Planting into cold, wet soil or fields prone to crusting increases the risk of poor emergence, requiring a higher seeding rate to compensate for anticipated seedling loss.
The efficiency of the planting equipment must also be factored into the final seeding rate. Not every seed dropped will successfully emerge; the average survival rate for a germinating seed is often around 85%. The final seeds-per-acre count is determined by dividing the desired final stand by the expected field emergence percentage. An additional buffer is often included for fields with known issues like high disease pressure or poor seedbed quality.
Converting Seed Count to Weight
The final step for purchasing and calibrating planting equipment is converting the calculated seeding rate into pounds per acre. This conversion is necessary because seed is typically bought by weight, but the planter is calibrated by seed count. The necessary information for this calculation is found on the seed tag, which details the quality and physical properties of the seed lot.
The seed tag lists the germination rate, which is the percentage of seeds expected to sprout under ideal conditions. This percentage, along with the expected field survival rate, must be factored into the calculation to determine the total number of seeds needed. The tag also provides the seed size, expressed as the number of seeds per pound. Seed size is highly variable, changing with the variety and prior growing conditions, and can range significantly from the common average of 2,800 seeds per pound.
To find the weight needed, the total number of seeds per acre is divided by the number of seeds contained in one pound of that specific seed lot. This final figure, expressed in pounds per acre, is the amount required for planting. Failing to account for germination rate or seed size will result in planting too few or too many pounds of seed, potentially compromising the final stand.