Soybeans are a major global commodity crop, providing both oil and protein for human and animal consumption. Measuring the productivity of this crop relies on a standard agricultural metric: bushels per acre (bu/ac). A bushel is a unit of volume, but for soybeans, it is defined by weight, with one bushel equating to a standardized 60 pounds of seed. The amount of soybeans harvested from a single acre varies dramatically based on numerous interacting factors. This variability means the answer to how many bushels an acre can produce is complex, relying on the interplay between environmental conditions, plant biology, and human management decisions.
Understanding Typical Soybean Yields
The national average yield provides a baseline for understanding typical soybean productivity. In recent years, the United States national average has consistently hovered around 50 to 51 bushels per acre. For instance, the 2023 national average was approximately 50.6 bushels per acre.
Individual states in high-production regions often report averages significantly above the national figure, reflecting favorable soil and climate conditions. Key producing states, such as Illinois, may see state-wide averages reaching 63 bushels per acre. A yield in the range of 65 to 70 bushels per acre is considered a strong performance for a commercial field in the US Midwest.
The theoretical maximum yield is far higher than farm averages, with specialized yield contests reporting results that can exceed 160 bushels per acre. This potential is governed by the crop’s yield components:
- The number of plants per acre
- The number of pods per plant
- The number of seeds per pod
- The final seed weight
Farmers consistently strive to push yields closer to this biological ceiling, resulting in a steady upward trend in national averages.
Environmental and Biological Factors Influencing Yield
The final yield is significantly shaped by environmental and biological elements. Weather is the greatest source of yield variability, particularly the timing and amount of rainfall. Soybeans are most vulnerable to water stress during the reproductive stages, specifically from the beginning of pod development (R3) through the seed filling stage (R6).
A lack of sufficient moisture during these reproductive phases causes the plant to abort flowers and young pods, reducing the total number of seeds per acre. High temperatures also compound this effect, as prolonged exposure above 95°F during flowering can significantly decrease pod set. Research indicates that yield loss can increase by approximately five percent for every one-degree rise in temperature during sensitive growth periods.
Soil quality is another determinant, as it governs nutrient and water availability for the plant. The optimal soil pH range for soybeans is slightly acidic to neutral, between 6.0 and 6.8. A pH outside this range can reduce the availability of essential nutrients, such as phosphorus, and hinder the effectiveness of the symbiotic relationship the plant forms for nitrogen acquisition.
Poorly drained or excessively sandy soils affect root health and nutrient uptake. The presence of pests and diseases can also limit the field’s maximum output. Soybean Cyst Nematode (SCN), a microscopic worm, is a persistent threat that reduces yield by establishing feeding sites within the roots, interfering with the plant’s ability to absorb water and nutrients.
Sudden Death Syndrome (SDS), caused by the soil-borne fungus Fusarium virguliforme, is a significant biological challenge. The fungus infects the roots and produces toxins that move into the leaves, causing premature defoliation. The earlier the foliar symptoms of SDS appear, particularly during the early pod-fill stages, the greater the resulting yield loss due to the disruption of photosynthesis and seed development.
Optimizing Yield Through Management Practices
Growers utilize management practices to mitigate environmental risks and maximize yield potential. A primary decision involves variety selection, where a farmer chooses a specific soybean cultivar based on its Maturity Group (MG) and genetic traits. Maturity groups are numbered classifications that determine the time required for a variety to reach maturity, regulated by the plant’s response to day length (photoperiodism).
Planting a range of maturity groups allows a grower to spread the risk of adverse weather or pest outbreaks during a single, sensitive growth window. Selecting varieties with specific resistance genes is routinely done to protect against local threats, such as SCN-resistant varieties or those tolerant to SDS. The selection of herbicide-resistance traits also facilitates effective weed control, reducing competition for light, water, and nutrients.
Planting density and row spacing are manipulated to ensure efficient light interception and canopy closure. Soybeans have a high degree of plasticity, adapting to lower populations by producing more branches and pods. Narrow row spacing (15 inches or less) often results in quicker canopy closure. This shading helps suppress weeds and conserves soil moisture, supporting a final harvest population typically ranging between 100,000 and 120,000 plants per acre.
Fertilization practices focus on providing adequate phosphorus (P) and potassium (K).
Phosphorus (P)
Phosphorus is essential for early root development and the later formation of pods and seeds, acting as a component in the plant’s energy storage and transfer.
Potassium (K)
Potassium regulates the movement of water, nutrients, and carbohydrates within the plant. It is heavily demanded from flowering through early pod development.
Soybeans, as a legume, acquire the majority of their nitrogen (N) through a symbiotic relationship with the soil bacterium Bradyrhizobium japonicum. The bacteria colonize the roots and form specialized structures called nodules. Inside the nodules, the enzyme nitrogenase converts atmospheric nitrogen gas (N₂) into plant-available ammonia (NH₃). This process can provide 40 to 70 percent of the plant’s total nitrogen requirement. Success is visually confirmed by the pink-red color of an actively fixing nodule, caused by the oxygen-binding protein leghemoglobin.