The soybean is one of the world’s most important global crops, valued primarily for its high protein and oil content. Its development from a planted seed to a mature, harvestable bean is a biological process dictated by genetics and the surrounding environment. The speed of this development is highly variable, changing significantly based on where and when the crop is grown. Understanding how quickly soybeans grow requires examining the standardized stages of development and the external factors that influence the process.
Measuring Soybean Development
Agricultural scientists track soybean growth using a standardized staging system divided into Vegetative (V) and Reproductive (R) phases.
The V stages begin when the seedling emerges from the soil. The speed of this vegetative phase is measured by the rate at which new nodes and leaves appear on the main stem. This stage establishes the plant’s structure, developing its root system and accumulating biomass before flowering begins. Vegetative growth often continues even after the plant enters the reproductive phase.
The Reproductive (R) stages mark the plant’s shift toward seed production and are the most sensitive periods for determining the growth timeline. The cycle begins with R1 (beginning bloom) and R2 (full bloom), followed by R3 (beginning pod) and R4 (full pod), where the physical structures for the beans are formed. The most resource-intensive periods are R5 (beginning seed) and R6 (full seed), when the plant rapidly fills the pods with developing beans. The process culminates at R8 (full maturity), when 95% of the pods have reached their mature, harvestable color.
Environmental Factors Determining Growth Rate
The speed at which a soybean plant moves through the V and R stages is regulated by temperature, moisture, and day length.
Temperature
Temperature directly controls the rate of metabolic activity. Growth is negligible below \(50^{\circ}\text{F}\) and optimal within a daytime range of \(68^{\circ}\text{F}\) to \(86^{\circ}\text{F}\). Temperatures consistently above \(95^{\circ}\text{F}\) during the reproductive stages stress the plant, causing it to accelerate development prematurely and potentially abort flowers or pods.
Photoperiod (Day Length)
The photoperiod, or the duration of light in a 24-hour cycle, determines the transition from the vegetative to the reproductive phase. Soybeans are classified as short-day plants, meaning they are triggered to flower when the period of darkness is long enough. As days shorten in late summer, this change signals the plant to shift energy from growing leaves to producing flowers and seeds, speeding up the maturity timeline.
Water Availability
Water availability is a major factor, and drought stress can both accelerate and decelerate development depending on its timing. During the early reproductive phases (R3 to R5), a lack of consistent moisture forces the plant to conserve resources by shedding flowers and pods, which slows mass accumulation. Severe and prolonged drought later in the season can cause the plant to rush to physiological maturity, resulting in smaller seeds and a shortened timeline.
The Total Timeline: Planting to Harvest
The total time required for a soybean plant to progress from emergence to a harvestable crop typically falls within a range of 90 to 140 days. This variation is primarily a function of the chosen soybean variety, which is categorized into Maturity Groups (MGs).
Maturity Groups dictate the plant’s sensitivity to the photoperiod, effectively programming the length of the growing season. Earlier Maturity Groups are highly sensitive to changes in day length and transition to reproductive stages sooner than later-maturing varieties. For example, a Group I soybean planted in a northern region may mature in approximately 100 days, while a Group V soybean planted in a southern region may require closer to 130 days. The actual number of days is always modified by the daily heat and moisture conditions experienced during the growing season.