The soybean plant, Glycine max, is a major global commodity whose visual appearance changes dramatically as it progresses through its life cycle. Observing these distinct stages is how farmers and agronomists track the plant’s development and manage the crop. The plant transitions from a delicate seedling to a lush, green bush before finally drying to a brown, brittle stalk ready for harvest.
The Initial Stages of Growth (Vegetative Phase)
Soybean growth begins with the emergence of the seedling from the soil, a process driven by the elongation of the hypocotyl, the stem section below the seed leaves. This growth pulls the two large cotyledons, or seed leaves, out of the ground, which are the plant’s initial food source and appear as thick, oval structures. Shortly after emergence, the first pair of true leaves, known as unifoliolate leaves, unfold at the first node above the cotyledons.
All subsequent leaves that develop on the main stem are compound leaves called trifoliates, each consisting of three separate leaflets attached to a single stalk. New vegetative stages are defined by the appearance of each new, fully unrolled trifoliate leaf. During this vegetative period, the plant develops an erect stem that is often covered with a fine layer of hairs, giving it a slightly fuzzy texture. By the time the plant reaches the V2 stage, typically around six to eight inches tall, root nodules begin to actively fix nitrogen, a process that continues as the plant rapidly increases in height and leaf area.
Transitioning to Reproduction and Pod Formation
The transition into the reproductive phase is marked by the appearance of the first flowers. These flowers are small and often appear as clusters at the nodes, typically starting on the lower third to sixth nodes of the main stem. Depending on the variety, these tiny blooms can be either white or a pale purple hue.
Once flowering begins, the plant starts to form its first pods, a stage known as beginning pod or R3, where tiny, fuzzy green structures about 3/16-inch long are visible on the upper parts of the stem. These pods rapidly increase in length and girth through the full pod stage (R4), reaching approximately 3/4 of an inch long and containing visibly developing seeds. The exterior of the pod remains a vibrant green and is covered in fine hairs.
Seed filling is most noticeable during the R5 and R6 stages, as the seeds swell inside the pod cavity. At the full seed stage (R6), the seeds are visibly filling the green pod. During this period, the plant begins to draw nutrients from its leaves and stems, redirecting them into the developing seeds, which is the precursor to the final stage of maturity.
Appearance at Maturity and Harvest
The final visual change in the soybean plant is senescence, the natural aging process where the plant loses its green color and begins to dry down. This process starts when the leaves begin to yellow and drop from the plant. The first sign of physiological maturity (R7) is when a single pod on the main stem has reached its mature color, typically a shade of tan or brown.
The plant’s color continues to change from green to a uniform yellow, and eventually to a brittle brown or tan. At the full maturity stage (R8), at least 95% of the pods across the entire plant have reached this mature, dry coloration. The leaves have mostly dropped, leaving a field of brown stalks and pods. This visual transformation from a green plant to a dry, brown one is the primary indicator that the seeds are ready for harvest. When fully dry, the seeds will audibly rattle when the pods are shaken.