The soybean plant, Glycine max, changes dramatically from planting to harvest, transitioning through distinct phases of growth and maturity. The field’s color, density, and structure offer direct visual cues about the plant’s current stage of development. This life cycle is broadly divided into vegetative and reproductive phases, and understanding these visual cues is important for managing the crop.
The Emerging Plant and Early Growth
The first visual sign of a soybean crop begins a few days to a week after planting when the seedling emerges from the soil. A hook-shaped stem, the hypocotyl, pulls the two pale green cotyledons (seed leaves) above the ground, which serve as the plant’s first energy source. Shortly after, the first true leaves appear as unifoliolate leaves—two single, oval-shaped leaves growing opposite each other. All subsequent leaves develop as trifoliolate leaves, the characteristic structure of the mature plant, consisting of three leaflets attached to a central stalk. At this early stage, the field appears sparsely covered with small, bright green plants scattered between the rows.
The Mature Green Plant
As the plant progresses through its vegetative stages, it develops a deep, uniform green color. During this peak growth period, the soybean plant typically reaches a height of two to four feet, depending on the variety and environmental factors. The stems are covered in fine, fuzzy hairs, and the numerous trifoliolate leaves create a thick canopy. This dense foliage often completely shades the ground between the rows, giving the field a lush appearance. The plant accumulates the majority of its biomass and nutrients during this phase, storing energy for the developing seeds.
Flowers and Developing Pods
The transition to reproductive growth is marked by the appearance of small flowers, typically white or pale purple. These flowers are usually inconspicuous and often hidden among the dense green foliage. Flowering begins on the lower nodes of the main stem and progresses upward, covering the plant with blossoms. Following pollination, the visual focus shifts to the developing pods. These pods initially appear as small, fuzzy, green structures clustered along the main stem and branches. As the reproductive stage continues, the pods rapidly lengthen to about three-quarters of an inch and begin to swell as the seeds inside grow. During this time, the field maintains its deep green color.
The Yellowing and Dried Crop
The final phase, known as senescence, signals the end of the growing season as the plant directs all remaining resources into the seeds. The most noticeable visual change is the loss of the deep green color as the plant stops producing chlorophyll. This process begins with the leaves turning a bright yellow as the nitrogen and other nutrients are mobilized out of the foliage and into the pods. The yellowing progresses quickly, eventually turning the leaves brown before they dry up and fall off the plant. The field is then left with the final, stark appearance of dry, brittle, brown or tan stalks holding the mature, dry pods. This visual transformation, where 95% of the pods have reached their mature color, indicates the crop has reached full maturity and is ready for mechanical harvesting.