Soybeans (Glycine max) are one of the world’s most significant legume crops, providing substantial sources of protein and oil for human consumption and animal feed. Understanding how this staple crop develops is foundational to agricultural science. A common point of confusion is whether the edible seed matures beneath the soil surface or above it. This article clarifies the development process and the definitive location where the soybean seed is formed and harvested.
The Definitive Answer: Above-Ground Development
The edible portion of the soybean plant, the seed, develops exclusively above the ground. These seeds are enclosed within small, hairy pods attached directly to the main stem and branches. Unlike peanuts (Arachis hypogaea), the soybean does not exhibit geocarpy, the process of pushing fertilized flowers into the soil to mature.
The confusion often stems from the subterranean growth habit of other high-protein crops. Soybeans complete seed production entirely in the open air, where the pods are visible throughout the latter half of the growing season.
Pods typically contain two to four seeds and are clustered along the stem nodes, remaining above the soil line. This growth pattern simplifies mechanical harvesting, allowing equipment to cut and thresh the aerial parts of the plant to extract the seeds.
Anatomy of the Soybean Plant
The mature soybean plant has a sturdy central stem that supports the developing pods. The stem is covered in fine hairs and ranges in height from 50 to 125 centimeters, depending on the variety and environment. Initial growth involves the emergence of the hypocotyl, which pulls the cotyledons (seed leaves) above the soil surface during the emergence (VE) stage.
The leaves are distinctive, appearing in a trifoliolate structure, meaning each leaf comprises three separate leaflets. These alternating leaves are the primary sites for photosynthesis, converting sunlight into energy for seed development.
Below the soil surface, the soybean develops a robust taproot system with many lateral branches that can reach depths of 75 to 150 centimeters. This structure anchors the plant firmly and efficiently absorbs water and mineral nutrients.
The roots also form specialized structures called nodules, which house symbiotic Bradyrhizobium japonicum bacteria. These bacteria perform nitrogen fixation, converting atmospheric nitrogen into a usable form, which is a defining characteristic of legumes.
The pods are an extension of the plant’s reproductive structures, forming where the small, self-pollinating flowers bloomed along the stem nodes. They are generally curved and densely covered in short, stiff hairs, protecting the developing seeds inside. Pods are initially green and change color as they dry and mature, signaling the completion of seed development.
The Soybean Growth Cycle
The development of the soybean plant is scientifically divided into two major phases: the Vegetative (V) stages and the Reproductive (R) stages. The V stages focus on building the plant’s infrastructure, starting with the emergence of cotyledons and the subsequent development of the unifoliolate and trifoliolate leaves.
Once sufficient vegetative growth is established, the plant transitions into the Reproductive stages, marked officially by the appearance of the first flower (R1). This transition shifts the plant’s energy allocation toward seed production. Flowering begins on the lower nodes and progresses upward, a pattern noticeable in indeterminate varieties.
Pod development begins shortly after flowering, generally around the R3 stage, when the pods reach about five millimeters in length at the upper nodes. The pods rapidly elongate and fill with liquid and then solid material as the plant moves toward the R4 stage, reaching a length of about 20 millimeters.
The most intensive phase is the seed fill period, spanning the R5 to R6 stages. During this time, the seeds rapidly accumulate mass, protein, and oil within the protective, above-ground pods. This accumulation requires large amounts of water and nutrients, making the period sensitive to environmental stress.
The plant reaches full maturity (R8) when 95% of the pods have dried and changed color, typically turning brown or tan. This confirms that the seed moisture content is low enough for mechanical harvesting.