Soybean Plant Biology: History, Plant Structure & Growth Cycles

By Dr. Harold Willis

The soybean is a truly amazing and versatile crop plant. It is one of the oldest food plants, domesticated by 1100 BC in northeastern China. Its ancestor is a wild vine-like plant that produces tiny, hard seeds that are useless for food unless properly prepared.


Over the next several hundred years the domesticated soybean (called Glycine max by botanists) spread throughout much of eastern Asia. It grew upright and yielded larger, more digestible seeds. A variety of foods was developed from the soybean, ranging from soybean sprouts to steamed raw beans to roasted seeds to soy milk to soy sauce to fermented soybean paste and cake to soy flour to the commonly eaten curd called tofu (or doufu).

Soybeans reached the western world by the early 1700s and were first grown in North America by 1804. Benjamin Franklin appears to have been involved in introducing soybeans from France to Philadelphia at that time. A number of varieties was grown and evaluated in the United States during the 1800s. The primary use for the crop was for forage, hay and green manure.

Soybean plant at harvest time
A soybean plant at harvest time. Courtesy How to Grow Super Soybeans.

In the 1880s, French scientists discovered that the soybean contains practically no starch, so its use in diabetic diets began. Later its high protein content was recognized.

Modern Uses of Soybeans

In the early 1900s the first processing of seeds for oil and meal was done in England. For the most part, soybeans were a neglected crop until World War II. Germany developed a soy oil lard substitute and a meat substitute. In the U.S. increasing amounts of soybean meal were used as livestock and poultry feed, especially after 1945, when consumption of meat increased dramatically. More recently, an increasing proportion of American soybean production has been used by the food processing industry—in such foods as margarine, shortening, ice cream, salad dressings and mayonnaise. Industry uses lesser amounts, in products including paint, ink, putty, caulking, wallpaper, rubber substitutes, adhesives, fire extinguisher foam, electrical insulation and gasoline. The versatile soybean is a part of everyone’s life in developed countries.

At present, most soybeans (over three-fourths of the world supply) are grown in the United States (especially in the corn belt and Mississippi Valley), in Brazil and Argentina. China produces most of the soybeans grown in the Orient, while only a few are grown in Europe. In the U.S., the soybean is third in production (corn and wheat are first and second) and second in value (corn is first) of crops grown.

Soybean Germination

After being planted in the soil, the seed absorbs moisture, changing from less than 13% moisture to about 50% in several hours. After one or two days the first root (called the radicle) emerges through the seed coat and begins growing downward to establish the root system.

The upper part of the young plant (the hypocotyl) begins to lengthen, pulling the remainder of the seed upward. About five to fifteen days after planting, the new plant arches through the soil, and the oval seed leaves (cotyledons) open up. The cotyledons provide the seedling with food (that was stored in them) for about a week, plus they soon turn green and begin making a little additional food by photosynthesis. Later they drop off.

Seed germination and emergence is a critical period in the life of a soybean because poor emergence due to a soil crust, cold temperatures or seedling pests or diseases can drastically cut yield.

Soybean Vegetative Growth

After the seedling has emerged from the soil, the young stem and first leaves begin to rapidly grow upward. The seedling is very tough and frost resistant. If the terminal bud (growing tip) of the stem is killed, side buds will take over.

After emergence, for the first six to eight weeks, the soybean grows its stem (and possibly branches) and leaves. This is called the vegetative period.

Young soybean plant diagram

The first two leaves that develop are called unifoliates, meaning that the leaf has a single flat surface, the blade, similar to the leaves of elm or maple trees. The remaining leaves are three-bladed, or trifoliates. Here, the total leaf has three divisions, all attached to a single “leaf stalk,” or petiole. The place where a leaf petiole attaches to the stem is called a node. Later, flowers will develop at the nodes, between the petiole and stem, and branches also grow out from here.

After the first few leaves develop, overall growth of the plant increases rapidly. If plants are spaced far apart, more side branches will grow outward to capture as much light as possible, producing a bushy-looking plant. Plants in dense stands tend to grow upward, with few or no branches. Some soybean varieties tend to branch more than others.

As new upper leaves begin to shade older, lower leaves, the lower leaves may turn yellow and fall off. This is nothing to be concerned about, since the plant is just getting rid of unproductive leaves.

Soybean Roots

While the stem and leaves are growing upward, the root system is growing deeper into the soil. At first, the plant grows a main taproot, but soon side roots branch off, and still others grow off from them. The deepest roots may reach down five feet or more in loose, well drained soil, but most of the roots are found in the upper one foot of soil.

Soybean roots.
Soybean roots.

The young roots start to develop root nodules within a week after emergence if the proper nitrogen-fixing bacteria are present in the soil. The nitrogen-fixing nodule bacteria, technically called Rhizobium, enter the nodules and after ten to fourteen days are able to supply most of the plant’s nitrogen needs, if the nodules are healthy. In favorable soil conditions, a couple dozen or so pea-sized nodules will develop on the upper roots of a plant. Healthy nodules will be pink or reddish inside.

 Soybean Flowering

In typical soybean plants, after six to ten trifoliate leaves have grown, the next main stage in the plant’s life begins, the reproductive period. From 3 to 15 flower buds develop at each node of the stem.

There are two main types of soybean, depending on how flowering occurs. Varieties called indeterminate continue growing upward at the tip of the stem for several weeks after flowering begins lower on the stem. Upper nodes will not flower until later. Most commercial varieties are indeterminate. They typically grow taller and do best in short growing seasons.

A few varieties are called determinate and complete their growth in height first, then all flowers bloom at about the same time. They are usually one-half to two-thirds as tall as indeterminate varieties and so are often called “semi­dwarfs.” There are also some intermediate varieties, called semideterminate, which grow taller during the first part of their flowering period.

The flowers of soybean are tiny (1/4 inch) and white, pink or purple. They resemble the flowers of pea or clover, since the soybean is in the same plant family, the legume family. Many more flowers are produced than eventu­ally produce seed pods. The extras drop off, anywhere from 50 to 80% of the total.

The flowers are self-pollinated; that is, the flower fertilizes itself, and insects are not required to carry pollen from one flower to another.

Soybean Light & Temperature

The beginning of the flowering period is hastened by high­er temperatures and a greater amount of vegetative growth, but a major factor that controls flowering is photoperiod—the length of the day. Flowering of a certain variety begins sooner when the days are shorter and later when the days are longer (if the plants are grown where there is artificial light during the night, they may never flower).

Each variety is adapted to flower and complete its life cycle at a certain geographic latitude (distance from the equator). Normally, if planted in the spring, the plants will begin flowering in mid-summer, after the days begin to get lon­ger (in the Northern Hemisphere, the longest day, the summer solstice, is about June 21). But the days are longer the closer one gets to the pole (the sun never sets above the Arctic Circle during the summer). This means that if you try to grow a variety adapted to a certain latitude, say around St. Louis, Missouri, at more northerly locations, say Minneapolis, Minnesota, the days will be longer and the plants will not begin to flower until later, and they may not mature before frost. If grown to the south, they will mature too soon and yield will be reduced.

Therefore, soybean varieties are grouped into 13 maturity groups, depending on the climate and latitude for which they are adapted. These maturity groups are given numbers, with numbers 000, 00, 0 and I being adapted to Canada and the northern United States, and numbers VII, VIII and IX being grown in the southern U.S. (Group X is tropical.) Be certain to plant a variety adapted to your area.

Soybean Pod Development

One or two weeks after the first flowers, the first seed pods appear, with most pods being set within the next three weeks. Inside the pod, three (or sometimes four) tiny seeds begin to grow and develop.

For the next 30 to 40 days, the seeds rapidly fill with food produced in the leaves. The seed-filling period is the most critical in the life of the soybean plant with regard to yield. If weather conditions are adverse, such as drought stress or leaf loss from hail, yields will be cut severely. At this time, the plant takes 30 to 40% of its total mineral needs from the soil, so soil fertility should be at a peak.

After most seeds have filled, the growth activities of the plant slow down rather suddenly (called senescence). The leaves slow down their photosynthesis and begin to turn yellow, eventually dropping off. Root nodules cease produc­ing nitrogen.

Soybean Maturity

The newly formed seeds contain about 90% moisture. As the seeds fill with food, moisture content decreases to about 60 to 65%. When seeds are mature (filled), the moisture content is 45 to 55% and the pods and stems of the plant are yellow or brown. The mature seed itself will also be completely yellow when mature (if it is a yellow-seeded variety).

In warm, dry weather, seed moisture will continue to drop to about 13 to 14%, when the crop can be harvested. In some varieties especially, the dying plants tend to lodge, making harvesting difficult, and in some varieties, pods tend to split open (shatter), dropping the seed and reducing harvestable yield.

As soybean seeds lose moisture they change from large, kidney bean shaped to smaller and nearly round. When dry, the seed contains about 40% protein, 21% oil, 34% carbohydrates and 5% ash.

Source: How to Grow Super Soybeans