The vegetable soybean (not the field soybean) can be used like green peas or lima beans or can be dried and used like navy beans or can be sprouted like Mung beans.
Soybeans are the lowest-starch, highest-protein bean.
The soybean is a sprawling bush legume much like a bush snap bean or lima bean. Plants grow 25-60 inches/70-150 cm tall.
Plant a soybean variety recommended for your growing region. A number of varieties have been developed to adapt to certain types of climates. Check with the cooperative extension or agriculture station closest to you for recommended varieties.
For soybean growing success, follow these growing tips:
- Soybeans require heat to produce well. (The soybean is native to tropical Asia.) There are varieties bred to grow in temperate zones, but not in cold regions.
- Soybean growth is optimal at temperatures between 68° and 77°F/20°C and 25°C. Temperatures between 53° to 68°F/12°C to 20°C are sufficient but germination, flowering, and pod development will be delayed. Temperatures greater than 86°F/30°C may hinder growth.
- For the best yield, choose a variety with the greatest number of days to maturity before the first killing frost.
- Warm weather and warm soil are required for the best yield. The soybean plant must make nearly all of its growth before it begins to bloom (blooming comes between 80 and 150 days after germination depending upon variety). Once the plant begins blooming and making seed pods, the plant stops growing up and out. Small plants at bloom time will not be large enough to yield well. (Optimal pod production per plant is about 50 pods—each with 2 or 3 oval-shaped beans.)
- Soybeans are sensitive to cold. Plant soybeans two to three weeks after the average last frost date.
- Do not plant soybeans in cool or cold soil. Pre-warm the soil by placing black plastic over the planting bed for two to three weeks before planting. The black plastic will help draw solar heat into the soil.
- The soil needs to be at least 65°F/18°C for soybean seed germination.
- Plant soybeans in full sun—at least 8 hours of direct sun each day. If you plant soybeans in partial shade you will not get the best yield.
- Prepare the planting bed by adding plenty of aged compost to the soil. Before your first planting turn the aged compost 12 inches (30.4 cm) into the soil. In following seasons, you can spread two inches of compost across the growing bed after each season and allow rain or irrigation to take the compost deep into the soil.
- Soybeans prefer a well-drained soil with plenty of organic matter and a pH of 6.5.
- You can add a commercial organic fertilizer to your growing beds. Use 5-10-10 fertilizer following directions. Avoid fertilizers with high nitrogen; nitrogen promotes foliage growth not bean growth. Use one pound of fertilizer for every 100 square feet.
- Plant seeds 1 inch deep (2.5 cm), 1 to 2 inches (2.5-5 cm) apart in rows 24 to 30 inches (61-76 cm) apart—not less than 12 inches (30.4 cm) apart. When the seedlings are 4 inches (10 cm) tall thin the plants from 2 to 4 inches (5-10 cm) apart. Cut the seedlings with scissors at ground level being careful not to disturb the roots of seedlings left to grow on. Soybean plants can grow close together for support.
- Do not soak seeds in water before sowing; too much moisture can cause soybean seeds to crack resulting in poor germination.
- Do not overwater soybeans after sowing. Once seedlings are up, keep the soil just moist. Do not overwater and do not allow the soil to go dry at the surface until plants are well rooted—at least 8 inches (20 cm) high.
- Water growing soybean plants regularly if there is no rain. Irrigate soybeans at the soil level not overhead. Overhead irrigation can cause flowers and small pods to fall off.
- Some soybean growers treat seeds before planting with a nitrogen-fixing bacteria inoculant to help the plant roots convert organic nitrogen compounds into usable organic compounds. Bacteria already in the soil will multiply quickly once plants are growing, so pre-treatment of seeds is not a requirement. However, inoculation may be helpful where beans have not been grown before.
- When the soil temperature reaches 60°F/20.5°C, add mulch around plants to conserve moisture. Do not add mulch until the soil has warmed.
- Do not handle soybeans when they are wet or covered with dew; this can spread fungus spores and disease.
- Keep weeds away from soybeans but be careful not to cultivate too close to plants; soybeans have shallow roots. Hand-weed when you can and mulch with compost to keep weeds down.
- Harvest soybeans to be used like green beans or peas when the pods are two-thirds to fully mature but before they start to turn yellow. The harvest time for a soybean plant is about 7 to 10 days.
- Beans are shelled—removed from their pods—by parboiling them for 5 minutes in the pods. Parboil a few at a time so that the shell is still warm when the beans are shelled—but the heat should not penetrate to the seed.
- Dry soybeans should be allowed to dry on the vines, but should be picked while the stems are still green—otherwise the shells will shatter and the seed will be lost.