Soil and Sun
Although tolerant of poor soil, soybeans will produce better if sown in fertile soil that is loose and well-drained (excess water encourages rot). Adding humus in the form of well-rotted leaf mold, aged manure or organic compost helps create the proper soil conditions. Always use legume inoculant. The ideal soil pH is 6.0 to 6.8. Your garden should be in full sun for best soybean production.
Like most legumes, soybeans can fix nitrogen from the soil. They also need adequate potassium and phosphorus. In the past, gardeners did not add supplemental nitrogen, although using an inoculant to promote growth of beneficial bacteria has always been recommended. Today’s higher-yielding soybeans, however, may benefit from nitrogen. Use an organic slow-release 10-10-10 fertilizer and place it eight inches deep.
Gardeners may do better with older heirloom varieties rather than commercial varieties, which often require pesticides and herbicides. Possible choices include:
- Beer Friend – Japanese, 70 days.
- Black Jet – for cooler climates, 90 days.
- Korean Brown – two seeds per pod, 100-110 days.
- Ohozyu – very high protein, 105-120 days.
- Velvet – high-yielding, 120 days.
When to Plant
Soybeans can germinate at slightly cooler soil temperatures than most beans – around 50°F (10°C). However, they are still warm-season plants and will germinate best at about 77°F (25°C). Sow soybeans at least two weeks after the last frost. If spring is cooler than usual, cover the soil with black plastic to increase the soil temperature before planting. Stagger planting times to ensure a continuous harvest.
Spacing and Thinning
Soybean plants (all are bush varieties) are about two feet tall when mature. They also take a little longer to germinate than most beans. Sow the seeds about one to two inches deep and two to four inches apart. If necessary, thin to four to six inches apart depending on how fertile your soil is – high fertility, closer spacing. Don’t pull plants, cut with a sharp pair of scissors.
It’s important to harvest soybeans before the pods begin to turn yellow unless you want to use them as dry beans. Look for full, plump pods that are about two to three inches long, which means the beans are about half mature. All the beans are ready at the same time, so pull the plant. You can shell the beans for cooking or cook pod and all – do not eat them raw.