By Dr. Harold Willis
If you want to replant your own soybean seeds, first, save out the needed amount. Seed should be dried to about 13% moisture and kept in ventilated containers (cloth bags, cardboard boxes, or glass jars with cheesecloth covers). It should be stored in a dry, ventilated area at cool temperatures (not higher than 70 degrees F. or lower than 32 degrees F.). Keep away from mice and rabbits.
Seed should maintain a good germination rate for the first year (80 to 85%), but after the second year of storage, germination may drop to 65%. Test the germination rate before planting.
Note on 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.
Source: How to Grow Super Soybeans