Seeds are living organisms in a state of quiescence, meaning that the seed is at rest until desirable conditions occur that trigger germination. Soybean seeds can be stored for years in cool, dry conditions without a significant reduction in viability.1
Upon being placed into the soil, the seed begins to absorb or imbibe water, and as a result, starts to swell. When enough water (approximately 50% of the seed’s weight) is taken in and with favorable temperatures, the radicle (primary root) breaks through the seed coat and rapidly develops into the primary seedling root. Lateral roots quickly emerge from the radicle as it elongates, and root hairs grow from the radicle and lateral roots. Root hairs are barely visible and should not be confused with later developing and easily seen branch roots. The root hairs become the main water- and nutrient-absorbing structures.
Soon after the radicle appears, the hypocotyl (seedling stem) starts elongating and forms a hook that pushes toward the surface. The cotyledons (seed leaves) are attached to the hypocotyl and progress upward with the growth of the hypocotyl. The hypocotyl can be easily broken if the soil surface is too hard or crusted. If the hypocotyl breaks, the seedling usually dies.
When the hypocotyl emerges it straightens, and in the process pulls the cotyledons out of the soil. The cotyledons start turning green from exposure to light, and as they open the epicotyl is revealed. The epicotyl contains the main growing point including the first true leaves, which are unifoliate (one leaf blade attached on opposite sides of the stem at the same node) (Figure 1).
Moisture. Planting into a moist seedbed with good seed-to-soil contact is necessary as moisture needs to move into the seed for germination to occur. If irrigation is required for good soil moisture, it should be applied ahead of soybean planting, and not immediately after planting. Planting into dry soil with rainfall or irrigation occurring too soon after can result in soil crusting and poor soybean emergence.
Soil Conditions. Soil crusting can delay or prevent seedling emergence and cause soybean hypocotyls to become swollen or break when trying to push through the crust. Fields with fine-textured soils, low organic matter, and little surface residue can be vulnerable to crusting, especially where excessive tillage has taken place.
Temperature. Soybean seed can begin to germinate when soil temperatures are less than 55° F; however, germination is likely to be slow until soil temperatures warm to the upper 70s. Cold soil temperatures can cause seeds to remain dormant, increasing their vulnerability to seed and seedling diseases and feeding by insects and wildlife. When soil temperatures are between 70° F and 90° F, seedling emergence should occur in less than a week. Soil temperatures above 95° F can also cause poor soybean germination and emergence, resulting in reduced stands.
Oxygen. Saturated, flooded, and compacted soils can reduce germination and emergence due to a lack of oxygen. Soil pore spaces filled with water have less oxygen available for seed respiration. Compaction reduces the availability of water and oxygen required for germination, root and plant growth, and nutrient uptake.
1Soybean as a crop. Modern corn and soybean production. MCSP, http://www.mcsp-pubs.com. Other sources: Hoeft, R.G., Nafziger, E.D., Johnson, R.R., and Aldrich, S.A. 2000. Modern corn and soybean production. First edition. MCSP Publications. Champaign, IL. Pedersen, P. 2007. Soybean growth stages. Soybean growth & development. PM 1945. Soybean Extension and Research Program. Iowa State University. http://extension.agron.iastate.edu. Pedersen, P. Soybean planting date. Iowa State University. http://extension.agron.iastate.edu