April 4, 2019
The sequence of events during germination begins with imbibition (absorbing water) followed by the development of the radicle (primary root) and emergence of the cotyledons (seed leaves).
Planting into a moist seedbed with good seed-to-soil contact is important for optimal germination.
Dry or flooded soils, cool temperatures, and lack of oxygen in soils are conditions that can hinder soybean germination and emergence.
Soybean Germination
After planting, a soybean seed will start to absorb or imbibe water and swell, and as a result, its moisture content will change from less than 13% to almost 50% in several hours. Within one or two days under favorable temperatures, the first root (radicle) of the swollen seed emerges through the seed coat (Figure 1) and grows downward to develop the primary root. Lateral roots quickly emerge from the radicle as it elongates and root hairs grow from the radicle and the 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.
Five to ten days after planting the new seedling emerges from below the soil surface. The hypocotyl (seedling stem) begins to elongate and forms a hook that pushes through the soil surface, pulling the cotyledons upward (Figure 1). The hypocotyl can be easily broken if the soil surface is too hard or crusted to push through. Seedlings usually die if the hypocotyl is broken.
The cotyledons (seed leaves) are a temporary source of stored food for the seedling. Shortly after emergence, they open and turn green due to exposure to light and start making additional food through photosynthesis. As the cotyledons 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) The cotyledons drop off shortly after the first set of true leaves is formed.
Conditions that Affect Germination and Emergence
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. 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. When soil temperatures are between 70° F and 90° F, seedling emergence should occur in less than a week. Cold soil temperatures can cause seeds to remain dormant, increasing their vulnerability to seed and seedling diseases and feeding by insects and wildlife. Conversely, soil temperatures above 95° F can also cause poor soybean germination and emergence, resulting in reduced stands.
Oxygen. Because seed respiration increases during the germination process, saturated, flooded, and compacted soils can reduce germination and emergence due to a lack of oxygen.
Sources:
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 4002_S1