- After planting a soybean seed will absorb about 50% of its weight in water and the radical or primary root will emerge.
- The cotyledons are the first leaves to appear and serve as food storage structures until the true leaves emerge.
- Soybean emergence depends on the temperature, moisture, and oxygen levels within the seed zone.
The Germination Process
Once planted into the soil, the seed begins to absorb water and as a result, begins to swell. When enough water (approximately 50% of the seed’s weight) is taken in and under favorable temperatures, the radicle breaks through the seedcoat (Figure 1) and rapidly develops into the primary root. Lateral roots quickly emerge from the radicle as it elongates and root hairs grow from the radicle and lateral roots. The root hairs become the main water- and nutrient-absorbing structures. Root hairs are barely visible and should not be confused with later developing and easily seen branch roots.
Soon after the radicle appears, the hypocotyl or “stem tissue” starts elongating and forms a hook that pushes toward the surface. The cotyledons 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 (Figure 1). Once the cotyledons reach the surface, they start turning green due to exposure to light and begin to open to expose the epicotyl, which contains the main growing point.
Contained in the epicotyl are the first leaves, which are unifoliate (one leaf blade attached on opposite sides of the stem at the same node) (Figure 1). They open quickly to start the photosynthetic process. After about a week, the cotyledons start to dry and drop off the plant because their stored energy has been depleted. Energy production is now dependent on photosynthesis by the leaves.
Challenges to Germination and Emergence
Soybean seedling emergence depends on the temperature, moisture, and oxygen levels within the seed zone. 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 inhibit soybean germination and emergence, resulting in reduced stands.
Planting into a moist seedbed with good seed-to-soil contact is necessary so moisture can 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 too soon after can result in crusting and poor soybean emergence.
Saturated, flooded, and compacted soils can reduce germination and emergence due to a lack of oxygen. When soil pore spaces are filled with water, less oxygen is available for seed respiration. Compacted soils have reduced availability of water and oxygen required for germination, root and plant growth, and nutrient uptake. Soil crusting can delay or prevent seedling emergence and cause soybean hypocotyls to become swollen or break when trying to push through the crust. If the hypocotyl breaks, the seedling usually dies. Fields with fine-textured soils, low organic matter, and little surface residue can be vulnerable to crusting, especially where excessive tillage has taken place.
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1Hoeft, 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