Factors that effect soybean germination and emergence

As a dicot, the soybean embryo has two cotyledons, which are the first leaves that develop within the seed. Temperature, moisture, oxygen, and soil conditions within the seed zone can affect soybean germination and emergence. The radicle root is the first part of the embryo to penetrate the seed coat followed by elongation of the hypocotyl, which pulls the cotyledons and epicotyl to the soil surface.

Seeds are living organisms that respire at a very low rate. They are in a state of quiescence or rest, where they stay dormant until desirable conditions that trigger germination occur. Seeds can stay viable for a year under cool, dry conditions.¹

Germination Process

Soybean seed germination is referred to as “epigeal”, because food storage structures (cotyledons) are pulled above the soil surface. In contrast, corn germination is considered “hypogeal” because the storage structure remains below the surface when the seed germinates.

Soybean seed can begin to germinate when soil temperatures are approximately 10° C (50° F); however, germination is likely to be slow until soil temperatures warm to near 25° C (77° F).² 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 favourable temperatures, the radicle breaks through the seed coat (Figure 1) and rapidly develops into the primary seedling root, which has the ability to force the cotyledons toward the surface (Figure 2).³ 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 absorbing structures. Soon after the radicle appears, the hypocotyl starts elongating and forms a hook that pushes toward the surface (Figure 3).

The cotyledons are attached to the hypocotyl and progress upward with the growth of the hypocotyl.⁴ 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 3). The cotyledons start turning green because of their exposure to light and as they open the epicotyl is revealed (Figure 2). The epicotyl contains small leaves, buds, and the growing point.

Factors 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 occurring too soon after can result in crusting and poor soybean emergence.

Soil Conditions. Soil crusting can delay or prevent seedling emergence and cause soybean hypocotyls to be swollen or broken 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. Cold soil temperatures can cause seeds to remain dormant, causing them to become increasingly vulnerable to feeding by wildlife that dig up the seeds, insects, and seed/seedling diseases. Once emerged, soybeans can tolerate a temperature dip down to -2.8° C (27° F) for a short period of time.³ However, the growing point of a soybean plant is near the top and newly emerged plants could suffer permanent frost damage with a late spring frost.

Oxygen. Saturated, flooded, and compacted soils can reduce germination and emergence due to the lack of oxygen. Soil pore spaces filled with water reduces the amount of oxygen available for seed respiration. Compacted soil reduces the availability of water and oxygen required for germination, root and plant growth, and nutrient uptake.

Summary

Germination is the time after a soybean seed is planted into the soil and before the young seedling emerges above the soil surface. With good seed-to-soil contact and favourable soil temperature, moisture, and oxygen concentration, seeds can germinate and seedlings can develop faster.