When it comes to planting soybeans, moisture and soil conditions matter.
Soybean seeds require more water to germinate than corn and must imbibe (absorb) half of their weight in water before they germinate. That is a lot of water. Corn, on the other hand, can germinate by only absorbing a quarter to a third of its weight in water. Soybeans will imbibe two to five times their weight in water while corn only imbibes 1.5 to two times its weight. So, you can plant corn into drier soils and expect successful establishment, but soybeans are another story.
The critical soil moisture for seed germination is only about 25 to 30% for corn, but 50% for soybeans. A quick way to estimate soil moisture is to try and form a ball between your fingers. At 30 to 50% moisture, a ball of soil formed will feel slightly moist to the touch and form a weak ball while gripped in your hand, but it won’t leave dirt stains. If soil is crumbly, but still moist to the touch, that is also a suitable condition. Drier soil won’t ball up at all and wetter soil will easily ball up and feel muddy.
While having ample moisture available is necessary, seed-to-soil contact is critical for imbibition. That is why planters have press wheels to firm the soil around the seed just enough to establish contact with soil. Water moves through the soil by capillary action and along water films. You can have moist soil, but if the seed is sitting in a ball of residue with no soil contact, then there isn’t a film of water extending from the soil to the seed and it can’t imbibe water. Planters have trash wheels to move trash away before the seed trench is made and the seed deposited.
Growers in Illinois like to till soils to manage residue and blacken it so it warms and dries in the spring. These results are beneficial. However, tilled soils, especially near the surface in the seed zone, will dry out sooner if doesn’t rain with 10 to 14 days before and after planting.
Having not enough moisture available can lead to false germination—the initiation but not completion of germination and then termination and poor stand establishment because of seed desiccation, especially when planting shallow (less than 1.5 inches).
On the other hand, planting deeper than 2.0 inches may also result in poor establishment if heavy rains occur after planting, but before emergence. The hypocotyl structure that breaks through the soil surface and pulls the growing point and cotyledons above ground is fragile and can be damaged. Consequently, the optimum soybean depth range is relatively narrow. Deep enough to hit moisture, but not so deep as to put the hypocotyl at risk.
A seed swelling with water isn’t necessarily an indicator of viability, because dead seed will also imbibe water. But when a seed swells and the two cotyledon seed leaves break apart due to germination, that is an indication of viability.
Agronomist Daniel Davidson, Ph.D., posts blogs on agronomy-related topics. Feel free to contact him at firstname.lastname@example.org or ring him at 402-649-5919.