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, says agronomist Dan Davidson, Illinois Soybean Association. 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.