With soybean planting just around the corner, it's time to consider planting depth. ISU Extension agronomists' recommendations for soybean planting depth can be found in the Soybean Growth and Development publication (PM 1945). Soybeans should be planted at 1 to 1.5 inches deep, but no deeper than 2 inches, they say.
Ultimately, soybean planting depth should be field specific and based more on soil conditions at the time of planting, notes ISU Extension agronomist Mark Licht. The publication, "Planting Soybean for High Yields", does not discuss planting depth but rather desired soil conditions for planting. For example, soil moisture, soil temperature, soil texture and the weather forecast following planting should all be considered when determining planting depth. Licht offers the following observations and suggestions to help farmers make their "depth of planting decision" for soybeans.
Soil moisture and management influence planting depth
Seeds, whether soybean or corn, are exposed to fluctuations of both soil temperature and moisture, and the magnitude of this fluctuation can be influenced by planting depth. Shallow planting depths (less than 1 inch) typically experience greater soil moisture and temperature fluctuations. However, soil temperature fluctuations are less problematic for soybeans due to later date of planting. Soil moisture influences on planting depth are described in greater detail in an ICM News article from 2013, "Soil and Management Factors Influence Seeding Depth".
Soil texture, tillage practices and the weather forecast following planting should be considered, too. Well-drained and lighter soils with low organic matter will dry out more quickly than high organic matter or poorly drained soils, potentially resulting in seeds placed in dry soil. A good rule of thumb is to place seeds into a zone of moisture soil, but not too deep, rather than into dry soil that would hinder imbibition of water by the seed.
Be willing to adjust planting depth for ideal germination in each field
Tilled fields can present the risk of soil crusting, especially with fine-textured soils such as silty clay or clay loam soils, after rain events. Crusted soils can reduce plant populations following planting because the hypocotyls may break from pressure against the soil crust or carbohydrate reserves may be exhausted prior to emerging through crusted soil.
Planting too deep (greater than 2 inches) puts the soybean seed at risk of running out of carbohydrate reserves before emergence and increased risk for infection from soil pathogens. Poor seed quality as well as some soybean varieties are at greater risk of pathogen infection or carbohydrate reserve depletion due to greater time to emergence from deeper planting depths.
A starting point for soybean seeding depth should be 1.5 inches but understand the influence of soil temperature, moisture and texture as well as tillage. Be willing to adjust planting depth for ideal germination in each of your fields.