The lack of significant rain this spring has allowed soybean planting to progress rapidly. In fact, 42% of the soybeans in Michigan have been planted according to the May 10, 2021, USDA Crop Progress and Condition report. This is 30% more than the 2016-2020 average. However, the lack of rain has also increased the potential for inadequate soil moisture to affect soybean germination and emergence. Because of this, planting depth will be an even more important management decision if the current weather trend continues.
Adequate soil moisture is the most important factor affecting soybean germination. The seed must imbibe (take in) 50% of its weight in moisture for the germination process to begin and remain above 20% moisture after the seed swells and the seed coat splits. This is why agronomists recommend planting soybeans into at least 0.5 inch of moist soil to ensure adequate moisture is available to complete the germination process. In theory, smaller seed should be able to germinate and emerge in drier soil conditions than larger seed.
When planting in dry soil conditions, producers have three planting depth choices:
- Plant deeper to place the seed into 0.5 inch of uniformly moist soil.
- Plant at your normal depth even though soil moisture is marginal and variable.
- Plant into dry soil even if this means planting slightly shallower than normal.
Planting seed deeper to reach uniform moisture is a viable option under most conditions and may be the best choice as planting can continue and germination is not delayed. Soybean seed can be planted up to 2.5 inches deep in most soils. Contact your seed dealer to determine the maximum planting depth for the varieties you intend to plant.
Soils that are prone to crusting pose the greatest risk with deep planting as the pushing power of the seedlings decreases as the hypocotyls elongate. Some producers may consider setting their row cleaners to run a little deeper to throw some dry soil out of the row, enabling them to plant into moisture without placing the seed deeper than 2.5 inches. This practice is an option for relatively flat, well-drained fields. However, it can create erosion in the rows in fields having slopes of 5% or more.
Planting into marginal and variable soil moisture carries significant risk as the seed may have enough moisture to initiate germination but not complete it. This scenario should be avoided unless rainfall is imminent within 24 to 48 hours of planting.
Planting seed a little shallower into soil that is too dry for the germination process to begin is also a viable option. The seed will remain dormant until a significant rain occurs. This practice allows planting to progress without the higher risk associated with placing the seed into marginal and variable moisture. The downside of this practice is that germination is delayed and is completely dependent on moisture from rainfall.
Because planters provide better depth control than box drills or air seeders, they should improve germination and emergence when planting into dry soils. Planters also provide better seed-to-soil contact and are often equipped with row cleaners and seed firmers. Planters will perform better than drills or air seeders when planting deep due to increased down pressure control.
Planting depth decisions are always important. However, they become even more important in dry soil conditions. Use the information provided in this article and consult your seed and equipment dealers for specific recommendations regarding your varieties and planting equipment. Also, remember to check your planting depth frequently to ensure you are placing the seed at your intended depth.
This article was produced by a partnership between Michigan State University Extension and the Michigan Soybean checkoff.