The standard soybean planting depth has been about 1.5 inches for a long time now. But recent data published in the last couple of years show that soybeans can be planted deeper, and that is a benefit.
Last spring (2014) Jim Specht, retired professor at the University of Nebraska, released results from his three year study which showed that 1.75 inches was the optimal depth regardless of soil conditions, planting date, population or tillage system. When I looked at his raw data, I felt that the depth ranged anywhere from 1.75 to 2 inches.
The agronomic theory holds that deeper planting depth has more stable moisture and temperature conditions than shallower planting depths while reducing the risk of freeze damage. For a long time producers and agronomists alike held to the belief that the correct planting depth was 1 to 1.5 inches. The rule of thumb was to plant deeper when soil is drier or lighter and shallower when soil is colder and wetter and heavier. But in general, the rule of thumb has been 1.5 inches.
Specht himself was surprised by the results and said that he used to recommend planting at 1 to 1.25 inches when planting early and 1.25 to 1.5 inches when planting late. But he realized that varieties and seed characteristics have changed.
What was more intriguing about Specht’s data was that planting at 1.75 inches was optimal regardless of plant population (30,000 to 210,000), tillage systems (no-till, conservation till or strip till) or planting date (early and late). But when planting at 210,000, depth isn’t important since there is a surplus of seeds. And planting at 30,000 to 70,000, planting deeper is better since there are so few seeds that getting each one to emerge counts.
So why does planting deep work? The seed is placed deeper into more consistent moisture and temperature conditions and further away from any residue, resulting in a better stand that emerges more evenly. Specht pointed out in our conversation a year ago that a soybean seed requires 50 percent more moisture to germinate than a corn seed. And today’s soybean seed has better tolerance of cold conditions and can withstand cooler and wet no-till soils and still emerge just fine. And we can’t forget the value of seed treatments in protecting the seed at this juncture of germination and emergence.
So the data and agronomic reasoning support planting deeper. Are you ready to take the plunge to 1.75 inches? How deep are you planting soybean seed today? Maybe it is time to consider going deeper than one and a half inches.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at firstname.lastname@example.org.