Agronomy: Soybean Planting Depth

Not too late to plant deeper!

For a long time producers and agronomists believed that the correct planting depth for soybeans is 1 to 1.5 inches while corn is 1.75 to 2.25 inches.

Soybeans can’t be planted as deep as corn because they generally do not have the strength in their hypocotyl to push out of the soil. Two other factors are that soybean seed requires more water to germinate and emergence can be more easily impeded when a soil crust develops. The rule of thumb is to plant deeper when soil is drier or lighter (sandy), and shallower when soil is colder, wetter and heavier (clays). But in general the practice has been 1.5 inches.

A report summarizing a three-year research project conducted at the University of Nebraska showed that the optimal planting depth is now 1.75 inches today regardless of planting date or populations. The thinking is that a deeper planting depth has more stable moisture and temperature conditions for the seed than the shallower planting depths, reducing the risk of freeze damage. Of course if you are planting early or deeper, you probably need to make sure to treat the seed.

For a long time the recommendation was to plant corn 1.75 inches deep, but growers have increasingly been putting seed down at 2 or 2.25 inches while planting earlier. We have to remember today’s corn seed lots have better cold tolerance and can be planted deeper and still have the strength and vigor to emerge. Also, seed treatments will protect the seed and seedling while this is happening.

So today some soybean growers also are increasing the depth of planting soybeans and it seems to be working fine. Soybean seed today has better tolerance of cold conditions and can withstand cooler and wetter no-till soils and still emerge just fine. And when treated with multiple fungicides and an insecticide, it can withstand harsher conditions and deeper planting depths.

Maybe the fact that seed companies have been selecting varieties in part based on the strength of the seed lot under no-till inadvertently means that these same seeds can be planted deeper under more stable conditions resulting in more even emergence.

So how deep do you plant your soybean seed? Are you planting a quarter or half-inch deeper than a decade or two ago? And just maybe—it is time to plant deeper.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.