Achieving the correct planting depth is more critical for soybean production than it is for corn production for several reasons. Soybean seed requires more water to germinate and soybean emergence is impeded to a greater extent when a soil crust develops.
In general, soybeans should be planted from 1 to 1½ inches deep under most conditions and always placed into at least ½ inch of moist soil. Michigan State University Extension recommends planting at the shallower end of the range under the following conditions: early planting; no-till/high residue situations; and where soils are prone to crusting. Producers should plant at the deeper end of the range when planting late or into coarse-textured soils. Soybean seed can be planted 2 inches deep in coarse-textured soils, but never planted deeper than 2½ inches.
But James Specht at the University of Nebraska summarized a 3-year project evaluating the effects of planting depth and planting populations on soybean yields. The research results have not been published yet, so actual data cannot be presented in this article. However, Specht has given permission to discuss the key results from the project.
Seven planting depths from 1 to 2½ inches deep were evaluated in 2011. In 2012 and 2013, a planting depth of 2¾ inches was added. Each planting depth was evaluated in six populations ranging from 35,000 to 210,000 plants per acre. All plots were planted in 30-inch rows. The plots were planted on April 29, 2013; May 1, 2012; and May 30 and June 4, 2011. A significant soil crust never developed at any of the research trials, so the following results apply to soils that are not susceptible to crusting.
The optimal planting depth across all populations and planting dates was 1¾ inches. This is surprising because the understanding was that planting shallow (1 inch) would be beneficial when planting early. However, the deeper planting depth provides more stable moisture and temperature conditions for the seed than the shallower planting depths. It also reduces the risk of frost or freeze damage to the emerged seedlings when planting early.
Consider having your seed treated with one of the fungicides (metalaxyl or mefenoxam) that provide excellent control of Pythium when planting early or into high residue conditions.
The optimal planting population was determined to be between 105,000 and 140,000 seeds per acre in 30-inch rows. Planting depth had a greater impact on soybean yield when planting populations were 105,000 seeds per acre or lower. Soybean yields were reduced when the lower plant populations were planted at depths of 1 to 1¼ inches. This information is especially beneficial for soybean producers experimenting with lower planting populations this spring.