By Mike Staton, Extension Soybean Educator
Soybean seed is very sensitive to planting depth, so growers need to be careful and get their crop off to a good start. Planting depth surveys revealed that only 20% of the fields planted with drills were planted at or near the intended depth. An even bigger concern is that in 68% of the fields, the seed was planted too deep, a condition known to delay emergence.
Under most conditions, soybeans should be planted between 1 and 1.5 inches deep. As a general rule, plant at the shallower end of the range under the following conditions:
- Early planting
- High residue conditions
- Fine-textured soils
- Moist soils
Planting at the deeper end of the range is recommended under the following conditions:
- Late planting
- Coarse-textured soils
- Dry soils
Soybean seed can be planted up to 2 inches deep in sandy soils.
Adequate soil moisture is the most important factor affecting soybean germination. The seed must imbibe (take-in) 50% of its weight in moisture in order for the germination process to begin and remain above 20% 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. This may require planting deeper than 1.5 inches under dry soil conditions.
If you must plant deeper than 1.5 inches in order to place the seed into uniform moisture, make sure the variety has an excellent emergence score or long hypocotyl.
Also, consider seed size when deciding how deep to plant. Large seed contains more stored energy and should be able to emerge from greater depths than small seed. This is true when large seed is planted in coarse-textured soils. However, the larger cotyledons on large seed are more difficult to pull through a soil crust, so plant large seed shallower when planting into soils prone to crusting.
Planters typically provide better depth control than drills or air seeders. However, depth control on drills and air seeders equipped with gauge wheels mounted on single disk openers can be greatly improved. A cheap and effective option is to reconfigure the planting units on drills or air seeders set up on 7.5-inch rows so that none of the gauge wheels run over the old corn row. This procedure is discussed in more detail in the article, “Reconfiguring planting units on no-till drills to improve soybean planting performance.”
Taking time to check planting depth is important regardless of the planting equipment used. I visited a field where a planter had failed to place the seed at the correct depth. The units mounted to the center frame of the planter placed the seed at 1.5 inches deep and into moisture while the units mounted to the outer wings planted the seed 1 inch deep and into dry or marginal moisture conditions. The shallower seed emerged 2 weeks later than the deeper seed and yielded 1.6 bushels per acre less.
Last spring, I visited a sandy field of drilled soybeans that were emerging slowly and unevenly. In this case, most of the seed was planted too deep (see photo). The two plants on the left were planted at least 1 inch deeper than the emerged plants on the right and the seed was placed about 3 inches deep. The variety had an excellent emergence rating and many of the plants were able to finally emerge, but with lower energy reserves and vigor than the plants that were planted at the correct depth.
Adjust your soybean planting equipment as soil and crop residue conditions change and dig up seed frequently to verify that it is placed at the intended depth and into at least 0.5 inch of moist soil.