Research repeated two years in a row proves that at least with warm soil, corn planted four inches deep emerges nearly as well and performs as well as corn planted at two or three inches deep. Sometimes it performs significantly better than corn planted one inch deep, depending upon conditions.
However, the same trend doesn't translate to soybeans. Shaun Casteel, Purdue Extension Soybean Specialist, has outlined in Indiana Prairie Farmer in the May and June issues that planting soybeans more than two inches deep trying to get to moist soil is risky, even when soils are warm. He does not recommend planting deeper than two inches below the soil surface for soybeans.
The difference is in how the plants germinate, he says. Corn produces a shoot, the coleoptile, that moves up through the soil and penetrates the surface. Starch within the kernel of the seed can keep it going until roots supply the energy needed. There is not bending needed by the corn plant. The only time it might get in trouble is if the surface is severely crusted, or if an herbicide reaction causes leafing out underground. Otherwise, it can emerge from as much as four inches deep.
Soybeans, on the other hands, are dicots, not monocots, like corn and most grasses. So the soybean splits in half and the hypocotyl, the part of the soybean stem that develops, must pull the halves, the cotyledons, above the ground. This requires force and is also difficult if the soil surface is hard or crusted. In those situations soybeans sometimes break their necks, which is the arched part of the hypocotyl, before it can pull the cotyledons through the soil. That plant is then done and will not produce a viable plant.