Planting depth for corn is extremely critical for proper development of the nodal root system. If the nodal root system (principal root mass for the corn plant) is located too close to the soil surface, then the nodal root system can be greatly reduced in size. The impact of a smaller nodal root system can be reduced nutrient uptake.
In recent years past, I have observed with a great frequency that corn planted at 1.5 inches or less is having a problem with nodal root development. I would recommend that corn be planted at 2 inches of depth and that soybeans be planted at only 1.0 to 1.25 inches of depth.