My own random use of a soil temperature probe in April has made me wonder whether this was really true, but I am a believer that corn should be planted at 2" deep, (more on that later) so I did not care what the temperature was like at 1".
As food for thought, along comes a study presented in the Pioneer Agronomy Journal discussing soil temperatures vs soil depth during the months of April and May.
Fascinating stuff. During the month of April the soil temperature was the same at 2" as it was at the shallower .4". Could it be that what we believe to be true is false? The relationship was the same no matter whether the soil was tilled or not. Tilled soil was slightly warmer than no-tilled which makes sense, but in April the depth made no difference. It was not until May when the air temperature warmed to 25-28C that the shallow soil began to warm faster. Why would the soil not be warmer at .4" in April?
Two reasons. Sunlight intensity and air temperatures are lower in April. The second reason is the deeper soil acts as "insulation". In April the soil is usually drying slowly and moisture is migrating up from lower depths. This water is still cold and it keeps the shallow soil from warming as quickly as we think it should.
Now the practical stuff. What impact does this have on planting depth decisions when we plant in April? As I said before I am a firm believer in planting at 2" deep regardless of conditions. The only time I would go deeper would be in extremely dry seedbeds, but I would never go shallower. Here's why.
This clearly shows the nodal roots getting off to a good start. Note how they start to grow at about 3/4 inch below the soil surface.
The seed on the right which was planted at 1" is going to struggle with nodal root development and will be slow to grow. As farmers we make more mistakes planting crops too shallow than we do planting them too deep.
Considering soil temperatures do not support shallow planting in April, we have no excuse not to get our seed into the 2" zone.