After two years of the Precision Planting/Indiana Prairie Farmer study at the Throckmorton Purdue University Research Center near Romney, one conclusion seems apparent. While it's not backed by statistical significance in every case, it appears that planting 4 inches deep is better than planting one inch deep for corn.
One year ago the study included three depths- one, two and three inches. The two and three inch depths were superior and were significantly better in yield than the one-inch depth. Jeff Phillips, Tippecanoe County Extension educator, theorized that for corn planted on May 30 at an inch deep, the soils dried out just enough that some of the kernels that were only one –inch deep started to germinate and ran out of moisture. Rain came about a week after planting.
This past year there were two plantings- May 20 and June 8. This time the researchers chose to include 4 inches deep, hoping to find the limit that would affect emergence, standard deviation of plant spacing, and most importantly, yield.
They didn't find it. The 4 -inch depth did as well as anything else, and better than the one-inch depth, although differences weren't significant. This year the one-inch depth, while slightly lower-yielding, wasn't significantly lower than the other depths, as it was a year ago.
The kicker, of course, is that soils were warm at the 4 -inch depth. Weather data collected at the farm indicated bare soil temperature as high as 104 degrees F at the 4-inch depth on the June 8 planting date. The air temperature that day maxed out at about 94 degrees. Temperatures at the 4- inch depth under sod at the same sight were slightly lower.
"What happens is that soils heat up and hold the heat," says Ken Scheeringa, assistant state climatologist. He likens it to the phenomenon that happens at the Indy 500 car race, where the black racing surface may be much hotter than the air temperature during the Memorial Day Race, especially in years when it's warm to hot and the sun is shining.
The temperature at the four-inch depth was some 20 to 30 degrees cooler at the May 20 planting, but corn still emerged from the 4-inch depth without issues. It leaves one thinking that it's better to error planting deep than shallow, Phillips notes.What remains to be seen is what happened to kernels planted at deep depths early in the season. While testing that concept was the goal this year, the weather didn't cooperate, and the plots were still planted later than hoped.