While farmers keep a close eye on the thermometer and their favorite weather app during planting season, Phil Long says the most important metric right now might be the one they can’t see: the temperature beneath the soil surface.
Long, a regional agronomist with Liqui-Grow, says growers in north-central Iowa are reporting sluggish emergence for corn and soybeans. That’s despite the fact the region accumulated roughly 197 Growing Degree Units (GDUs) from April 10 to May 1, outpacing the 30-year average of 121 GDUs.
“It takes about 130 or so GDUs to get corn or beans out of the ground,” says Iowa-based Long. “So why aren’t more crops emerged?”
The discrepancy, he contends, lies in the difference between air GDUs and soil GDUs. While air temperatures are important, seed reacts almost totally to the heat of the soil surrounding it. For a seed to germinate and push through the soil surface, it requires consistent warmth that hasn’t materialized during recent chilly conditions in some areas.
“What’s most important to the corn and beans out there in the ground is soil GDUs,” Long says. “Even corn up to V6 is regulated primarily off the heat in the ground.”
Why Some Crops Have ‘Just Sat There’
The formula for calculating GDUs relies on a base temperature of 50 degrees Fahrenheit and a ceiling of 86 degrees. When nighttime temperatures dip into the 30s, as they have recently in Iowa and parts of the Eastern Corn Belt, the soil temperature can linger in the 40s and 50s. At those levels, the “heat engine” for the seed essentially stalls.
“We’re not getting that soil temperature up there very far,” Long explains. “That does not stack up GDUs very quickly.”
Long notes that along with the chilly weather conditions, two additional factors can act as “buffers” against soil warming: crop residue and cloud cover.
While heavy residue is often a benefit in the heat of July, it can act as an insulator in the spring, preventing the sun from reaching the soil. In some cases, high-residue fields can see a 50% reduction in GDU accumulation compared to conventionally tilled ground, Long notes.
Furthermore, a stretch of overcast days will rob the soil of solar radiation.
“If there’s heavy cloud cover, that can reduce solar radiation by 80%,” Long says. He explains that even on a cool 55-degree day, direct sunlight can push soil temperatures much higher. But persistent clouds have kept the ground locked in a cool cycle.
As planting continues into the heart of May, Long advises farmers to look beyond the air temperature and keep in mind the micro-climate of the seedbed as they plant.
“Although we’re ahead in terms of air temperature GDUs for this year compared to the ‘average’ year, we’re probably behind in terms of those seeds sitting in the ground,” Long says. “That soil GDU is a big factor when it comes to getting crops out of the ground.”
