Why’s it so miserably hot? Blame corn sweat

Source: Donnelle Eller, Des Moines Register • Posted: Tuesday, July 26, 2016

It’s not the heat, Iowa, it’s the corn sweat.

Yes, one of Iowa’s most well-known crops is getting blamed for adding to the oppressive humidity that’s making Iowa and big parts of the nation so miserable this week.

How miserable? Well, the heat index through Saturday is supposed to be well over 100 degrees throughout Iowa and big chunks of the Midwest.

No, the nation’s 94 million acres of corn don’t really sweat, said Mark Licht, an Iowa State University Extension agronomist. But they do transpire moisture — or water vapor — as they drink in massive quantities of water through their roots deep in the ground, Licht said.

At the same time, corn plants breathe in air, absorbing carbon dioxide and converting it into sugar that enables them to grow.

Right now is a peak time for water use, with corn plants pollinating and forming rows of kernels within developing ears. The hotter the temps get, the more transpiration — or corn sweat — that occurs, Licht said.

State climatologist Harry Hillaker said Iowa’s 13.6 million acres of corn, and its nearly 9.7 million acres of soybeans, add to Iowa’s humidity.

“All vegetation does, to some degree,” including soybeans, trees, shrubs and lawns, he said.

During the growing season, an acre of corn sweats off about 3,000 to 4,000 gallons of water each day, according to the U.S. Geological Survey.

Iowans grow corn, soybeans and other crops on about 25 million of the state’s 36 million acres. And it led the nation in corn and soybean production last year, according to the U.S. Department of Agriculture.

“We’d be humid, anyway, but we’re more so with the row crops that we have,” said Hillaker, who said the dew point, a measure of humidity, has been “very, very high.”

“The warmer the air is, the higher the capacity it has to hold moisture,” Hillaker said. “That can increase very dramatically as it gets warmer.”

Recent rain adds to the water reservoir to draw from, he said. Some places in Iowa received up to 5 inches of rain this week.

“It’s a cycle that builds on itself,” Hillaker said. “Rain begets more rain.”

The precipitation has helped some parts of southern Iowa move out of drought conditions, the U.S. Drought Monitor showed.

Transpiration from corn is “minimal compared to the evaporation that we’ll have from the soil,” Licht said, especially with recent storms.

“That will drive humidity much more than plant transpiration,” he said.

Hillaker said Iowa’s soils can hold as much as a foot of water, which helps crops grow in the summer. He said Iowa’s humidity isn’t affected much by shifts in the acres planted between corn to soybeans.

“The numbers are pretty stable year to year. It might be a million acres here or there, but percentagewise, it’s not that large of a change,” Hillaker said.

Iowa farmers this year planted the fifth-largest corn crop since the 1920s, USDA estimates show.

Nationally, it’s the third-largest number of corn acres planted since  since 1944.