Cold soil, delayed planting
Brian Lacey would like to be done planting small grains by the end of April. This spring, however, the Wendell, Minn., farmer doesn’t expect to even start planting until late April.
“We’re not worried. But we’d like to be further along,” he says.
Planting of wheat and other crops is off to a slow start in the Upper Midwest, in large part because soils aren’t warm enough yet.
Wheat, the first of the region’s three major crops to be planted, needs a soil temperature of about 40 degrees to germinate. Lacey and other area farmers often plant wheat before soil temperatures reach that level, especially when they’re reasonably confident the air temperature soon will be high enough to pull up soil temperatures to 40 degrees or above.
“With wheat, we go by the calendar more than soil temperatures,” he says, noting low soil temperatures are a bigger factor when planting corn and soybeans, the region’s other two major crops.
Low soil temperatures are particularly common this spring in northwest Minnesota, northern South Dakota, North Dakota and eastern Montana, where soil temperatures were still stuck in the low and mid 30s in the middle of April.
For instance, average bare soil temperatures on April 17 stood at 32 degrees in Baker, Mont., 30 degrees in Britton, S.D., 32 degrees in Minot, N.D., and 31 degrees in Humboldt, Minn., according to the North Dakota Agricultural Weather Network.
The long, cold winter gets most of the blame.
“There is a deep layer of frost, which is affecting soil temperatures,” says North Dakota State Climatologist Adnan Akyuz.
Unusually cool weather the week of April 13 didn’t help, either. Many areas of the Upper Midwest had freezing temperatures in the first half of the week, which limited how much soil temperatures could rise.
“I had thought we might get started with field work in this area by April 25. But after what’s happening this week (the week of April 13), I think it will be into May before we get started,” says Keith Brown, extension agent in northwest North Dakota’s Divide County.
Recently, he used a soil probe to measure soil temperatures. “There were places I couldn’t even get the probe in 6 inches” because of frost in the soil, he says.
The rule of thumb, according to information from the North Dakota State University Extension Service, is that spring wheat, durum, barley, canola, mustard, safflower, field pea and lentils germinate at about 40 degrees, with corn, dry beans and soybeans germinating at about 50 degrees. Optimal soil temperatures for germination and emergence are about 5 to 10 degrees warmer.
Farmers don’t necessarily wait to plant until soil temperatures have reached those levels. But planting into cool soil means a longer wait until germination, which increases seeds’ exposure to soil pathogens. The concern is greater for corn and soybeans than wheat, Lacey and others say.
Soil that’s thawed responds much more quickly than frozen soil to changes in air temperature, says Laura Edwards, climate field specialist with South Dakota State University Extension.
Thawed soil warms up faster when air temperatures rise, but also cools faster when the air temperature drops below freezing, she says.
Duration is another factor in how much and how fast air temperature pulls up soil temperature, she says.
A long stretch with modestly warm air temperatures often does more to warm the soil than a short stretch with high air temperatures, she says.
Sunlight also helps warm the soil. For instance, soil temperatures will rise more during a sunny 60-degree day than an overcast 60-degree day, she says.
Air temperatures across much of the Upper Midwest are expected to be warmer this week, she says.
In general, Upper Midwest farmers would do well to delay planting corn until soil temperatures rise, Edwards says.
A number of organizations provide information on soil temperatures in the Upper Midwest.
The Minnesota Department of Agriculture: www.mda.state.mn.us/pro tecting/soilprotection/soiltemp.aspx.
The South Dakota Office of Climatology: www.climate.sdstate.edu.
The National Weather Service: www.weather.gov.
The High Plains Regional Climate Center: www.hprcc.unl.edu.