Wallaces Farmer

ISU agronomists are fielding questions about terminating cover crops, best depth to plant soybeans, nitrogen application and more.

Rod Swoboda 1, Editor, Wallaces Farmer

May 6, 2014

6 Min Read

Every Monday morning Iowa State University Extension field agronomists across the state participate in a phone call conference with ISU agronomists on campus at Ames. On Monday May 5 they reported corn planting was well underway, especially in most of western and southern Iowa. And yes, even some soybeans were planted before the wet and cold weather of last week shut down most planting activity across the state.


North central, northeast and east central Iowa were really too cold or too wet to get much corn planted last week, the agronomists reported. But fields were starting to dry out on May 5 and 6 and its likely corn planting will get started again by mid-week in these areas that are lagging behind—unless it rains again!

ISU Extension field agronomist Mark Licht has heard reports of corn that was planted April 11 and other early planted corn that is now starting to emerge. That should be no surprise looking at the accumulated growing degree (GDD) days in Iowa since April 11. It normally takes 90 to 120 GDD from planting to emergence for corn, says Licht. Julius Schaaf, farming near Randolph in Fremont County in Iowa's southwest corner, says as of May 6 corn planting is about finished in Fremont County, where it's been dry all spring. He says a lot of corn is now emerging in that part of the state.

More questions about when to terminate cover crops
Questions the ISU Extension field agronomists have been getting the past week from farmers and crop consultants include two that are being asked most often: 1) What about the timing for termination of cereal rye cover crops? Planting is delayed, should I go ahead and plant the corn into the cover crop and then terminate it? and 2) soybean planting depth. What is the best depth to plant soybeans?


"It is very important to terminate cereal rye cover crops 7 to 14 days prior to planting corn," says Licht. "This still holds true even as weather conditions are pushing fieldwork into May. It is my belief that the penalty for not terminating a cereal rye cover crop 7 to 14 days prior to corn planting is going to be greater than planting corn later."

ISU Extension weed management specialist Bob Hartzler has written a very helpful article in the ISU ICM newsletter regarding cover crop termination. You can read it here. Licht says: "My advice for terminating cereal rye that's getting more than 6 to 8 inches tall is to use appropriate herbicide rates. The taller the cereal rye gets, the harder it is to terminate it."

How deep should you plant soybeans?
Soybean planting depth is another popular question—farmers wonder about planting them to get to moisture. How deep can you safely plant them? Or should you just plant them in dry soil and wait for rain? Licht has recommended soybean planting depths of 1.5 to 2 inches. But he also notes that in the Soybean Growth and Development publication from ISU Extension, the recommendation is to put soybeans at a 1 to 1.5 inch planting depth ideally, and go no deeper than 2 inches.

In another ISU publication, Planting Soybean For High Yields, "we don't talk about planting depth but rather discuss the need to plant into good soil conditions," says Licht. "Generally, soil conditions of concern are soil moisture and temperature. Both soil moisture and temperature should influence planting depth but soil texture and the weather forecast should as well."


In dryer, non-irrigated soils, planting soybeans deeper might be recommended to ensure planting into moist soil, especially if the forecast for rain is slim. However, planting too depth into clay or clay loam soils can result in stand losses, especially if weather creates crusted soil conditions, he adds. Planting too shallow results in greater temperature and moisture fluctuations with minimal time to emergence differences. "Bottom line, planting depth should be approximately 1.5 inches but minor adjustments should be made based on soil moisture, temperature, soil type and weather forecast."

Go ahead and get your corn planted, and apply N later
Some farmers who have been delayed on corn planting due to wet weather this spring, and who don't have their nitrogen applied yet, are going ahead and planting the corn. Rather than applying nitrogen first, they want to get the corn in the ground and will sidedress their nitrogen later. "You can sidedress corn as soon as possible, you can sidedress following the corn planter while the planter tracks can still be seen," says Mark Johnson, ISU Extension field agronomist in central Iowa. "Last year some farmers chose to work on their N application rather than plant corn, and then the rains continued following that short window of opportunity, and planting was delayed even more."

Sudden Death Syndrome aggravated by compaction
Sudden Death Syndrome, a soybean disease, is caused by a fungus that lives in the soil and enters plants through their roots, causing the roots to rot. SDS is more prevalent if soil is wet, cold and compacted during germination and early reproductive stages. "This is another reason you want to avoid causing compaction while planting," notes Johnson.


Soil moisture supply short in northwest, southwest Iowa
Northwest and Southwest Iowa are the driest areas of the state this spring. Winter snow and spring rains have brought less moisture to northwest Iowa than to much of the rest of the state this year. "But, that isn't atypical. Northwest Iowa has the lowest average rainfall amount for the state over the years," says Joel DeJong, ISU Extension field agronomist at Le Mars. "Because of that, there is probably more annual interest in how much moisture is stored in the soil going into each growing season in northwest Iowa than there is in other parts of Iowa."

Paul Kassel, ISU field agronomist at Spencer, along with DeJong, both working with research farm staff in northwest Iowa, each fall and each spring pull and analyze soil moisture levels at several sites in northwest and west central Iowa. These are sites that ISU staff has sampled for many years – some more than 50 years. "So, we have a good idea of what 'normal' spring moisture levels would be," notes DeJong.

"These samples are pulled at one foot increments down to 5 feet below the soil surface – normally considered the rooting depth of corn and soybeans," says DeJong. "However, many of these sites are deep loess soils, and roots can be found over eight feet deep in some locations and some years. These soils can hold up to 11 inches of water for crops to use later when at field capacity – almost half of the crop need for the season. The smaller the amount found in storage, the more important timely rainfall during the season becomes. Long term averages for this time of year are about 6.5 (west) to 7.5 (east) inches of plant available water in mid-April."

About the Author(s)

Rod Swoboda 1

Editor, Wallaces Farmer

Rod, who has been a member of the editorial staff of Wallaces Farmer magazine since 1976, was appointed editor of the magazine in April 2003. He is widely recognized around the state, especially for his articles on crop production and soil conservation topics, and has won several writing awards, in addition to honors from farm, commodity and conservation organizations.

"As only the tenth person to hold the position of Wallaces Farmer editor in the past 100 years, I take seriously my responsibility to provide readers with timely articles useful to them in their farming operations," Rod says.

Raised on a farm that is still owned and operated by his family, Rod enjoys writing and interviewing farmers and others involved in agriculture, as well as planning and editing the magazine. You can also find Rod at other Farm Progress Company activities where he has responsibilities associated with the magazine, including hosting the Farm Progress Show, Farm Progress Hay Expo and the Iowa Master Farmer program.

A University of Illinois grad with a Bachelors of Science degree in agriculture (ag journalism major), Rod joined Wallaces Farmer after working several years in Washington D.C. as a writer for Farm Business Incorporated.

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