In early April, producers are eagerly awaiting or have already started their spring planting. Though other considerations like soil moisture, air temperature, and the weather forecast play important roles, the adage of waiting for morning 4-inch soil temperatures to hit the 50-degree-Fahrenheit mark is important, too. That line is currently crossing into the southern Corn Belt, and the coming forecast should bring that northward throughout the next week.

The 50-degree mark is considered the minimum for corn and soybean seeds to germinate, though as the University of Nebraska-Lincoln pointed out, corn can withstand soil temperatures down to 40 degrees F without damage. (https://cropwatch.unl.edu/…)

The higher the temperature, the faster crops develop and emerge from the soil. According to Iowa State University, corn planted at soil temperatures at 50 degrees F take 21 days for emergence, while it only takes seven days at 70 degrees. (https://crops.extension.iastate.edu/…)

Soil temperature therefore affects development and consistency in stands. Small differences in temperature can have larger effects in total corn stands at lower temperature than at higher temperature. On the other hand, planting early has also been shown to increase yields for various reasons, but especially because the plant can capture more sunlight for a longer period of time. (https://cropwatch.unl.edu/…)

The 50-degree line should be advancing northward during the next week. The current forecast calls for above-normal temperatures for much of the United States through at least April 15. However, cold air residing in Canada may halt or even reverse that line back toward the south.

Soil temperatures increase and decrease with fluctuations in energy input. Rises can come from increasing air temperatures, with conditions being fairly warm during the last several weeks, or by increasing sunlight, which is now increasing rapidly as we are past the spring equinox. That gets soil out of the cold and into the right conditions for seeds to germinate and roots to grow.

However, falling temperatures and frosts are still significant risks in April, especially for the northern and central Corn Belt. Colder air is forecast to move down through the northern tier of the U.S. behind a system starting April 16. Models are still unsure of how cold it will be, or how far south that cold air may get, but it may produce effects that are detrimental to planting.

If the 50-degree mark pushes north into the Dakotas and Minnesota during the next week, it could quickly fall below that again. A consistent temperature over 50 degrees F is more important than where that line is on any given day. As an example, central Iowa consistently rises and stays above 50 degrees F by the end of April. But even on 14% of the years, or about one in seven, soil temperatures fall below 45 degrees F on April 30. The Iowa Environmental Mesonet has a handy table for those living in Iowa that showcase how often soil temperatures fall below a certain threshold. You can view that here: https://mesonet.agron.iastate.edu/…. For those outside of Iowa, you can infer similarities based on your proximity. If you are north of Iowa, you can reasonably expect temperatures to be lower for longer than the table. If you are south of Iowa, you can reasonably expect temperatures to be lower for shorter than the table.

Another risk is what happens after emergence. Low temperatures can produce frosts and damage crops, particularly cotyledonous plants like soybeans, where the growing point emerges above the soil surface. Though soil temperatures may support planting and the short-range forecast may favor it as well, late frosts could be an issue, especially for those across the north. Some long-range forecasts call for widespread significant cold bursts in early May that may cause frosts deep into the Corn Belt.

While planting early when soil warms may maximize yield potential, it should be balanced with frost risks and crop insurance eligibility dates.

You can always check your average last-frost date here: https://www.ncei.noaa.gov/…. There is a long way to go for most areas across the north.

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John Baranick can be reached at john.baranick@dtn.com

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