Ag Weather Forum

As we enter April, American farmers get antsy to get into their fields to start the new season. The old adage of a 50-degree soil temperature at planting depth to get going with planting has been debated but is generally agreed-upon by most farmers in the U.S to start planting corn.

But, according to Purdue University Extension reports, the 50-degree Fahrenheit mark does not accurately allow the necessary heat to build up around the seed to germinate. For corn, it takes roughly 115 growing degree days (GDDs) for emergence. But those GDDs should be calculated using soil temperature, not air temperature.

Learn more about emergence and soil temperature from Purdue University Extension here: https://www.agry.purdue.edu/…. A rough estimate from this article suggests that average soil temperatures should average about 61 to 62 F for 10 straight days to expect corn to emerge. And hitting 50 F does not mean soil cannot get colder afterward. Quite the contrary, actually. A forecast for continued favorable weather is preferred to ensure good germination.

However, where you measure that temperature, the time of day -- and weather conditions -- can all play a part in the actual number and how it may change over time. We'll go through a few of the ways that soil temperatures are affected by weather conditions to help give a sense of whether planting will be significantly delayed this year, and if so, where.

Most computer models and soil temperature gauges are set at 4 inches (10 centimeters) below the soil surface, but that is below the normal planting depth for most crops. Generally, you should use the early morning temperature, before the sun rises, to get the overall coldest time of day. This gives you a good baseline toward hitting and sustaining that 50-degree mark.

The biggest factor in getting soil to warm up is exposure to the elements. A dense snowpack insulates the ground from the sun and surrounding air, keeping it roughly steady around the temperature of the snowpack above. The thicker the snowpack, the more insulated it is.

Nowhere is this more prevalent than in the Northern Plains and Canadian Prairies, which have had a good snowpack around all winter and have just received another round of heavy snow from a strong storm system this week. Most of the same areas still have that deep snowpack that I reported on a two weeks ago: https://www.dtnpf.com/…. To find the current snowpack, visit https://www.nohrsc.noaa.gov/…. A couple of months'-worth of rainfall is locked away in this snowpack and will take a while to melt off. But as you can see in this satellite loop video (https://www.dtnpf.com/…) from April 1-2, the April sun can be quite powerful, and melting can be quick when exposed.

Coupled with rising temperatures, the snow can melt off rather quickly. A forecast for increasing temperatures is coming to this region this weekend and lasts into next week. Some areas will be warmer than others, but the melting process will commence in earnest. That will likely produce overland flooding, and delay the rise in temperatures further.

That is because water has a higher heat capacity than soil particles. This is easy to imagine if you think about a nice sunny day. If you sit next to a pond and a rock, which one is going to be warmer in the afternoon? You might burn your bottom sitting on that rock, but the pond will feel much cooler. That is an oversimplification but illustrates the point. The same is true for soils. Wetter soils need more energy to increase temperature than drier soils. The energy that goes into the soil is partly used to raise the temperatures, but also works to evaporate water out of the soil. Wet soils are found across the majority of the Corn Belt, Delta, and Southeast due to the active weather pattern during the last several months. It may take some time for soil temperatures to rise and be maintained above that 50 F mark. The southwestern Corn Belt and western Plains have been drier and have responded more to the conditions.

Soil texture also has an effect. Sandy soils warm quicker than clay soils. But sandy soils also cool more rapidly than clay soils. Sandy soils are less dense and have more air between soil particles than heavy clay soils. Therefore, the same amount of energy going into the soil will be spread out in a denser, clay soil as opposed to a less dense, sandy soil, and will take longer for all of those particles to heat up.

The heat comes mostly from the sun. Sunny days lead to a faster rise in soil temperatures than cloudy ones. With the active weather pattern we have seen lately, more areas have been under cloud cover than have had sun.

Soil temperatures also vary quite wildly during the course of a day. The sun causes temperatures to rise rapidly and the lack of it at night causes them to cool. Variation in the soil column is generally greater toward the surface and more steady with depth, but it does vary quite a bit during the course of 24 hours. You can find an example from the Iowa Environmental Mesonet here: https://mesonet.agron.iastate.edu/….

But air temperature plays the most important role. With lower temperatures comes cooler soils, and higher temperatures lead to warmer soils. It is a very intuitive concept that all farmers know. Cooler air has been situated across the West and Northern Plains, partly due to the snowpack, but also due to the conditions in the upper atmosphere.

A trough of low pressure has been a prominent feature for the West, western Canada, and the Northern Plains for quite some time. The cooler air has kept snowmelt down or added to it when systems come by with snow. Meanwhile, the Southeast has had a near-continuous ridge of higher pressure. Temperatures there have been above normal and would be more optimal to begin planting if equipment can make it into the fields between rainstorms.

We are going to see that ridge of higher pressure extend farther west later this week and into next, a sign that springtime temperatures will return to the colder areas of North America. The question will be for how long? Models do predict a trough to move back into the western states by the middle or end of next week. However, the air from this trough is more Pacific in nature instead of Arctic. While temperatures will cool down a bit, they should at least be closer to normal. And if the snowmelt starts to take place, it will continue through the lower temperatures due to the more-intense sun.

In the East, the ridge of high pressure and warm weather will continue and cause soil temperatures to rise higher for the rest of April. This all bodes well for getting into the fields and planting generally on time. However, wetter soils will likely keep temperatures down in the Northern Plains, but if the snowmelt is quick, we can be optimistic about planting being done before it gets too late.

To find updated radar and analysis from DTN, head over to https://www.dtnpf.com/…

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

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