For Planting Corn, Soil Temperature Of 50 Degrees Is Magic Number

How important is soil temperature for determining when to start planting corn?

Published on: Apr 15, 2013

Agronomists say you shouldn't plant corn until the soil temperature is 50 degrees F at the 4 inch depth. How important is that number when you look at the calendar and see it's the 15th of April?

"That's kind of a magic number, 50 degrees, for planting corn," observes Jim Fawcett, an Iowa State University Extension field agronomist in eastern Iowa. "That's when corn will germinate, with a soil temperature of 50 degrees or higher. The reason for not planting before that is the seed actually will take up water with soil temperatures cooler than 50 degrees. The seed will swell, but it doesn't germinate. That's not a good combination because the seed will tend to rot if it's just sitting there swelling in the soil but not germinating. The seed imbibes the water, takes it in, but doesn't germinate because it's too cold."

EARLY PLANTING TIPS: Soil temperature and field conditions outweigh calendar date when deciding when to start planting corn. It is generally recommended to plant when soil temperatures are at or above 50 degrees F and the near-term forecast shows a warming trend. Planting into cold, wet soils puts stress on corn seed emergence, as does planting just ahead of a cold spell, says ISU agronomist Jim Fawcett.
EARLY PLANTING TIPS: Soil temperature and field conditions outweigh calendar date when deciding when to start planting corn. It is generally recommended to plant when soil temperatures are at or above 50 degrees F and the near-term forecast shows a warming trend. Planting into cold, wet soils puts stress on corn seed emergence, as does planting just ahead of a cold spell, says ISU agronomist Jim Fawcett.

Fawcett adds, "Particularly in April, I would definitely be paying attention to soil temperature before I decide to plant corn."

Especially in April, pay attention to soil temperature before deciding to plant
The week of April 7 Iowa received some much needed rain. In some localized areas of the state the rain came too much, too fast. Are there any concerns that the soil isn't going to be ready to go, when the temperature and the calendar finally say it's time to plant corn? Fawcett and other ISU Extension field agronomists don't think we'll be seeing many planters going this week in Iowa, the week beginning April 14. "We are pretty wet now here in eastern Iowa, and actually do have some excess moisture in places," he noted on April 10. "But I don't hear farmers complaining—not after going through the historic drought of last year."

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.

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Keep in mind that it's still early yet. It's easy to think that Iowa is running late on planting—when you compare this spring to what happened last year when it was so warm in April. The ideal corn planting window is April 20 to May 5 in Iowa. "So we still have time," notes Fawcett. "A lot of farmers have big planters and enough help that they can get their corn acreage planted in 3 to 5 days. If you can do that, there's no sense in trying to push things along before soil conditions are right, no reason to try to get everything planted on April 20. We have plenty of time yet."

How long can seed sit in the ground waiting for soil to warm up to germinate?
How long will that seed wait in the ground? If you plant corn early, say during the first half of April, how long should you expect that seed to be good? "That's part of the problem of putting it in too early," says Fawcett. "You can expect poorer stands and a lower plant population surviving when you do that because the seed will imbibe the water that is in the soil. And if you don't hit that 50 degree soil temperature soon, you will have some seed starting to rot. Also, it increases the chances of having other things happen, like fungal diseases moving in and damping-off occurring. It just puts more stress on the crop when you plant corn when the soil temperature is too cool."

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.

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"If you look at the calendar today, it's too early to be planting corn right now," Fawcett noted on April 10. "But we have received quite a lot of rain this week. I've had close to three inches in my gauge in the Iowa City-Cedar Rapids area this past week and there are some farmers who received more than that in some areas of the state."

Water was flowing out of farm drainage tiles in eastern Iowa earlier this week
He added, "We have field drainage tile running in a lot of fields in southeast Iowa now. You don't have to go too far west of Cedar Rapids though, and you see that those drier areas of the state in central and western Iowa could still use some more rain. It just depends on where you are located, but we are getting on the wet-side of things in terms of soil moisture here in some areas of eastern Iowa."

It isn't fair to make comparisons to a year ago. If you look back over last year's history, 2012 certainly was an abnormal year. This spring Iowa weather has swung back in the other direction to where it is abnormally cool. "But it's not as cool as we're thinking because we're comparing it to what happened last year," he points out.

Are farmers losing some carryover nitrogen? Can you count on carryover N?
ISU Extension agronomists have been saying this past winter that given the drought in 2012, farmers need to do a soil test to check the nitrogen carryover in the top two or three feet of soil this spring. Now, with three inches of rain in areas of the state this past week and a half, are we losing some nitrogen now? Will farmers have to apply more fertilizer nitrogen than they had previously anticipated?

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.

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"We are losing some nitrate-N from the soil due to this rain in the wetter areas that are now receiving rain," says Fawcett. "But I still think there is potential for many fields to have more carryover nitrogen than we have in a normal year just because it was so dry last year. It's hard to guess how much carryover nitrogen is there in the soil. That's why you really need to be pulling some soil samples if you want to adjust your nitrogen fertilizer application rate this spring. And the time to do that would be after this rain—before you apply nitrogen fertilizer in the spring. Then this rain would be accounted for and we could see how much N was still there in the soil."

What about potential for herbicide carryover this year due to 2012 drought?
It's not too late to get a soil sample and have it tested for N content before you plant corn this spring. If you haven't yet applied your spring nitrogen fertilizer it shouldn't take more than a day or two the soil test results back from the testing lab. "You can pull some soil samples and see where you are in terms of Nitrate N content in the soil and see if you can reduced your nitrogen application rate somewhat this spring," says Fawcett. "But for the farmers who haven't received a lot of rain this past week, that's where the potential is for cutting back on your nitrogen application rate this spring."

Any potential for any herbicide carryover this spring--since last year was so dry? "That's another concern, and another reason to not be pushing it on the planting date and not be planting when soil temperatures are cold. Corn may be taking a long time to germinate if it's planted in those cooler than desirable soils. "If we do have a little bit of herbicide carryover from last year's soybeans, you're just putting another stress on the corn you are planting in that field this spring," says Fawcett. "So whatever you can do to make sure you are not putting undue stress on that corn seedling is just going to make the crop better able to handle some potential herbicide carryover in certain fields."

Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.