Corn and soybean planting progress has been slow so far in Illinois, with 7 percent of the corn crop and 5 percent of the soybean crop planted by May 1. These numbers should increase modestly by May 8, but this will not be an early-planting year. With warm temperatures returning next week, planting progress should accelerate.
Soil temperatures at the 2-inch depth over the last ten days ranged from around 60 in southern Illinois to 55 in central Illinois to 50 in northern Illinois. In central and northern Illinois, where maximum air temperatures have been in the 50s and 60s, that means slow germination and emergence.
Soils in many areas have worked up well this spring, and surface soils have relatively small particle (aggregate) size. This wasn’t due to extra tillage, but may have resulted from repeated cycles of freezing and thawing over the past six weeks or so. With frequent rains in April, there also was not a lot of early tillage to cause clods to form that then dried out.
The forecast is for the weather to warm quickly, with high temperatures in the 80s for most of next week. This will greatly accelerate soil warming, and will move daily growing degree day accumulations to around 25 GDD per day. It takes 110-115 GDDs to get planted seeds to emerge, which means that fields planted now will have enough GDDs to emerge next week. Fields planted early next week should emerge within about five days, only a day or two later than those planted in late April.
This combination of soil and weather conditions may result in more soil crusting than we normally see; and the crust may form in time to cause problems with crop emergence. Warm temperatures act to both push the pace of germination and emergence, and, along with sunshine and wind, to cause soil crusts to “bake” quickly. This won’t happen in all planted fields, but the potential is greater than usual over the next week.
This is a bigger issue for soybeans than for corn, because their relatively large cotyledons can get trapped in a soil crust, which can cause the plant not to emerge. The key will be to examine fields daily after the first plants emerge, and to judge the pace of emergence over several days. The “48-hour rule”—that all plants in a field should emerge within 48 hours—is not very helpful when the weather is cool, but when highs are in the 80s and daily GDD numbers are in the mid-20s, it is a realistic guideline.
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If soybean plants continue to struggle up over several (warm) days, and many have their cotyledons trapped in the soil crust, those that failed to emerge early may not emerge at all. Rain to soften the soil can help plants to emerge, but many of these may be damaged—for example, their cotyledons may have broken off—and may not grow into healthy plants. The best remedy in fields with fewer than half the plants emerged and with some of those injured may be to “repair-plant”—plant additional rows at an adjusted seeding rate—as soon as field conditions allow.
While corn has better ability to emerge as a crust is developing, we also need a larger percentage of corn seeds to develop healthy plants than we do with soybeans. If corn seedlings don’t show thickening of the coleoptile or have their first leaf breaking out of the coleoptile before emergence, they should have the potential to emerge.
Corn planted deeper than 2 inches will reach the surface a little later and may be unable to break through the crust. A rotary hoe will sometimes help more corn seedlings emerge. Without cotyledons to move through the soil or a hypocotyl to break, corn seedlings can wait a little longer for rain to soften the crust to improve emergence.