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Issue No. 3, Article 9/April 13, 2007

Corn and Cold Soils

Though official statistics put the amount of corn planted in Illinois at less than 1% by April 9, there are areas where some fields were planted as much as two or more weeks ago. There have been some reports of corn that has emerged. There was also some corn planted on April 9 and 10, in areas where soils were dry enough. With cool, wet conditions (including the possibility of snow) forecast for the coming days, concern will increase about the health and emergence ability of corn seeds.

Temperatures over last weekend (April 6 to 9) dropped into the 20s or lower at least once in almost all of Illinois, and daytime temperatures barely reached 40 in much of the state. As a result, soil temperature went from the 50s or even 60s at the beginning of the cold spell down to the 30s (at least at night) and 40s, where it is likely to remain for several more days.

Wet soils and soil temperatures in the 30s are very hard on corn seed and seedlings. Even the "stress" cold test is conducted at 50 degrees, so it probably does little to predict seed's ability to emerge when temperatures are 40 or less. Thus, while we are confident that corn seed can lie in cold soil for several weeks without great harm if the soil is dry, we have no such confidence when the soil is wet. Had soil temperatures recovered quickly, seed might have germinated and emerged well. The ability of seed to do that is increasingly in question as we wait for the return of warmer air and soil temperatures.

For corn seedlings to emerge, about 110 growing degree days (GDD--base 50) in air temperature after planting need to accumulate. When soils start cold, it might take more than this, since the soil needs to warm up before anything can happen. During the last 10 days of March at Urbana, we accumulated 136 GDD. During the first 10 days of April, we accumulated 36 GDD. So corn planted by about March 25 might well have emerged, while that planted April 1 is unlikely to emerge much before April 25 or so, and even then only if temperatures return to normal after April 15. This lack of temperature high enough to cause germination means that it is difficult to assess the progress of the germination and emergence processes.

Those who were able to plant by March 25 have likely seen serious damage on plants that were emerged or were just starting to emerge. Soils may have frozen far enough down to kill the seedling above the seed, in which case all growth will stop. There is almost no chance for such plants to come back, at least not uniformly enough to produce a productive stand. The absorption of cold water by roots can result in abnormal growth and physiological damage to seedlings, so plants may not be normal even if they make some additional growth. If plants are somehow still alive and growth seems normal after warmer temperatures return, it's possible that healthy plants could develop, but this is difficult to predict with accuracy.

Most of the corn planted after March 26 or 27 has probably not emerged yet, but that does not mean it is safe. The absorption of cold water into the seed and seedling can cause injury from which the small plants are unable to recover. Signs of such injury include abnormal growth of seedlings, such as corkscrew growth and other signs of disorientation of growth. Affected seedlings might leaf out underground or simply stop growing, with no chance to develop a normal plant. It probably does not help much to dig up seeds and bring them into warm temperatures to see how they grow, since this does not duplicate what they are experiencing in the field. All we can do is to watch them, especially after GDD accumulations reach more than 100, to see if they are developing normally. It might help to line up replant seed in the meantime.

I'll deal next week with the effect of planting delays on corn yield potential. If it's any comfort, not having seed planted during the kind of weather we are experiencing is no disadvantage, when the prospects for planted seed are so dim and when plants in the field are not able to grow anyway.--Emerson Nafziger

Emerson Nafziger

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