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Issue No. 7, Article 11/May 6, 2005

All OK Underground?

I had a couple of reports this week that suggested that some corn planted into soils that remain cold might not be germinating and developing normally. Lyle Paul reported that at DeKalb the soil temperature under bare soil at the 4-inch depth was 39 degrees this morning (May 4). At the 2-inch depth, soil temperatures have dropped into the low to mid-30s the past two mornings. Such temperatures are much lower than even the temperatures used in the cold test for seed vigor, and they are certainly lower than corn seed is ever expected to experience.

One of the unusual phenomena observed was growth of the small shoots (the plumule) of the plant growing not straight up toward the soil surface, but rather growing sideways or even starting to curve downward. One hypothesis suggested was that since germinating plants normally grow toward warmer temperatures (at the soil surface), perhaps these were growing toward the higher temperatures beneath, rather than above, the seed under the cold air and surface soil temperatures. While such a temperature "inversion" is uncommon, we don't think that it disorients seedlings so they don't know which way is up. Plants have a system that senses what direction is up and which is down, and growth normally responds to this system; we say plants show "geotropic" growth, meaning roots normally grow down and shoots grow up.

Why might plants show disoriented growth? In some cases, seeds germinate and grow abnormally due to mechanical damage, and perhaps in some cases due to things like seed treatment, though this has not been shown very convincingly. Growth regulator herbicides like 2,4-D can directly affect growth rate of one side of the seedling more than the other, resulting in sideways or even circular growth. It's possible that slow seedling growth under cool soil temperatures might amplify such growth abnormalities.

As we have said elsewhere, much of the unemerged crop in Illinois has simply not had enough growing degree days to emerge--it normally takes about 110 to 120 GDD for emergence, and corn planted after April 20 may have accumulated only half that amount. Once GDD are sufficient for emergence, check the crop daily to see if seedlings are showing normal growth. Any tendency for sideways growth or leafing out before emergence of the coleoptile tip represents a danger to the establishment of adequate stands. While we don't think that cool soils themselves will increase this danger by much, we are trying to establish plants under unusual conditions this year, and it pays to keep checking.

Finally, some soils may be forming crusts as they dry following rainfall earlier. If coleoptile tips are showing signs of swelling and the crust is physically hard to break, rotary hoeing might be necessary.

A final note on soybean planting: There is a report on a Web site of some research in Ontario indicating that planting soybean seed into soils with temperatures in the 40s might cause serious loss of germination and emergence. They did this by chilling trays of soil into which soybean seed was planted. This might not exactly duplicate field conditions, but they concluded that when soybean seed takes up cold water in the hours after it is planted, the seed might never recover to germinate and form normal plants. They referenced some other information suggesting that planting in midday can produce better stands than planting in early morning or in the evening. Again, the soil (water) temperature during the 5 or 6 hours after planting seemed to be the important factor.

If we accept this as operating in our fields, it might be appropriate to wait to plant soybeans until soil temperatures return to 50 or so, which should happen by later this week. We did not observe this in the planting date work we did recently in Illinois, with planting as early as April 1. But soybean seed planted into soil with temperatures of 45 or so will not germinate and emerge anyway, so there's no particular advantage to such planting. Up to May 15 or 20, we remain within the planting time range considered to be optimum for soybean.--Emerson Nafziger

Author:
Emerson Nafziger

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