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Issue No. 9, Article 11/May 25, 2007

Seeds and Plants in Dry Soil

Anxiety about wet fields and delaying planting in April has turned in many parts of Illinois to anxiety about lack of rainfall in May and the resulting dry surface soils. Many places have received less than a half inch of rain so far this month, while air temperatures have averaged above normal.

Many soybean fields and some late-planted corn fields were planted into soils that were marginally dry, and soils dried more after planting. Emergence has been less than complete in some of these fields, especially soybean fields. The reasonable decision not to "plant to moisture" if that meant planting 2-1/2 inches deep or deeper has meant, where there has not been enough rainfall, ragged emergence patterns in some fields. There is little to do in such cases except wait for rain.

In some fields there was just enough water to cause seeds to germinate, followed by drying of the soils and the seeds. On their own, surface soils dry slowly due to being "self-mulched"--water movement toward the surface is slow, especially where there has been tillage that helps break up connections between soil particles. In the vicinity of the seeds, water also moves slowly to replace that taken up by seeds, so seed swelling can create its own moisture-deficient soil "capsule" that stops water movement to and into the seed before germination is complete. Thus water uptake by the seed can create its own deficit.

Seeds that swell by taking on water but cannot complete germination due to lack of adequate water uptake may be unable to germinate again once more water is available. Some of this may be due to invasion of pathogens, but some is physiological, in that biochemical events initiated at the start of germination aren't normally reversible. If seeds swell only slightly and the seed coat is not broken, they may be able to germinate if they get more water within a few days. But after the seed coat is broken and the root emerges, lack of adequate water to complete the process usually means death of the seed. To find out if seeds whose germination was suspended due to drying soil are still capable of germination, you can dig them up and put them in a moistened paper towel in a warm place to see if they germinate.

In many fields, due to plentiful soil moisture in April and the slow loss of water from the soil while plants are small, there is plenty of soil water to keep small plants growing well even when it has rained little in May. The problem is that we often manage soils and planting such that, when it fails to rain soon after planting, plants may not be able to reach the water that is present. One way this happens is when we till the soil too wet and hard clods form. These may have water in the soil inside, but seed or roots in contact with the dry surface of clods cannot reach this water, so often they cannot germinate or grow. Clods also dry out faster than loose soil because they allow air to circulate around them, and they conduct water to the outside of the clod due to good connections between soil particles. The only remedy for clods is rainfall to melt them down.

Another problem we are seeing and hearing about is failure of the root system to develop normally on plants that have emerged and made some growth. I visited some fields on May 22 that were planted at the end of April and have reached the V3-V4 stage (3 or 4 collars visible). The visible problem in these fields is plants that are starting to flop over, staying attached to the soil by the mesocotyl (which joins the seed and its root system to the base of the stem) and in some cases by a few nodal roots.

"Floppy" corn in a field in southwest Piatt County on May 22.

A floppy corn plant removed from the soil, showing the seminal root system and lack of nodal roots.

In these fields, even the plants that are standing straight have the base of the plant very unusually close to the soil surface. This is a phenomenon that we have previously called the "high crown syndrome." Though we have seen this symptom occasionally for years, we still do not understand why it happens. Different fields showing it this year had different herbicides, or none, applied to date, so we can probably rule out herbicide effects. The symptoms have tended to be greater in lower-lying areas, suggesting that these fields, which were planted after a rain delay of several days, might have been worked and planted a little on the wet side, with the lower-lying areas more affected.

Some hybrids seemed to show this symptom more than others, but few seemed to have the base of the stem located as deep as 3/4 to 1 inch beneath the surface that we consider the normal depth. While only some of the plants were "flopping" at the time of the visit, the fact that nearly every plant had the base of its stem located more shallowly than normal tends to suggest that weather conditions may have been the most important cause. The explanation that we have offered before is that warm temperatures after planting into good soil moisture mean rapid growth and emergence, and that when coleoptile growth stops after its tip is hit by light after emergence, the base of the stem is "fixed" higher in the soil than usual. This means that the mesocotyl is longer than normal.

When the base of the stem is placed high in the soil, the nodal roots have to form near or at the soil surface. This means that these roots try to grow into soil near the surface that dries out most quickly. In some cases, the sidewall after planting into somewhat wet soils is also smeared, and this can create an additional barrier when it dries out. Plants that are not well anchored like this tend to move in the wind, and as a result nodal roots are continually "sanded off" by moving back and forth in the soil. Nodal roots also tend to be exposed to the air, and some even turn green from exposure to sunlight. None of this is conducive to development of a healthy nodal root system.

As plants move past the 4-leaf stage, the seminal root system--that arising from the seed--can no longer supply enough water to the plant, especially on warm, windy days. In cases where the nodal roots have failed to develop well by this time, water deficits start to develop. The lack of an anchoring by the nodal root system also means that the plant has limited ability to stand upright, and so plants can flop onto the soil surface. In some cases, the mesocotyl can break or shrivel, meaning that the connection between the seminal root system and the plant was broken. Such plants are truly rootless and are unlikely to survive.

While cultivating some loose soil into the row to try to prop up floppy plants might keep a few plants from breaking over and wilting, this is probably not very practical. If you need to try something and this is the only activity that seems to have promise, cultivate in the morning when plants are better hydrated and so standing better. Trying this when the wind is blowing will often mean burying plants and doing harm. If such "precision" cultivating seems to do little good, then it's best to stop. The only practical course of action is to wait for rain, then assess plant loss after that.

Rainfall should result in very rapid nodal root growth, and so the problem should solve itself quickly after rain. The fact that the roots start to grow late and that they grow from a "perched" stem may mean that total root depth and density could be less than normal in these plants. If dry weather happens later in the season, this could be a problem. In the meantime, be sure not to damage roots if cultivation is for some reason necessary.

Because we can't predict very well if it will be abnormally warm and dry after planting, it is difficult to prevent this type of problem. I found the problem in a small percentage of corn plants planted on April 20 and where the crop was at V5 and growing very well. Most of those plants were also perched, with the base of the stems near rye soil surface, but most had good nodal root systems.

If you have not looked at your corn planted in the last week or so of April, you might want to check to see whether plants show any of this perching. It is a problem of degree, in that some plants may have the base of the stem relatively shallow but show no flopping. Where there has been rain after emergence, nodal root growth may be fine even with higher stem placement. One thing to watch is whether plant stems tend to move when the wind blows. Such movement says that their nodal system is not well anchored.

We are also seeing a great deal of purpling in some hybrids that were planted somewhat late and where soils are very dry on the surface. This has in common the problem discussed earlier, in that the proximate cause is lack of nodal root growth. These plants do not have unusually high stem placement, but the soil around the base of the stem is very dry, and roots cannot grow into dry soil. Lack of root growth and moisture limitations on top growth means that there is not enough place for sugars produced by the plant to go, so sugars accumulate and purple pigments develop. This is far more common where fields were tilled before planting, such that surface soils were already fairly dry at the time of emergence. Rainfall that wets the soil to allow root growth will cure this problem quickly, and there should be no lingering effects except for the reduced growth that will need to be made up later.

Purple corn plants in dry soil.

Finally, we saw in a field near Berlin some rather unusual symptoms in what was a good stand planted in a strip-tilled field following soybean. Matt Montgomery, Extension crop systems educator in Sangamon-Menard counties, had visited this field and noted the presence of grape colaspis, which might be a contributing factor. But nearly every plant shows some "tubing," or elongation of leaves while still in the whorl, and there were symptoms of mild leaf burn, probably as a result of herbicide (plus surfactant) application soon after emergence. Leaf symptoms were more likely related to additives and carrier than to herbicides, since these symptoms were relatively minor.

The primary cause for concern in this field, though, was the darkening of the interior of the stem tissue. Some who saw this expressed concern about the fact that the "growing point" had darkened, and they felt that this meant that the plants were doomed. I do not believe this is the case. The growing point is the tip of the cone at the upper end of the stem, and the darkened area of the stem is below the growing point. We do not know the source of this dark material, but it is probably a result of some plant damage (probably from herbicide application here), and I do not believe it represents a continuing threat to plants. The best support for that conclusion in this field was that leaves continued to appear at a normal rate, and there were no signs that the growing point or the new leaf tissue above it was damaged.

A V3 corn plant with a darkening of the lower stem tissue. The "growing point" at the tip of the stem appears unaffected.

--Emerson Nafziger

Emerson Nafziger

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