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Looking at Crops in Midseason

July 10, 2003

Two weeks ago, Bob Hoeft and I made some observations about nitrogen deficiency symptoms. Since that time, we have gotten several calls on this problem, and I have visited some fields, mostly northwest of Peoria, in or near Stark County. The affected areas of such fields are stunted and have the unmistakable K-deficiency symptom of yellowing of the edges of leaves, especially on the older leaves. This problem remains a conundrum: It appears in fields that seem otherwise in excellent condition; some fields where we see it have high enough K soil test values that this symptom should not appear; and there are either no indications of mechanical pattern, or in some cases the problem is actually less where there has been some soil compaction (such as in the end rows). Some hybrids might be less likely to show the symptom, and insect damage to roots accompanies this in some fields, but not all.

One of the more puzzling aspects in some fields is the occurrence of small "islands" of healthy plants, often only 5 to 10 plants, in the middle of stunted, yellow plants. Although this problem is generally more common on soils with medium or marginally low soil test K, George Rehm of the University of Minnesota has suggested that the dry soil conditions at and after planting might have resulted in slow movement of K through the soil solution and perhaps some increased tie-up of this nutrient in the soil. The seeming inability of the most affected plants to grow out of this condition, however, makes us think that the problem is often more than simple K deficiency. The one uniform aspect of the problem is small nodal root systems and restricted growth of belowground nodes of the stem. If this were simply K deficiency, we think plants ought to have grown out of it, especially where there has been rainfall. It is possible that K deficiency is simply the most visible result, rather than the cause, of a restricted root system. Stay tuned as we try to figure out what might be happening in these fields, and let us know whether you have some insight on it.

In most early-planted fields, the corn crop looks good or even outstanding. Tassel appearance is a little slower than normal, reflecting the lower GDD accumulation and, in some areas, slightly slower growth from lack of adequate soil moisture. Crop height is average or above for the growth stage in most fields, however, which tells us that dry soil has not greatly decreased growth rates. Rainfall is needed for full potential to be reached as we enter pollination, and so we'll be hoping for more than the hit-or-miss rainfall pat-tern that we have had over most of the southern half of Illinois since mid-June. We may receive some much-needed rain this week, but less than an inch of rain has fallen here at Urbana from mid-June through the first week of July.

Corn at this stage of growth has the potential to use about 0.25 to 0.3 inch of water per day under warm, sunny conditions with some wind. Although root systems are clearly tapped into soil water quite well, such a high rate of use without rainfall to replenish soil water supplies can result in very rapid onset of moisture shortage in plants. Appearance of tassels followed within a day or two by silks is the best indication that this is not happening, that water is not limiting to plant growth and function. The corn we planted here on March 24 is now fully silked, and there was little or no delay in silk appearance, so soil water was adequate even with little rainfall. Let's hope that's the case in most fields or that rainfall returns to normal.

Like corn tasseling, soybean flowering has been delayed somewhat from normal, even though the warm temperatures recently have speeded up the process. Maturity Group 3 soybeans here, planted around May 24, are just now flowering and are at about the V6 stage, about 16 to 18 inches tall. Leaf color and health are quite good, so we believe that yield potential remains high. The crop is not yet at the stage when stress cuts into yield, however, so looking at the soybean crop now is similar to looking at the corn crop about a month earlier; excellent crop condition can still be wiped out by weather-related stress a month from now.--Emerson D. Nafziger

Author: Emerson Nafziger

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