NDSU Crop and Pest Report

ISSUE 5  May 29, 2003


Within the past week, many areas within small grain fields have turned yellow. Yellowing is a symptom of many problems, some nutritional, some environmental. The following is a ranking of the most and least common causes, with most common ranked as number 1, and the rarest causes at the bottom.

1. Cold soils. Often, soils are cold, but air temperatures are warm. This disparity encourages top growth, but retards root growth. The root systems under many small grain fields is poor. When we sustain warmer temperatures, many of these fields will return to their normal green color on their own.

2. Wet soil conditions. We all know that water is important for plants to grow well, but we sometimes forget that roots need air also. Soils with low permeability, such as clays, clay loams and silty clay loams, retain high amounts of soil water for a long time after rains. When soil pores are filled with water, air exchange is poor and levels of carbon dioxide rise with a corresponding decrease in oxygen content. Roots need to respire just as above ground portions of the plant. When soils dry out, the green color will return to many fields.

3. Nitrogen. Looking out at local fields, the most yellow areas appear in drainage ways and depressions. Part of this difference in color is probably due to wet soil conditions. However, some of it is probably also due to lower nitrogen levels. Denitrification does not need total inundation with water to be present. Any pores that are saturated with water are subject to anaerobic transformation to nitrogen gas through the activity of an assortment of soil bacteria. In the Valley, these organisms appear to be particularly active. West of the Valley, the activity of these bacteria appear to grow less and less important. In the Valley, under our temperature conditions, transformations and loss of N of about 10 lbs/acre/day would be likely, while in the far west, losses of about 1/10 that rate would be more the rule. Fields in between would be somewhere in between. I have seen evidence for these estimates in our precision ag studies over the years. If crops are yellow in sandier areas of the field, particularly on ridge tops, then some degree of leaching has occurred. A combination of soil testing and plant analysis would help to verify any deficiency. If the crop has not reached jointing, and the areas are extensive enough, then supplementing their N supply through topdress would be one option to consider.

4. Phosphorus. Review the P fertilization of the field. It is surprising to me that some growers do not apply P to their wheat. Wheat yields are nearly always increased with seed-placed or at planting time placed P, regardless of soil test level. One of the symptoms of poor P nutrition of wheat is yellowing of the crop. If P is not in the input list, perhaps it is time to think about putting it back into the plan.

5. Sulfur. Sulfur deficiency in our region would generally not be found in heavier textured soils, nor would it be seen in depressions. It is most likely found in sandier soils on ridge tops . The symptoms would be found mostly in upper leaves, not lower leaves; the opposite of an N deficiency. Plant analysis would help to identify this cause.

6. High soil salt levels. Small grains, especially barley, can tolerate very high levels of soluble salts, but they have their limits. At EC levels higher than 3 mmohs/cm, wheat begins to suffer. At EC levels higher than 4 mmohs/cm, barley growth is reduced.

7. Potassium. It would be a rare thing to see a potassium deficiency in wheat, but if soil levels were around 50 ppm, some yellowing could be seen. Most likely on neglected sandy ridges.

8. Copper. On rare occasion, very sandy, very low organic matter ridge/hilltops may become yellow due to copper deficiency. These areas would probably be small in size. If they were found, a copper chelate would take care of the problem.

9. Iron. On very rare occasions, a narrow ring of very yellow small grain plants around a pothole may be seen. This is the same iron chlorosis we see regularly in soybean. The area is more of a curiosity than a real problem. If the soils dry, the problem will go away, although the salts present in the area may reduce yield.

Things not to consider at all- manganese, zinc, boron, magnesium, calcium. I have not seen any evidence that these nutrients are a problem in small grains in the region.

Dave Franzen
NDSU Extension Soil Specialist

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