June 13, 2003

 

NDSU Soil Specialist Explains Possible Reasons for Yellowing Crops

Yellowing crops found in many small grain fields this year may be caused by either nutritional or environmental problems, according to Dave Franzen, NDSU Extension soil specialist.

Franzen offers the following list of the most to least common causes for yellow crops:

• Cold soils. Cold soils combined with warmer air temperatures encourage top growth, but inhibit root growth. When the soil becomes warmer, many fields naturally return to their normal color.
• Wet soil conditions. Soils with low permeability retain water for a long time. When soil pores are filled with water, air exchange is poor and levels of carbon dioxide rise, decreasing oxygen content. Roots need to respire, just as other parts of the plant do. When soils dry out, the green color returns.
• Nitrogen. Most yellow crops appear in drainage ways and depressions. In addition to wet soils, nitrogen levels may be lower in these areas. Any pores that are saturated with water are subject to anaerobic transformation to nitrogen gas through the activity of an assortment of soil bacteria.

If crops are yellow in sandy areas, particularly on ridge tops, it is likely that leaching has occurred. A combination of soil testing and plant analysis helps verify deficiency. If the crop has not yet reached jointing, and the yellowing is severe enough, supplementing nitrogen through topdress is an option.

• Phosphorus. One of the symptoms of poor phosphorus nutrition of wheat is yellowing of the crop. Wheat yields are almost always increased with seed-placed or planting-time placed phosphorus.
• Sulfur. Sulfur deficiency is usually found in sandier soils on ridge tops. The symptoms are found mostly in upper leaves, not lower leaves, contrary to a nitrogen deficiency. Plant analysis helps identify this cause.
• High soil salt levels. Small grains, especially barley, can tolerate very high levels of soluble salts, but they have limits. At electrical conductivity levels higher than 3 millimhos per centimeter, wheat begins to suffer. At levels higher than 4 millimhos per centimeter, barley growth is reduced.
• Potassium. Rarely do you see a potassium deficiency in wheat, but if soil levels are around 50 parts per million, some yellowing can be seen. This deficiency is most likely to be found on neglected sandy ridges.
• Copper. On a rare occasion, very sandy, very low organic matter ridge/hilltops become yellow because of copper deficiency. These areas are generally lesser in size. A copper chelate will take care of the problem.
• Iron. Seldom, a narrow ring of very yellow small grain plants may be seen around a pothole. This is the same iron chlorosis seen in soybean. If the soils dry, the problem will disappear, although the salts may reduce yield.

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

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Source: Dave Franzen, (701) 231-8884, dfranzen@ndsuext.nodak.edu
Writer: Teresa Oe, (701) 879-6270, teresa.oe@ndsu.nodak.edu
Editor: Rich Mattern, (701) 231-6136, richard.mattern@ndsu.nodak.edu