NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
July 6, 2000
Some soybean and flax fields have turned yellow following recent heavy rains. David Franzen, extension soils specialist at North Dakota State University, says soybean and flax plants turn yellow, or chlorotic, because they have trouble getting iron, an essential nutrient.
In some cases a pale yellow color may result from a moderate nitrogen deficiency rather than iron chlorosis. True chlorosis is not just a pale yellow color but shades of yellowness from subtle to bright yellow between the veins of the leaf.
Many factors contribute to chlorosis, Franzen says. The first is presence of free carbonates, especially bicarbonate, resulting from a combination of water and carbonate minerals in the soil.
"Bicarbonate solubility is low, and contrary to logic increases in concentration as the soil becomes wet," says Franzen. "So in dry periods, chlorosis can be relatively minor, because bicarbonate levels are low. As the soil becomes moister, bicarbonate concentrations increase and the plants become less able to take up iron."
Stress factors that make plants less able to take up iron are cool weather and soluble salts. Cool weather slows down plant systems, including iron uptake. Soluble salts in the soil contribute to chlorosis in supposedly chlorosis tolerant varieties. There are wide differences between soybean varieties in their tolerance to chlorosis.
Another concern of soybean growers is the impact of herbicide stress. Studies indicate that herbicides may further stress already stressed soybeans. Soybeans growing on higher salt soils seem to perform most poorly when treated with harsh contact herbicides or herbicides with systemic activity.
Franzen says if growers have several herbicide choices for controlling weeds, using one of the less stressful herbicides on chlorosis-stressed beans could improve yields. However, there will be situations where the less stressful herbicides are not an option because of the weeds that must be controlled.
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NDSU Agriculture Communication
Source: David Franzen, (701) 231-8884
Editor: Gary Moran, (701) 231-7865