ISSUE 11 July 13, 2005
CHLOROSIS OF SOYBEANS AND MORE
A few weeks ago as it became wetter and the temperature was still relatively cool, I expected soybeans to turn yellow with chlorosis immediately. Surprisingly, it took about a week for soybeans to turn chlorotic. Now, many fields that are still saturated with water or at least moist below the surface are most definitely chlorotic. True chlorosis is yellowing between the leaf veinal tissue, with the veins remaining green unless the chlorosis is extremely severe. If the plants just look generally yellow, look at the nodules to see if there are any there. Generaly yellowing may be caused by low N levels or just by fields being too wet. Chlorosis strikes newly developing leaves, not older leaves initially.
The chlorosis in this area is a result of unusual soil conditions compared with those in states to the south and east. Soil pH will be higher than 7, with significant soil carbonate content. Once these basic soil conditions are met, other environmental and soil factors combine to increase the severity of the chlorosis.
Soil moisture content increases the solubility of bicarbonate, which is a solubility product of soil carbonate minerals. As bicarbonate solubility increases, roots have a harder time maintaining an acidic aura around the roots which support the activity of an iron reducing substance excreted by the soybean roots. The iron reducing substance transforms highly insoluble iron III to very soluble iron II, but it only works in an acid environment. When bicarbonate neutralizes the root acidity, plants have to try to take up iron that is a trillion times less soluble than if the reducing substance was working well.
In addition to high soil water, higher levels of soluble salts put an additional stress on the soybeans and reduce the plants capacity to take up iron. High levels of soil nitrates also have a negative physiological effect on iron utilization within the plant. Herbicides are also a stress factor that may push a plant into deeper chlorosis and sometimes prolong the effect. Certain herbicides with longer soil residual or a harsh contact effect may also reduce yield compared to other herbicide options, although the effect of the weeds on eventual yield must be considered and weighed before spray decisions are made.
Iron foliar sprays may help to green the plants up in some cases, but yield increases are not consistently seen. Many growers have seeded highly tolerant varieties, based on Dr. Goos’s tolerance screening available through the North Dakota Soybean Association, and so the effects of chlorosis on many fields may not be as great as it used to be. Screening fields before planting and avoiding those with high salt levels and severe drainage problems will help avoid future problems.
Flax has also exhibited iron deficiency chlorosis this year across the state. Similar mechanisms as soybeans are causal factors in flax as well. Corn is less likely to show chlorosis, however, I have seen some chlorosis on corn around potholes in very wet parts of the state.
Dr. Dave Franzen
NDSU Extension Soil Specialist