Editor’s note: The following was written by Antonio Mallarino, Iowa State University professor and nutrient management Extension specialist, for the ISU Extension Integrated Crop Management website Aug. 21.
Since early August, farmers and consultants have been reporting what they believed were potassium (K) deficiency symptoms in soybean leaves located in the middle or upper canopy.
This is not surprising in fields or portions of fields with soil-test values in the very low or low K interpretation categories. Moreover, K deficiency symptoms could develop at these growth stages with drought conditions, even in fields with adequate soil-test K levels.
Potassium deficiency symptoms are very common and well known at early growth stages, but due to poorly understood reasons, in the last couple of decades deficiency symptoms in upper leaves at middle to late reproductive stages also have become common.
In low-testing soils or droughty soils, K deficiency symptoms may develop from the V3 stage up to more advanced vegetative stages, mainly in the older leaves, but severe deficiency symptoms may progress to the upper leaves. The symptom is yellowing of the leaflet margins with mild deficiency that may become brown and necrotic with extreme deficiency.
The physiological reasons for late-season development of deficiency symptoms during the last couple of decades are not entirely clear. Reasons might be that with increasing soybean yield potential there is more translocation from leaves near developing pods and grain, resulting in deficiency symptoms.
Observations during many years have shown that severe K deficiency can advance soybean maturity. Therefore, it is not surprising to see senescing soybean, with most leaves yellow or brown, in low-testing field areas a few days before plants in other parts of a field.
It should be remembered, however, that deficiency of other nutrients or conditions such as excessively wet or dry soil also can advance soybean senescence.
Several soybean diseases caused by fungi and viruses can also produce yellowing of soybean upper leaves, which also may advance plant and leaf senesce. Sometimes, the disease symptoms and K deficiency symptoms occur at the same time.
This should not be surprising because Iowa research has demonstrated that K deficiency aggravates the incidence or severity of several soybean leaf diseases. Additional field observations suggest possible interactions with soybean cyst nematode (SCN) and aphid infestation levels. That is, upper canopy K deficiency symptoms can develop in field areas associated with SCN or aphids.
Sometimes it is difficult to distinguish between K or disease symptoms unless the plants or leaves are submitted to a plant pathology lab for study. Soil and leaf K testing of apparently normal and affected field areas also may help identify the cause for the symptoms.