Since early August, farmers and crop consultants have been reporting what they believed were potassium (K) deficiency symptoms in soybean leaves in the middle or upper canopy. This isn’t surprising in fields or portions of fields with soil-test values in the very low- or low-K interpretation categories, says Antonio Mallarino, Iowa State University Extension agronomist specializing in soil fertility.

“Moreover, K deficiency symptoms could develop at these growth stages with drought conditions, even in fields with adequate soil-test K levels,” he explains. “Sometimes symptoms occur in late summer with rainfall after a dry period. 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.”

Early growth stage vs. later symptoms
In low-testing soils or droughty soils, K deficiency symptoms may develop from the V3 growth stage up to more advanced vegetative stages mainly in the older leaves, but with severe deficiency, symptoms may progress to the upper leaves, says Mallarino. The symptom is yellowing of the leaflet margins, with mild deficiency that may become brown and necrotic with extreme deficiency.

On older leaves, K deficiency symptoms sometimes remain until the reproductive stages of the plant but often may not be seen because the leaves have been shed or partially decomposed. The reason symptoms are observed mainly in the older leaves at early vegetative growth stages is because K is very mobile within the plant, and with a deficiency, it is translocated from older leaves to new leaves.

These two photos with this story show examples of typical soybean K deficiency symptoms at early growth stages.

EARLY SIGNS: These soybean plants show potassium deficiency symptoms at early vegetative growth stages.

The K deficiency symptoms at early vegetative stages should not be confused with soybean iron deficiency chlorosis, which often occurs in high-pH (calcareous) soils. In contrast to K deficiency, IDC symptoms are yellowing of the interveinal area of mainly entire young leaflets, says Mallarino. With extreme iron deficiency, however, brown and necrosis may occur in leaf margins, he notes. The ICM News article “Is It Iron or Potassium Deficiency?” refers to IDC symptoms in soybeans.

The K deficiency symptoms in soybean middle or upper leaves at intermediate to late reproductive stages are similar to symptoms observed early in older leaves. “The physiological reasons for late-season development of deficiency symptoms 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,” says Mallarino.

LATER SYMPTOMS: During soybean reproductive stages, potassium deficiency symptoms look like the plants shown here.

Observations during many years have shown that severe K deficiency can advance soybean maturity. Thus, it’s not surprising to see senescing soybeans, with most leaves yellow or brown, in low-testing field areas a few days before plants in other parts of a field. The photo above shows an example observed in research plots. Remember, however, deficiency of other nutrients or conditions such as excessively wet or dry soil also can advance soybean senescence.

Disease symptoms can also show up
Several soybean diseases caused by fungi and viruses can also produce yellowing of soybean upper leaves, which also may advance plant and leaf senescence, Mallarino adds. Sometimes, the disease symptoms and K deficiency symptoms occur at the same time. This shouldn’t be surprising, because Iowa research has demonstrated that K deficiency aggravates the incidence or severity of several soybean leaf diseases.

Part of this research is summarized in the article “Watch potassium management - It also affects corn response to nitrogen and soybean diseases.” Additional field observations suggest possible interactions with soybean cyst nematode and aphid infestation levels. That is, upper-canopy K deficiency symptoms can develop in field areas associated with SCN or aphids.

“Sometimes it’s very difficult to distinguish between K or disease symptoms unless the plants or leaves are submitted to a plant pathology lab for study,” says Mallarino. “Soil and leaf K testing of apparently normal and affected field areas also may help identify the cause for the symptoms. Recently published interpretations for K tissue testing can be useful for soybean plants at the V5 to V6 vegetative growth stages or for upper leaves at the R2 to R3 reproductive growth stages, but not for later growth stages. This is because leaf K concentrations decline during later growth stages.”

You can see the tissue test interpretations in ISU Extension and Outreach publication CROP 3153, “Phosphorus and Potassium Tissue Testing in Corn and Soybean.”

Source: Iowa State University