While we typically think of potassium deficiency as showing up in the lower crop canopy, it often shows on the upper leaves late in the season. We saw late-season potassium deficiency in some fields this year and last.

Some plants may have interveinal chlorosis (yellowing) and necrosis (death) on leaves in the upper canopy. There are a number of potential causes, and an article from the Crop Protection Network does a nice job of outlining the causes and how they differ.

In Iowa beginning in early August, soybeans in some fields began showing typical symptoms of potassium deficiency on leaves located in the middle to upper canopy. That’s not surprising in fields or parts of fields with soil-test values in the very low or low categories that didn’t receive adequate preplant fertilization.

Antonio Mallarino, Iowa State University soil fertility and nutrient management specialist, says during the last couple of decades K deficiency symptoms in the upper soybean leaves also have become common at the middle to late reproductive stages. He says K deficiency symptoms can develop in upper leaves in well-fertilized soybeans when no deficiency was observed at the early stages of growth, mainly when drought conditions develop during late spring or in the summer.

Watch for yellowing of leaf margins

In low-testing or drought-impacted soils, K deficiency symptoms may develop from the V3 stage to more advanced vegetative stages. The symptom is yellowing of the leaf margins with mild deficiency, and it becomes brown or necrotic with extreme deficiency.

The symptoms of these leaves often remain until the reproductive stages but may not be seen because the leaves have been shed or are partially decomposed. The main reason symptoms are seen mainly on older leaves at early vegetative growth stages is because K is very mobile within the plant and is translocated from older leaves to new leaves.

The K deficiency symptoms in soybeans during middle to late reproductive stages are similar to those observed earlier in the season on older leaves.

The physiological reasons for late-season development of deficiency symptoms during the last couple of decades are not entirely clear, Mallarino says. Reasons might be that with increasing soybean yield potential, more K translocates from the middle or upper leaves to developing pods and grain.

Severe K deficiency speeds maturity

Observations over many years have shown severe K deficiency can advance soybean maturation. So, it isn’t surprising to see soybeans beginning to lose their leaves, with most leaves yellow or brown, in low-testing areas a few days before plants in other parts of a field.

Keep in mind that deficiency of other nutrients or conditions such as excessively wet or dry soil can also advance soybean senescence, the process of leaves dying on the plants. Several soybean diseases can also produce yellowing of upper leaves, which may advance senescence. Sometimes, the disease symptoms and K deficiency symptoms occur at the same time.

This is not surprising. Iowa research has shown that K deficiency aggravates the incidence or severity of several soybean leaf diseases. Field observations also suggest possible interactions with soybean cyst nematode and aphid infestation levels. Potassium deficiency symptoms in soybeans can develop or be worse in field areas associated with SCN or aphids.

Soil and tissue testing

It is difficult to distinguish between K deficiency and disease symptoms during reproductive stages unless the plants or leaves are submitted to a lab for analysis. Mallarino says soil testing and soybean leaf testing of apparently normal and affected field areas also may help identify the cause of the symptoms.

Recently published interpretations for K tissue testing are useful for soybean plants at V5 to V6 vegetative growth stages or for upper leaves at the R2 to R3 reproductive growth stages, but not for later growth stages because leaf K concentrations decline. Tissue test interpretations are available in an ISU publication Phosphorus and Potassium Tissue Testing in Corn and Soybean, CROP 3153.

To gain the maximum benefit of tissue sampling and testing, be sure to take a soil sample at the same locations. A tissue sample is only a snapshot in time. A tissue test may identify that a given nutrient is low but doesn’t answer why. Also, a tissue test may show adequate levels in the plant, but it can’t estimate the soil reserves of a given nutrient to support the plant through the remainder of the growing season. A soil test in conjunction with a tissue test can answer these questions and bring greater interpretive value to the tissue test.

Suppose the tissue test is low, but the soil test is not. Then the plant is unable to access the nutrient, and additional soil-applied fertilizer will not be effective. If both the soil test and tissue test are low, then addressing soil fertility levels should be the focus of future management. Fall is a good time to sample soil and have it tested and get a handle on whether potassium fertilizer is needed in these “suspicious yellow areas” of soybean fields.

McGrath is the research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Email him at [email protected].