Editor’s note: The following was written by Antonio Mallarino, Iowa State University professor of soil fertility and nutrient management, for the Extension Integrated Crop Management website Aug. 23.
Since early August, soybeans in several fields began showing typical potassium (K) deficiency symptoms on leaves located in the middle to upper canopy.
This is not surprising in fields or portions of fields with soil-test values in the very low or low K soil-test interpretation categories that did not receive adequate preplant K fertilization. Potassium deficiency symptoms are well-known and very common in older leaves during early growth stages.
Due to poorly understood reasons, during the last couple of decades K deficiency symptoms in upper soybean leaves also have become common at middle to late reproductive stages.
Moreover, K deficiency symptoms can develop in upper leaves in well-fertilized soybean when no deficiency was observed at early stages, mainly when drought conditions develop during late spring or summer.
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 leaflet margins with mild deficiency that 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 partially decomposed.
The reason symptoms are observed mainly on older leaves at early vegetative growth stages is because K is very mobile within the plant and K is translocated from older leaves to new leaves.
The K deficiency symptoms in soybean 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. Reasons might be that with increasing soybean yield potential there is more K translocation from the middle or upper leaves to developing pods and grain.
Observations over many years have shown severe K deficiency can advance soybean maturation. 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.
Keep in mind that deficiency of other nutrients or conditions such as excessively wet or dry soil can also advance soybean senescence.
Several soybean diseases can also produce yellowing of upper leaves, which also may advance senescence. Sometimes, the disease symptoms and K deficiency symptoms occur at the same time.
This is not surprising because Iowa research has demonstrated 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. Potassium deficiency symptoms in soybean can develop or be worse in field areas associated with SCN or aphids.
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.