Since early August, soybean 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 draughty soils, K deficiency symptoms may develop from the V3 stage to more advanced vegetative stages mainly in the older leaves, but with severe deficiency, symptoms may progress to the upper leaves. Figure 1 shows typical soybean K deficiency symptoms at early growth 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 at early vegetative stages should not be confounded with soybean iron deficiency chlorosis (IDC), which often occurs in high-pH (calcareous) soils. In contrast to chlorosis or necrosis of leaf margins associated with K deficiency, IDC symptoms are yellowing of the interveinal area of mainly entire young leaflets. With extreme iron deficiency, browning and necrosis may also occur in leaf margins. The ICM News article “Is It Iron or Potassium Deficiency?” describes IDC symptoms in soybean.
The K deficiency symptoms in soybean during middle to late reproductive stages are similar to those observed earlier in the season on older leaves. Figure 2 shows typical examples. 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.
For more information, check out the full story from Iowa State University.