Yellow soybean areas within fields are being noticed in some areas of the state. There are six factors which could be causing the soybean plant yellowing: nitrogen (N), potassium (K), or sulfur (S) deficiency, iron deficiency chlorosis (IDC), soybean cyst nematode (SCN), or yellow flash from Roundup application. The following bulleted list contains a brief review of each possible cause.
- The symptoms of nitrogen (N) deficiency are lower leaves that are usually lighter green or chlorotic (yellowing) with upper leaves darker green. Action steps: dig roots and check for nodulation. Larger nodules are more efficient if lower soil moisture and reduced temperatures have occurred.
- The symptoms of potassium (K) deficiency are lower leaves exhibit leaf margin yellowing or chlorosis. Action steps: obtain 0-6 inch soil samples and plant tissue from good and poor areas for nutrient analysis, primarily for K.
- The symptoms of sulfur (S) deficiency is usually yellowing of whole plant and somewhat less growth compared with unaffected area. Action steps: obtain 0-6 and 6-24 inch soil samples and plant tissue from good and poor areas for nutrient analysis, primarily S.
- The symptoms of IDC is interveinal yellowing or chlorosis of younger leaves. Leaf veins dark green. Action steps: obtain 0-6 inch soil samples for pH and calcium carbonate determination. Each is usually high where IDC occurs. Select soybean varieties with tolerance, increase planting density, consider using Fe chelates such as FeEDDHA in furrow or foliar, reduce and monitor residual nitrate-N in corn, consider crop rotations and cover crops.
- The visual symptoms of SCN include stunted plant growth, yellowing plants, and soybean rows which are uncovered and uneven (canopy fails to close). Visual symptoms don’t have to occur for SCN to cause yield reduction. Action steps: dig and check roots for cysts. Submit soil samples for SCN determination*. Select soybean varieties for resistance and practice crop rotation to non-host crops in order to reduce SCN populations. Minimize soil movement from field to field as SCN is transported with the soil.
- The symptoms of yellow flash is overall plant yellowing after Roundup application. Specific indicator would be where spray boom started or stopped resulting in small area of over application. Action steps: some studies have indicated Manganese (Mn) deficiency after Roundup application and usually associated with fields that have low Mn soil test results. Soybean Mn deficiency has not been confirmed in South Dakota.
* SCN soil testing is sponsored by the South Dakota Soybean Research and Promotion Council and is free of charge for South Dakota soybean growers. Soil sampling can occur any time of year as long as the ground is not frozen or completely saturated but soil sampling immediately following soybean harvest is suggested for an accurate picture of what your SCN numbers are in your field.