There are six things that could cause the yellowing of soybeans, according to Anthony Bly and Connie Strunk, SDSU Extension field specialists.
1. Nitrogen deficiency -- Lower leaves usually lighter green or chlorotic (yellowing) with upper leaves darker green. Dig roots and check for nodulation. Larger nodules are more efficient if lower soil moisture and reduced temperatures have occurred.
2. Potassium deficiency -- Lower leaves exhibit leaf margin yellowing or chlorosis. Obtain 0-6 inch soil samples and plant tissue from good and poor areas for nutrient analysis, primarily for K.
3. Sulfur deficiency -- Usually yellowing of whole plant and somewhat less growth compared with unaffected area. Obtain 0-6 and 6-24 inch soil samples and plant tissue from good and poor areas for nutrient analysis, primarily S.
4. Iron chlorosis deficiency -- Interveinal yellowing or chlorosis of younger leaves. Leaf veins dark green. 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.
5. Soybean cyst nematode -- Visual symptoms 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. 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.
6. Yellow flash -- Overall plant yellowing after Roundup application. Specific indicator would be where spray boom started or stopped resulting in small area of over application. 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.
See more at Igrow.Source: SDSU Extension