I have had a good number of calls, emails and texts with photos of soybean plants with classic potassium deficiency symptoms showing in the upper leaves.  Some fields may also have some SDS as well which is a separate issue.  After a little investigating, it appears that the plants are experiencing a K deficiency in the top nodes that is caused by impaired root uptake of K in beans that are trying to fill pods (i.e. K demand is higher than roots can supply).  Each location may be slightly different as to the cause, but some contributing factors appear to be one or more of the following:

• Water logged root systems, shallow rooted plants with necrotic root hairs will have a more limited interception of K;  This seems to be more common in low lying areas in fields and bottom fields that have stayed wet through much of the season.  In some fields, at least 25% of the small root hairs are necrotic from growing in saturated soil conditions, and plants may be shorter than beans growing in higher ground.
• Soybean cyst nematode;  Dr. Kelly will be screening a number of samples next week to determine SCN levels, but in some fields, we have been able to spot cysts on root hairs with a hand lens.  SCN root damage in addition to an already reduced root system in wet soils may be reducing K uptake.
• Low soil potassium levels– Fields showing symptoms that have not been fertilized in years may need to have K level checked.  Also, where K was just applied this spring, only 40 to 60% will be available to plants this first season. Where K spread was not uniform, symptoms may look more streaked in the field instead of typical irregular shaped areas more common to wet conditions.