Friday, June 27, 2014

Kentucky's No. 1 Nutrient Deficiency in Soybeans: Potassium

Chad Lee, Extension Agronomist, University of Kentucky

K deficient soybean.
Every year we see soybeans somewhere in Kentucky with deficiency to potassium (K). Causes for the deficiency can be dry weather, soil compaction and/or low soil test levels for K. For the fields that I visit or receive calls, the reasons tend to be 1) soil compaction, 2) dry weather and 3) low soil test K. The symptoms in this image seem to generate to most phone calls, text messages and emails, which is why I've labeled it number one for soybean.

Evidence of K deficiency occurs when the leaf margins turn yellow (chlorotic) and that yellow moves towards the center of the leaves. On more advanced stages, the margins will turn brown (necrotic) as the yellowing advances towards the leaf center. On very small plants, all the leaves may show the symptoms. On older plants, the symptoms normally occur on the older leaves first and move to leaves up the plant.

The soybeans in this image have a bit of compaction - since this is at the edge of the field - and dry weather. That combination results in low K on the soybeans. Since these few soybeans are on the edge of the field where the equipment enters and exits, there is no need to do anything here. The yield potential for these particular plants is low and the overall area affected is very small.

If the field in these images had a low soil test for K, then an application
of potassium muriate (KCl or 0-0-60) can still be applied. If the field gets rain, then the soybean plants will benefit from the potassium. Some farmers will be tempted to apply a foliar fertilizer with potassium. The foliar products will contain 1 to 2 pounds of K2O at the most. However, 50 bushels of soybean seed will have about 55 pounds of K2O (AGR-1, Table 5). The lowest K2O recommendation in AGR-1 is 30 lbs K2O per acre (AGR-1, Table 15). So, a foliar product might help make the leaves green for a few days, but applied once will not be near enough for the crop demand.

If these symptoms appear in a soybean field, dig a few plants and look for compaction, first. If no compaction, look at the weather. If it has been dry, then get the plants water. While you are waiting for the rain, pull a soil sample if one hasn't been done in the last two years. That soil test will confirm if dry 0-0-60 should be applied to the field.

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