As growers scout their fields for potential disease, some are also noticing spots showing surprising nutrient deficiencies. One common deficiency is potassium, particularly in areas receiving limited rainfall. Many growers are puzzled to find potassium deficiencies when their soil samples test high for K. However, high soil potassium levels and fields showing potassium deficiencies are not mutually exclusive occurrences.
Potassium is held on the edge of soil clay colloids. As the potassium ions are removed from the edges they need to be replaced from the pool of potassium that is held within the clay lattice layers. As soils dry, the clay lattice tends to collapse trapping the potassium between the sheets that would otherwise migrate to the edge of the colloids and be available for plant uptake. As a result, areas of fields that would test for high levels of potassium begin to show deficiencies.
Another reason that potassium or any other deficiency may show up in dry conditions can be related to root growth, or lack thereof. Anything that limits root growths such as a smeared seedwall or hot/dry soils that limit root growth early on can cause deficiencies, although soil tests would indicate the soils have ample fertility.
The immediate reaction may be to apply supplemental fertility via foliar applications to correct the issue, but in the case of potassium both corn and soybeans require such large amounts of the nutrient to produce a crop that it is difficult to supply the needs via foliar applications. In the case of potassium, the best cure is for rainfall to rewet and expand the clay lattice and release available potassium back onto the edge of the soil colloids and into solution.
In dry years it can also be true that some micronutrients rely on mineralization of organic matter to be made available for plant uptake. Under dry conditions, mineralization of organic matter slows. Because by definition micronutrients are required in smaller amounts it may indeed be feasible to supplement the crop needs for micros via foliar applications. Be sure to scout your fields and contact your local retail agronomist to run tissue tests and see if foliar applications of micronutrients could supplement your corn or soybean fields to help maintain yield.