Correction of Potassium Deficiency In Soybean Production In Kansas.

Over the last decade low soil test K levels and K deficiency, especially in soybeans, has become a major concern in the claypan soils of southeast Kansas.  The use of more intense crop rotations (corn/wheat/double crop soybeans) and increased production of high K extracting crops such as soybeans has increased K removal.  The traditional use of nutrient sufficiency based fertilizer recommendations has also resulted in K application rates substantially below crop removal.  In addition, farmers and crop consultants who maintain good soil test records are reporting substantial variation in soil test K from year to year in the same soil sampling areas.  This project was initiated in 2009 to determine the extent of the K soil test variation seen between years and during the crop year, and if this variability was impacting K availability as indicated by leaf K content and crop yield.  Monthly soil tests during the past three crop years at multiple locations have shown the ammonium acetate exchangeable K levels to change as much as 50% during the growing season.  The changes seem to follow soil moisture and precipitation trends.  As soils dry, soil tests drop and rewetting increases K levels.  K content of the leaf tissue tends to follow soil test K levels.  However, no impact on corn or soybean yield, or response to spring applied K has been observed, if the spring soil test K level was initially above the current critical level of 130 mg kg^-1.