Effects of Sampling Time, Soil Moisture Content, and Extractant On Soil Test Potassium Levels.

An accurate prediction of plant-available potassium (K) requires a thorough understanding of the mechanisms that might impact soil test K.  A three year field study was developed to determine quantitative relationships between soil test K levels and time of soil sampling, soil moisture content, and extractant.  Five field sites were established in 2006 throughout Wisconsin at Arlington, Hancock, Marshfield, and Lancaster Agricultural Research Stations, and a private farm in Fond du Lac County each representing a unique pedogenic region of the state.  The sites were cropped for three years in a corn-soybean-corn rotation.  Potassium fertilizer was applied in the first year only at application rates of 0, 62, 125, 187, 249, 311, and 374 kg K ha^-1.  Soil samples were collected preplant (and prior to K application in 2006) and post harvest in 2006, 2007, and 2008.  Three and two additional soil samplings occurred throughout the growing season in 2006 and 2007, respectively.  Each soil sample was sieved (2mm) and split into three subsamples to undergo different drying methods (moist, air dry, and oven dry at 35°C) and analyzed for extractable K by both Bray 1 and ammonium acetate.  In 2008, all soil samples were oven dried and analyzed for K using the Bray 1 extractant.  Differences in soil test K levels between sampling times and drying methods are currently being examined.  The effect of location, sampling times, and drying methods on the relationships between Bray 1 and ammonium acetate extractants are also being investigated.