Soil tests are an important tool to guide farmers in determining an appropriate fertilizer application rate. The interpretation of K soil test results are complicated by the fact that STK levels are known to fluctuate throughout the year (Blakemore, 1966; Childs and Jencks, 1967; Liebhardt and Teel, 1977). Therefore, the time of soil sampling may impact fertilizer recommendations. Fluctuations in soil test K (STK) have been attributed to clay mineralogy and environmental conditions, like soil moisture status, wetting and drying cycles, and freezing and thawing cycles (Childs and Jencks, 1967).
Soils high in 2:1 type clay minerals (micas and vermiculites) have the ability to fix K (i.e., trap K in the clay interlayer) or release potassium depending on the STK level and soil moisture status (Goulding, 1987). Soil tests only measure the solution and exchangeable forms of soil potassium, and do not measure the potassium that is ‘fixed’ in the interlayer of 2:1 clay minerals. Leubs et al. (1956) measured exchangeable K levels in the top ½ inch of two Iowa fields from June through August and found exchangeable K to be inversely related to soil moisture. In laboratory investigations, an increase in the number of wetting and drying or freezing and thawing cycles has been found to either increase or decrease the magnitude of fixation or release of potassium (Graham and Lopez, 1969; Zeng and Brown, 2000). However, the response of STK levels to environmental conditions differs widely among different soils; therefore it is important to evaluate how STK levels may fluctuate in the major soil groups of Wisconsin.