Soil test potassium changes over time following fall potassium application in three western Kentucky soils

Field experiments were conducted under bare-ground conditions to investigate changes in soil test K levels over 2 yr following fall K applications to Crider, Zanesville, and Grenada soils in western Kentucky. Additional laboratory studies investigated particle size and clay mineralogy of these soils. In field experiments, highest soil test K in the Crider and Zanesville soils occurred 12–15 mo after fall K application. However, seasonal fluctuations in soil test K occurred in all three soils, likely due to fluctuations in precipitation prior to sampling. The Crider and Zanesville soils showed a much more substantial drop in soil test K at 9 and 21 mo after December K application than did the Grenada soil. The 9- and 21-mo samplings were both preceded by 1–2 mo of dry conditions where precipitation was well below the 30-yr average at the Crider and Zanesville sites. Particle size analysis showed that the Crider and Zanesville soils had a higher clay content than the Grenada soil. Soils with higher amounts of clay particles such as vermiculite have greater potential to trap K during dry periods, making soil test K levels lower than they might be under better soil moisture conditions. Over the course of these experiments, the Crider soil appeared to be more responsive to fall K application than the other soils, showing the greatest increase in soil test K by the following spring. This research illustrates the seasonal fluctuations in soil test K levels that occur naturally in these soils and how precipitation and clay content may influence these fluctuations.