The Potassium Crusades: On the Cusp of the Age of Enlightenment.

In 1995, the 3-state IN-OH-MI region published new potassium (K) recommendations; in 1997, we implemented a 5-year, 5-location field study with the intent of (1) addressing known limitations of the new recommendations for certain soils, (2) assessing whether soil K levels for optimum soybean (Glycine max Merr.) growth were higher than those for maize (Zea mays L.) and (3) assessing whether soil critical levels (CLs) developed for heavily tilled soils were relevant in systems with reduced tillage and nutrient stratification. Fields were specifically selected for soil type and K test values at or near the expected CLs for a maize-soybean rotation. Seventeen years later, the field experiments remain under study reflecting inherent difficulties in using a simple soil assay for a nutrient whose presence in soil is spatially variable at meso- to micro-scales (vertically and horizontally) and whose uptake and function is so strongly interactive with plant-soil water dynamics. This presentation will be a retrospective synthesis of key findings that collectively suggest the existing recommendation framework - based solely on a single soil CL modified by soil-specific cation exchange capacity (CEC) and a single buildup/drawdown function for soil test change with K addition and removal - is likely not suited to further refinement. Highlights include: the repeated failure of unfertilized controls to exhibit yield reductions when at or below expected CLs, soil- and crop-specific CLs that do not correspond to variation in CEC across locations, and differing rates of change in soils test values with net addition as compared to net removal of K. It is important to note that, because of the richness of the database, analyses completed to date constitute only a preliminary synthesis. Analytical activities are ongoing and opportunities to access the database and collaborate on further analyses alone or in combination with results from other experiments will be discussed.  Soil samples are archived and further chemical and mineralogical analyses represent an additional opportunity to derive value from this long-term experiment.