Soil Testing for Potassium in Relation to Sample Handling and Mineralogy.

Soil-testing for K is a useful tool to assess plant-available K in soils and fertilization needs. However, there is a great of variation in critical levels across regions or soils, temporal variability, and uncertainty in K testing than P testing due to several factors. Soil moisture regimes at the field, sample moisture handling, and mineralogy are important factors because can greatly influence the relative distribution of K among various soil pools of different plant availability. Ongoing Iowa research is confirming that K testing of undried soil samples provides a more reliable estimate of plant-available soil K by eliminating the confusing effect of variation in field moisture conditions and sample drying on exchangeable K levels. Even with moist testing, however, relationships between crop yield response and soil-test K levels across Iowa soils still are poorer compared with soil P testing. This presentation provides results of ongoing field and laboratory research useful to better understand how soil sampling handling and mineralogy influence estimates of soil-test K critical concentrations and interpretations.