Revision of Potassium Soil-test Interpretations and Fertilizer Recommendations

There is a long history of potassium (K) fertilization research in the North-Central Region. Sustained Iowa field research efforts have focused on developing soil-test K (STK) interpretation and studying the effect of K fertilization strategies on grain yield and STK. Because of changes in the soil-test K method used in Iowa, information published over time should be evaluated separately. Published research conducted from the 1960s until 1991 (the last publications by Mallarino et al., 1991a and 1991b) involved extracting K from field-moist soil samples using the ammonium-acetate test. Research during the 1960s and 1970s showed that extracting K from the soil without drying the samples gave more consistent results than using air-dried or oven-dried samples. Evaluations of STK results from field-moist samples are different from those for dried samples because less K is extracted from moist samples. The Iowa State University Soil and Plant Analysis Laboratory discontinued analyzing samples with the moist K test in 1988 based solely on practical considerations for laboratory work. Laboratory procedures are simpler for dried soil samples. Moreover, although the moist test was used in Iowa and was among tests recommend for the North-Central Region by the NCR-13 soil testing committee, it was not adopted by other private or public soil testing laboratories.