Soil K extraction with neutral 1 M ammonium-acetate (NH₄OAc) based on air-dried or oven-dried samples is the most widely used soil test for K. It has long been recognized that sample drying often increases K extracted by this test. An NH₄OAc K test based on field-moist samples (MK) was used until 1988 by the Iowa State University laboratory but was abandoned because no private laboratory adopted it. In the 1960's work was done to look at different chemical treatments that prevent K from being released upon sample drying or revert the drying effect before K extraction. The objectives of this study were to assess the impact of sample drying and two post-drying treatments on extracted soil K and its relationships with crop yield response to K fertilizer. Sample treatments before measuring K with the NH₄OAc extractant were no treatment (test on field-moist samples), drying at 40 C (DK), water incubation (Rewet K), and water-octanol incubation (Octanol K). Soil samples were collected from single-year and multi-year K response trials with corn (Zea mays L.) or soybean [Glycine max (L.) Merr.] that were conducted at 19 Iowa fields from 2001 to 2004. Soil K extracted with DK was higher than for MK, and these tests were linearly correlated (r2 = 0.53). The K extracted with Rewet-K and Octanol-K tests often was less than for DK, and Octanol-K values and were as low as values for MK. Rewet-K and MK-values were linearly related (r2 = 0.59). Octanol-K was linearly but poorly related to MK (r2 = 0.35). The difference between K extracted by MK and that extracted by DK, Rewet K, or Octanol K tended to be inversely related to the soil K level. Soil types and measured soil properties (texture, cation exchange capacity, pH, and organic matter) did not clearly explain the differences between K tests. The Octanol-K test did not measure plant-available appropriately and was the worst method for prediction of crop response to K fertilization. The Rewet K test was approximately similar to DK and MK and would be more practical for routine testing than MK test. Further field calibration research including a wider range of field conditions is needed to better compare these three K testing methods.

Abbreviations: CN, Cate-Nelson; DK, oven-dried K test; LP, linear-plateau; MK, field-moist K test; NH₄OAc, ammonium-acetate; Octanol K, K extracted after water-octanol incubation; QP; quadratic-plateau; Rewet-K, K extracted after incubation with water.