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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 test based on fieldmoist samples (MK) was used until 1988 by the Iowa public laboratory but was abandoned because no other laboratory adopted it, and a factor of 1.25 was used to adjust upwards soiltest interpretation classes. The objectives of this study were to assess the impact of sample drying on soil K extraction from several Iowa soil series, conduct field correlations of NH4OAC tests based on oven-dried samples (DK) and a modified version of the MK test for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Single-yr and multi-yr K response trials were conducted at 54 locations from 2001 to 2004. Soil K extracted with the DK test was higher than for the MK test and the difference increased with increasing drying temperature (air dried to 50 °C). However, no single factor can be used to relate DK and MK test results because the difference was inversely related to the soil K level and the drying effect varied greatly between soil series. The MK test correlated better with yield response and showed a better defined critical K concentration compared with DK. Results showed that different calibrations are needed for different soils and (or) growing conditions for DK but not for MK. Critical concentration ranges defined by Cate-Nelson (CN) and linearplateau (LP) models across all soils for com were (mg K kg"1, 15 cm sampling depth) 144 to

Evaluation of potassium soil tests and methods for mapping soil fertility properties in Iowa corn and soybean fields