Evaluation of Field-Moist and Dry Soil Extraction on Mehlich-3 Potassium Availability on Loamy Soils from Arkansas.

Insufficient information exists to support the widespread use of field-moist soil (FMS), rather than oven-dry soil (ODS), for routine soil testing and additional laboratory comparisons and field correlation are needed. Our objectives were to compare Mehlich-3 extractable potassium (K) concentrations from FMS and ODS soil from long-term fertilization trails and a set of 35 composite soil samples representing multiple soil series from Arkansas.  The correlation between field-moist and oven-dry K concentrations were determined using linear regression. Selected soils were gradually air-dried to determine the moisture content that Mehlich-3 extractable K changes from drying. The relationship between ODS and FMS was significant and described by linear equations for the Calhoun soil (ODS-K = 22 + 0.78 x FMS-K, R²=0.95), the Dewitt soil (ODS-K = 44 + 0.71 x FMS-K, R²=0.88) and the set of 35 soils (ODS-K = 30 + 0.81 x FMS-K, R²=0.93). The relationship showed that K extracted from FMS was initially lower than from ODS, but as the amount of extractable K increased the FMS-K concentrations eventually became greater than from ODS. The FMS-K concentrations where the predicted difference was 0 was 102, 151, 158 mg kg^-1 for the Calhoun, Dewitt, and 35-soil set, respectively.  The soil moisture where drying caused the amount of extractable K to change (increase or decrease depending on soil K concentration) was <5% moisture.  Although soils appear to respond to drying in the same general manner, the equilibrium between FMS and ODS extractable K concentrations is different among soils and a simple correlation between FMS and ODS us unlikely to improve soil-test K based recommendations.  Field correlation of FMS-extractable K is needed to fully evaluate whether FMS can improve the accuracy of K-fertilizer recommendations.