Comparing Extraction Methods for Soil Test Potassium (STK) Relative to Soybean [Glycine max (L.) Merr] Yield.

Supplemental potassium recommendations are made from soil test correlation/calibration studies.  Fertilizer recommendations from some soil testing laboratories are based on studies using older soybean [Glycine max (L.) Merr] varieties.  As a result, there is no compensation for “yield lag” between older conventional varieties and new genetically enhanced varieties.  Therefore, the objective of this three year study was to evaluate no-till soybean yield from conventional and genetically enhanced varieties produced in irrigated and rain-fed environments relative to soil test potassium (STK) as determined by four common methods.  Experiments were conducted in 2013, 2014 and 2015 at the Pontotoc Flat Ridge-Flatwoods Branch Experiment Station on a Falkner silt loam (fine- silty, siliceous, active, thermic Aquic Paleudalf) soil.  Four extraction methods (ammonium acetate, Mehlich I, Mehlich III, and Lancaster) were used to determine STK.  Two soybean varieties (‘Hutcheson’ conventional variety and ‘Asgrow 5332’ genetically enhanced variety with high yield potential) were grown in both irrigated and rain-fed environments.  Yields were compared to each STK method by variety and irrigation treatment using SAS 9.4.  There were interactions between STK and soybean yield among extraction methods, varieties and environments.  Levels of STK extracted with the ammonium acetate and Mehlich III methods were greater than STK levels obtained with Mehlich I and Lancaster extractions for both irrigated and rain-fed environments.  Therefore, recommendations may vary based on soil testing method and soil fertility maintenance for a given crop.