Soybean [Glycine max (L.) Merr] Yield Relative to Extractable Soil Test Potassium (STK) Levels Attained Using Various Extraction Methods.

Soybean yield can be limited by the ability to accurately determine soil test K (STK).  Several soil extraction methods are currently utilized to estimate STK.  Research was conducted to determine which soil extraction method most accurately estimated available K for plant uptake and how K level relates to yield.  A correlation calibration process was used to evaluate the effect of STK determined by five soil extraction methods (ammonium acetate, Bray P-1, Mehlich 1, Mehlich 3, and Lancaster) on crop yield.  Studies were conducted at the Pontotoc Flat Ridge – Flatwoods Branch Experiment Station during 2014 and 2015 on a Falkner silt loam (fine- silty, siliceous, active, thermic Aquic Paleudalfs).  One conventional and one high yielding soybean cultivar were used in both irrigated and non- irrigated environments.  Higher yields were significantly correlated with higher STK levels attained from soil extraction analyses.  The ammonium acetate extraction method extracted significantly higher STK than other methods in relation to percent relative yield.  Mehlich 1 and Lancaster were second and third.  Soybean grown on soils with higher K levels have been proven to have higher yields.  Improvements in soil testing methods could be the key to maximizing crop production.