Comparison of Seven SO4-S Extraction Methods for Analysis by Turbidimetry or ICP Spectrometry

Soil sulfur (S) analyses for fertilizer recommendations in the northern Great Plains often do not reflect crop S requirements. Seven SO4-S extraction methods with S determination by either turbidometry or inductively coupled plasma emission spectroscopy including Ca(H2PO4)(2) and KH2PO4 (both containing 500 ug/l P), 0.25M KCl (40 degrees C) and 0.25M KCl (room temperature), H2O, DTPA, and Mehlich 3 extractants. Three horizon depths of three soils from a previous field study were used for these comparisons. Average standard deviations for turbidometric determinations were 4.3 times greater than ICP determinations. With turbidometry, S values were H2O > KH2PO4 > Ca(H2PO4)(2) > KCl (40 degrees C)=KCl, while with ICP, the values were Mehlich 3 > KCl (40 degrees C)=KCl > DTPA (diethylenetriaminepentaacetic acid) > KH2PO4 > H2O > Ca(H2PO4)(2). Extraction with KCl at room temperature with ICP determination appears to show promise, but further method evaluation is necessary before it can be recommended as a SO4-S test method.