A Comparison of Soil Sulfur Extraction Methods

There are currently no field calibration data for interpretation of soil S tests in the U.S. Northeast and limited data elsewhere. Given the reduction in the S deposition rate during the past decade in the Northeast, it is important to evaluate the potential for soil testing as a tool for S management in the region. An incubation study was conducted to compare: (i) the accuracy and effectiveness of six S extraction methods to identify an increase in available S; and (ii) S detection in solution using inductively coupled plasma atomic emission spectroscopy (ICP–AES) at 182 nm with turbidimetric determination of S using BaCl 2 ×2H 2 O and a spectrophotometer at 420 nm. The six extraction solutions were 1.0 mol L −1 NH 4 OAc, 0.016 mol L −1 KH 2 PO 4 , 0.01 mol L −1 Ca(H 2 PO 4 ) 2 , 0.01 mol L −1 CaCl 2 , Morgan, and the Mehlich‐3 solution. Four New York soils were treated with six S rates (0, 28, 56, 84, 112, 168 kg ha −1 ) applied as gypsum and incubated for 4 wk (in the dark, at room temperature, 75% of field capacity, four replications). Sulfur test results correlated with S fertilizer application for all extraction and detection methods; however, the 0.01 mol L −1 CaCl 2 extraction with ICP–AES detection of S in solution was best correlated with S added (highest r 2 , smallest RMSE across all four soils), the regression equation had the largest slope, and this method showed the greatest consistency between the two detection methods. We conclude 0.01 mol L −1 CaCl 2 extraction with ICP–AES detection of S is most promising and should be evaluated in field trials.