A Comparison of Soil Sulfur Testing Methods On NY Soil.

Sulfur (S) is one of the eighteen essential plant nutrients. It is required in relatively large amounts for processes like photosynthesis and nitrogen (N) fixation. A recent survey has indicated a potential for S deficiency for alfalfa grown in sandy, low organic matter soils in New York. Current tools for S management in New York are limited to tissue analysis. Our objectives were to: 1) compare the accuracy and effectiveness of six S extraction methods to identify and increase in available S through fertilizer S addition; and 2) compare inductively coupled plasma (ICP) spectrophotometry at 182 nm with a turbidimetric S detection method using BaCl₂.2H₂O and a spectrophotometer at 420 nm. The extraction methods were 1.0 M NH₄Oac, 0.016 M KH₂PO₄, 0.01 M Ca(H₂PO₄)₂, 0.01 M CaCl₂, the Morgan, and the Mehlich-3 extractions. Four New York soils were treated with six S rates (0, 28, 56, 84, 112, and 168 kg ha^-1 applied as gypsum) and incubated in the dark for four weeks at room temperature and 75% of field capacity (four replications). Sulfur content of the different extraction methods correlated with S fertilizer application for all extraction and detection methods. However, the 0.01 M CaCl₂ extraction with ICP detection of S in solution showed the greatest promise as a soil S test; in linear regression across the four different soil types, this method was best correlated with S added (highest r², smallest RMSE), the regression equation had the largest slope, and this method showed the greatest consistency between the two detection methods. Fieldwork is ongoing to determine critical values for this new soil test.