Assessment of Factors Contributing to Variability and Inaccuracy in Soil Test Sulfate Results.

Changes in industrial practices have led to reduced atmospheric deposition of sulfur. This fact, considered along with increased yield responses to sulfur fertilization, leads to the quantification of soil sulfate-sulfur as an important step in environmental monitoring and fertility research. This poster details efforts to modify and improve laboratory techniques for soil sulfate-sulfur measurement in agronomic systems. Concurrent with a recent upgrade from manual determinations of extractable soil sulfate-S via a Klett colorimeter (ca. 1950), to an automated turbidimetric procedure using a Lachat Quikchem 8500 series 2 Flow Injection Analyzer, we embarked on an ongoing series of assessments looking at factors influencing precision and accuracy of results within the NCR-13 standardized method currently in use. Our data shows a marked and relatively consistent decrease in sulfate recovery associated with the use of activated carbon (used to clarify sample filtrate), and a further decrease in recovery associated with acid-washed activated carbon. We also observed that the use of activated carbon is related to a significant increase in variability.