Interlaboratory and Intralaboratory Testing of Soil Sulfate Analysis in Mollisols of the Pampas

Sulfur (S) deficiencies in grain and forage crops have been detected in many agricultural regions of the world, but soil tests are not commonly used as the basis for S fertilizer recommendation programs. Errors of measurements of soil sulfate were determined to assess whether the variation among and within soil-testing laboratories could be a factor that prevent the adoption of soil testing to assess soil sulfate availability. Subsamples of 10 selected soils (Mollisols) from the Pampas (Argentina) were sent in two batches to five soil-testing laboratories. Laboratories were unaware of the existence of subsamples and performed routine sulfate analysis as if these soils came from 60 different fields. Soil sulfate ranged from 3.3 to 20.6 mg kg−1. One laboratory reported sulfate values greater than the other ones, having a mean bias of 4.1 mg kg−1 S sulfate (SO4). The other four laboratories reported similar sulfate values when soils had low sulfate availability (less than 10 mg S kg−1), even when they used d...