Sulfur Soil Testing and You

(For proper impact please read the title in a monotone voice with the crackle of a film projector audio)

A variety of questions have recently come into the lab regarding sulfur soil tests and how to interpret the results. Sulfur soil tests are often misunderstood and are frequently criticized for producing inconsistent or questionable results. Much of this perception stems from a lack of understanding that the soil test chemistry of sulfur differs from that of the other primary, secondary, and micronutrients. Analytically, sulfur behaves much like nitrate and should be managed more like nitrogen than phosphorus or potassium.

Unlike phosphorus or potassium soil tests, sulfur soil tests are not intended to measure a nutrient reserve that should be built or maintained. Instead, their primary purpose is to estimate the amount of sulfur currently being supplied from natural sources and determine whether additional sulfur fertilizer is needed for the crop. Understanding this distinction is essential for interpreting sulfur soil test results.

Of the various forms of sulfur, the two predominant soluble forms are sulfate (water- and acid-soluble) and sulfide (acid-soluble). Sulfide is not stable in most soil environments. During soil testing, an acid extractant such as Mehlich 3 is used to solubilize and extract sulfur. Only sulfate forms are extracted. If the primary sulfur source is sulfate fertilizer, it will be reflected in higher soil test values, provided the soil sample was collected before the sulfate leached below the sampling depth. Elemental sulfur, however, will not be detected until it has been oxidized to sulfate.

Sulfate leaches through the soil profile only slightly more slowly than nitrate. Like nitrate, sulfur soil test results can vary throughout the year, making it impractical to build and maintain elevated soil test sulfur levels. More than 50% of sulfate present in the soil profile during the fall may be lost through leaching before the following growing season. Consequently, sulfur soil test values represent the sulfur available at the time of sampling rather than a stable reservoir of plant-available sulfur.

The dynamics of sulfur soil testing and interpretation have changed considerably over the past 50 years. The time frame of an interpretation therefore has a major influence on its relevance. Before about 1990, atmospheric sulfur deposition on agricultural fields in the eastern half of the United States commonly exceeded 20 lb S/acre annually, primarily in the sulfate form. By the early 2000s, atmospheric deposition had declined to less than 5 lb S/acre annually and is currently less than 2 lb S/acre in many areas. Most publicly available sulfur soil test interpretations were developed before this dramatic decline in atmospheric sulfur deposition.

Although atmospheric sulfur deposition has changed dramatically, the purpose of the sulfur soil test has not. The sulfur soil test is intended to estimate the sulfur supplied from natural sources—not to establish a target soil test level to achieve or maintain. While atmospheric deposition has declined substantially, sulfur can still be supplied from soil organic matter, manure, sulfate fertilizers, soil minerals, and irrigation water. The sulfur soil test helps estimate the contribution of these sources and whether supplemental sulfur fertilizer is needed.

Modern interpretation of sulfur soil test results reflects these principles. An example of sulfur fertilizer recommendations based on Mehlich 3 sulfur soil test results is shown below.

• Over 30 ppm Mehlich 3 sulfur: No additional sulfur is recommended.
• 25–30 ppm Mehlich 3 sulfur: Apply sulfur to replace approximately 50% of annual crop sulfur removal.
• 20–25 ppm Mehlich 3 sulfur: Apply sulfur to replace approximately 100% of annual crop sulfur removal.
• 15–20 ppm Mehlich 3 sulfur: Apply sulfur to supply approximately 75% of annual crop sulfur uptake.
• Under 15 ppm Mehlich 3 sulfur: Apply sulfur to supply approximately 100% of annual crop sulfur uptake.