Elemental Sulfur

What are elemental sulfur and sulfurous acid? How are they allowed for use in organic farming?

By Cesar Cartagena

Sulfur is the fifth most common element on Earth when measured according to mass. “Elemental sulfur” refers to the chemically simple form that is not combined with other elements in compounds (e.g., sulfates, sulfites or sulfur dioxide). Since sulfur is so abundant, the extraction of pure sulfur from volcanic regions was the primary method for production in the United States until the late 20th century. Today, most elemental sulfur is a synthetic substance with a high level of purity that is obtained as a desulfurization by-product from refining coal, natural gas and petroleum. This increased level of purity is required for sulfurous acid production, and allows for the generation of sulfurous acid through the burning of elemental sulfur. Both elemental sulfur and sulfurous acid have allowances in organic crop production.

Synthetic elemental sulfur is allowed with restrictions per the National Organic Program (NOP) regulations at §205.601(e)(5) as an insecticide, and at §205.601(i)(10) for plant disease control, if the requirements of §205.206(e) are met. Synthetic elemental sulfur is also allowed without restrictions at §205.601(j)(2) as a plant or soil amendment, and nonsynthetic sulfur is allowed without use restrictions per NOP Guidance 5034-1. The OMRI Generic Materials List© (GML) includes the categories Sulfur – elemental and Sulfurous Acid that address the respective NOP allowances and restrictions.

Synthetic and nonsynthetic forms of elemental sulfur require months or years before sulfur becomes bioavailable for crop production, and large-scale applications may not prove to be cost-effective. Sulfur reacts with most other elements, yet it cannot be used readily by plants and must be oxidized to a sulfate to be effective. For this reason, it can be helpful to understand the compliant uses of forms of sulfur which are more suitable for plant uptake, specifically sulfurous acid. Sulfurous acid (CAS # 7782-99-2) is commonly used to produce immediate results in crop applications. In essence, sulfurous acid is water that has been sprayed through fumes produced by burning elemental sulfur, typically under closed conditions in a sulfurous acid generator. While burning sulfur emits sulfur dioxide, when dissolved in aqueous solution the sulfur dioxide becomes sulfurous acid. In July 2010, the NOP added sulfurous acid to the National List at §205.601(j)(9) as an allowed synthetic for use as a plant or soil amendment “for on-farm generation of substance utilizing 99% purity elemental sulfur per paragraph (j)(2) of this section.” This annotation could be interpreted to suggest that the 99% purity requirement also applies to elemental sulfur at §205.601(j)(2). However, OMRI currently does not hold other elemental sulfur products that are not marketed for sulfurous acid production to that standard, as described in the various Sulfur – elemental categories in the GML.

This article was originally published in the Winter 2019 edition of the OMRI Materials Review newsletter.