Sulfurous Acid

Is sulfurous acid acceptable as a soil amendment in organic crop production?

By Jennifer Schomp

Sulfurous acid is a gaseous compound made up of sulfite and bisulfite (hydrogen sulfite) anions that are an effective pH adjuster when suspended in an aqueous solution. It was considered by the NOSB in 2009 for addition to the National List, and in July 2010 the National Organic Program (NOP) added sulfurous acid to the National List as an allowed synthetic for use as a plant or soil amendment, where it remains, having completed a sunset review in fall 2018. Elemental sulfur of at least 99% purity is acceptable for use in on-farm generation of sulfurous acid.    

Sulfurous acid generation involves oxidation of elemental sulfur in a heat chamber to produce sulfur dioxide. Sulfur dioxide is then injected into a regulated flow of irrigation water, forming sulfurous acid. The unstable nature of the compound makes manufacture off-site and transport to farms impossible. On-farm generation is therefore necessary. This form of sulfur is preferable to large-scale broadcast applications of elemental sulfur, as it has less of an impact on soil biota. Also, while soil pH adjustment is immediate with sulfurous acid, the same effect can takes months or years when elemental sulfur is applied. Sulfurous acid generators are currently available on the market for use in on-farm production.    

Elemental sulfur is not currently commercially mined. It is primarily available as a by-product of oil and gas manufacture, since air pollution regulations require the removal of elemental sulfur from commercial gasoline products. The NOSB recommendation to approve generation of sulfurous acid cited the benefits of its use in organic agriculture. These include a reduced need for direct addition of elemental sulfur for soil pH adjustment, which may cause accumulation of carbonates and bicarbonates and alkalinity extremes that affect soils in arid climates. Use of sulfurous acid may also reduce the use of some other compliant acids that require significant resources for transportation, including citric acid, peracetic acid, vinegar, and other acids used for pH modification.

This article was originally published in the winter 2011 edition of the OMRI Materials Review newsletter, and was revised and updated in May 2021 by Technical Director Doug Currier.