"What is Nitrate?"
Sodium nitrate is a naturally-occurring substance in vegetables, water, soil, and even the air. Originally discovered as an impurity in salt, small amounts have been used for thousands of years to cure meats.
Rust, Robert E.
Iowa State University Veterinarian: Vol. 40
, Article 7.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol40/iss1/7