In horticulture, lime sulfur (British spelling lime sulphur) is a mixture of calcium polysulfides formed by reacting calcium hydroxide with sulfur. It can be prepared by boiling calcium hydroxide and sulfur together with a small amount of surfactant. It is normally used as an aqueous solution, which is reddish-yellow in colour and has a distinctive offensive odour.
Lime sulfur is sold as a spray for deciduous trees to control fungi, bacteria and insects living or dormant on the surface of the bark. Lime sulfur burns leaves so it is not as useful for evergreen plants.
Bonsai enthusiasts use undiluted lime sulfur to bleach and sterilise portions of trees to give an aged look known as Jin.
Diluted solutions of lime sulfur (between 1:16 and 1:32) are also used as a dip for pets to help control ringworm. (Note that undiluted lime sulfur is corrosive to the skin and will cause serious injury.)
Lime sulfur reacts with strong acids (including stomach acid) to produce highly toxic hydrogen sulfide (rotten egg gas) and indeed usually has a distinct "rotten egg" odour to it. Lime sulfur is not extremely flammable but combustion produces highly irritating sulfur dioxide gas.
Safety goggles and gloves should be worn while handling lime sulfur. Lime sulfur solutions are strongly alkaline (typical commercial concentrates have a pH over 11.5), and so it is corrosive to living things and can cause blindness if splashed in the eyes.
Lime sulfur is believed to be the earliest synthetic chemical used as a pesticide, being used in the 1840s in France to control grape vine powdery mildew Uncinula necator, which had been introduced from the USA in 1845 and reduced wine production by 80%. In 1886 it was first used in California to control San Jose scale. Commencing around 1904, commercial suppliers began to manufacturer lime sulfur; prior to that time, gardeners were expected to manufacture their own. By the 1920s essentially all commercial orchards in western countries were protected by regular spraying with lime sulfur. However by the 1940s, lime sulfur began to be replaced by synthetic organic fungicides which risked less damage to the crop's foliage.
- Chronological History of the Development of Insecticides and Control Equipment from 1854 through 1954
- Background on History of Pesticide Use and Regulation in the United States, Part Two (PDF, 54 kB).
- The Value of Fungicides in U.S. Crop Production, (PDF, 1.1 MB)