In horticulture, lime sulfur (British spelling lime sulphur) is a mixture of calcium polysulfides formed by reacting calcium hydroxide with sulfur, used in pest control. 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.
A (New York State Agricultural Experiment Station) recipe for the concentrate is 80 lb. sulfur, 36 lb. high grade quicklime, and 50 gallons of water. 2.25:1 is the ratio (by weight) for compounding sulfur and quicklime; this makes the highest proportion of pentasulfide. If hydrated lime is used, an increase by 1/3 or more (to 48 lb. or more) might be used with the 80 lb. of sulfur. If the quicklime is 85%, 90%, or 95% pure, use 42 lb, 40 lb, or 38 lb; if impure hydrated lime is used, similarly increase its quantity. Avoid using lime that is less than 90% pure. Boil for an hour, stirring and adding small amounts of hot water to compensate for evaporation.
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, sterilise, and preserve deadwood on bonsai trees while giving an aged look. Rather than spraying the entire tree, as with the pesticidal usage, lime sulfur is painted directly onto the exposed deadwood, and is often colored with a small amount of dark paint to make it look more natural. Without paint pigments, the lime-sulfur solution bleaches wood to a bone-white color that takes time to weather and become natural-looking. Because the lime sulfur does not contact the leaves or needles, this technique can be used on evergreen trees as well as other types of trees.
Diluted solutions of lime sulfur (between 1:16 and 1:32) are also used as a dip for pets to help control ringworm (fungus), mange and other dermatoses and parasites. (Undiluted lime sulfur is corrosive to the skin and can 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.
"Chemical Investigation of Best Conditions for Making the Lime-Sulfur Wash." L.L. Van Slyke, A.W. Bosworth, & C.C. Hedges, New York Agricultural Experiment Station Bulletin No. 329, December 1910, Geneva, New York