Sulfur is one of the world's oldest remedies for gardens and human ailments. It occurs naturally as a pure element in the form of a yellow powder and as crystals. Sulfur is a product of volcanic emissions and ancient societies mined sulfur around volcanoes. In modern times, most sulfur production is a by-product of crude oil and gas processing. Sulfur readily forms compounds such as pyrite (iron sulfide) and gypsum (calcium sulfate). It is an essential element, as all living cells contain sulfur.

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Volcanic eruptions in Indonesia deposited yellow sulfur.

Lowering pH

Most plants prefer soils of pH range between 5.8 and 7.0. Some plants such as azaleas, gardenias and blueberries prefer a more acidic soil. Soils in limestone regions may have a pH as much as 8.0 and consist of 50 percent lime. Such alkaline soils induce deficiencies in iron, zinc and manganese. Leafy plants may become yellow in these conditions. Sulfate-based fertilizer may solve these problems but in more extreme cases, elemental powdered sulfur is required. The sulfur powder must be mixed well in moist and aerated soil for the treatment to work.

Fungicide

Sulfur controls powdery mildew and fungus on leaves. It works best when applied on dry leaves. Sulfur prevents mold and has to be applied before the fungus settles. Reapplication is required after heavy rain. Sulfur halts the rot on cuttings from fleshy plants like begonias. The stem is dipped in sulfur before propagating. It also stops mold on bulbs and rhizomes. Its drawback is that at temperatures above 90 degrees Fahrenheit, it can burn leaves.

Pesticide

Sulfur is one of the oldest known pesticides. Its best application is for vegetables and fruit. It controls psyllids, spider mites and thrips but is toxic to cucumber, raspberry and apricots. Sulfur powder cannot be mixed with horticultural oils and applied to plants as a pesticide, as this combination will kill the plants. Some gardeners can suffer bronchial problems if they do not wear a face mask when applying powdered sulfur.

Nutrient

Sulfur is an essential plant nutrient like nitrogen, potassium and phosphorus. In addition to its own nutritional properties, it improves the soil and assists in fixing nitrogen and phosphorus. Intensive agriculture worldwide has lead to a deficiency of sulfur in soil. Measures to halt air pollution by reducing or eliminating sulfur dioxide emissions from electricity-generating plants has reduced sulfur absorption by soils.