Sulfur is most often found in forms such as sulphate and sulfide and in organic compounds such as amino acids. Pure sulfur, a yellow crystalline solid, is used to acidify soils and is essential to plant growth. However, as sulfur dioxide, it reacts with gaseous water and oxygen to form sulphuric acid, one of the components of acid rain, and may be damaging to soils unless converted into other sulfur compounds.
Sulfur is found as a pure element, S, in hot springs and near volcanoes but is usually found as part of a chemical compound. It is part of proteins, amino acids and other organic materials and is essential to both animal and plant life.
Sulfur is also found in inorganic forms such as sulfur dioxide (SO2), a gas produced by volcanoes and by human activities. It can be oxidized into sulfate (SO4) in the atmosphere and in that form can cause acidic rain. Hydrogen sulfide (H2S) is produced by organic decay in the absence of oxygen and can also be transformed into sulfate.
Sulfur And Plants
Plants use substantial quantities of sulfur for the formation of proteins, as a catalyst in chlorophyll production and in other processes, but are unable to absorb it in elemental form. The only form they can use is sulfate, which is soluble and able to be absorbed by the roots. Broccoli, cabbage, cauliflower, corn and beets have especially high needs for sulfur.
Sulfur And Soil pH
Elemental sulfur is added to neutral or alkaline soils to make them more acidic, but the effect of an application or of rain containing sulfate depends on a number of factors. The measurement of these factors, including the amount of clay, organic matter and lime, is called the "buffer capacity," the capacity to resist a change in pH. Lime, calcium carbonate, neutralizes acidity and both clay and organic matter hold hydrogen ions, removing them from the soil solution. Plants also neutralize acidity as their roots absorb sulfate.
Soil Microbes And Sulfur
Microbes in the soil break down organic matter during the process of decay and, in a well-aerated soil, oxidize organic sulfur compounds to gain energy, releasing sulfate as a by-product. Because of their role in the cycle, sulfur is most available when the soil is warm, moist and well-drained, conditions that favor microbial action.
Sulfur And Sand
Being soluble, sulfate can be leached from the soil, lost as water carries it downward and away from the roots of crops. Sandy soils are particularly subject to leaching, having little clay or organic matter to absorb water, so excess sulfur compounds are removed more quickly than they would be in silt or clay soil.