Which form of 'sulfur' is readily available in plants?

Sulfur is an essential nutrient for plants and is now becoming recognized as the fourth macronutrient, along with nitrogen, phosphorous and potassium.

Most of the sulfur absorbed by plants (about 90%) is used for building blocks of proteins. Sulfur is essential for chlorophyll (green pigment of plants) formation. Sulfur is found in soil in the organic matter and is not available to plants in this form. In order to become available to plants, the sulfur must be first released from the organic matter and go through the mineralization process. This process is a result of microbial activity. By the mineralization process, the transfer of organic sulfur into inorganic forms is the first step. The inorganic forms are sulfide (S), sulfite (SO3), hydrogen sulfide (H2S) and elemental sulfur (S). These forms undergo oxidation to form sulfate (SO4-2). This SO4-2 form is readily available in plants. Due to its negative charge, the sulfate ion is mobile in soils.

Today, there are various fertilizers are available containing significant amounts of sulfur. The most common ones are ammonium sulfate, magnesium sulfate and potassium sulfate.