One of the 16 essential elements for plant growth and development, sulfur is used by all plant tissue for protein synthesis and is a component of some of the amino acids that are the building blocks of proteins. Soil tests for sulfur are not always reliable. Sulfur exists in several oxidation states in the soil. Only sulfate (SO4) is available to plants, so soil test correlations are difficult since its mobility in the soil is similar to nitrate nitrogen. Sulfur recommendations often are based on crop removal data. Some state crop specialists suggest an application of 20 pounds per acre annually as a standard practice. If alfalfa or another high-quality, high-oil crop is grown, a rate of 30 pounds per acre annually is needed. In plant tissue, a 1:1 phosphorus-to-sulfur ratio is required. When the nitrogen-to-sulfur ratio is greater than 15:1 in plant tissue, low sulfur levels are indicated.