Because it is dissolved in the soil solution, gypsum readily migrates into the lower depths in the soil. Most western soils contain abundant calcium, but as very insoluble calcium carbonate. Where pH is 7 or more, the limestone is insoluble and the calcium it contains is not available to plant roots. Again, the calcium in applied gypsum is immediately available. The other component of gypsum, sulfate, and supplies plants with sulfur, which is essential to protein synthesis.
Sulfur is a constituent of the amino acids cystine, methionine and cysteine.
It is also essential for nodule formation on legume roots, and for the characteristic odors of plants such as garlic and onions. Sulfur deficiencies are less common than calcium deficiencies, but can occur throughout North America, particularly in high-rainfall areas. If sulfur is applied as elemental sulfur, it is not available until the soil bacteria oxidize it to sulfuric acid. Surface-applied sulfur is oxidized faster than incorporated sulfur, but because the required bacteria are frequently not present in alkaline soils, the oxidation may be very slow. Gypsum supplies sulfur as sulfate, the form plant roots can absorb.