Sulfate is a major ion found in water. It has the highest oxidation state of Sulfide. Sulfates exist in almost all kinds of natural water. Metal sulfides such as pyrites in igneous rocks and sedimentary rocks are oxidized to sulfate by dissolved oxygen through the weathering process. Sulfate has low toxicity. However, high concentrations would cause calcium deposits and indirectly poison the crops with sodium. Sulfate in the water would be reduced to hydrogen sulfide by microorganisms in anaerobic environments.