Iodine (I) is an essential nutrient for mammals including humans.  Iodine deficiency is still the number one preventable cause of mental retardation worldwide.  The greatest source of iodine to soils is I that has been released into the atmosphere from sea water. Thus, areas far removed from oceans are generally deficient in I and people living in these areas require I supplements to offset the mental and developmental problems associated with its deficiency. Mild I deficiency is appearing again in some developed countries and is thought to be the result of current health trends that omit iodized salt from the diet. Iodine deficiency will become an even greater concern with the movement toward organic farming and the consumption of more localized produce, especially if crops are produced on soils lacking I. The dominant forms of I in soil solutions are the inorganic species iodate (IO3-), iodide (I-), and molecular iodine (I2).  Thermodynamically, under typical pH and Eh ranges found in natural soil environments, I- is the most prevalent phase, while IO3- is only present under more oxidizing conditions. Proper management of these systems will require an understanding of the behavior of I in soil environments in order to safeguard against any potential threats to public health and possible toxicity to plants and animals. Likewise, proper management of I-based soil and water biocides warrants a thorough understanding of the biogeochemical cycling of I in soils.