Ferrous iron may be distinguished as water-soluble, exchangeable, complexed, and precipitated. pH plays an important role in determining the relative proportions among the various forms. The water-soluble iron exists in the form of chelates carrying a negative or positive charge or ions. The stability constants of ferrous chelates as well as manganese chelates are of the order of 2.5 to 5.5. pH is determinative with respect to the concentration of both molecular H2S and ionic S2-. There is a close relationship between Eh and the amount of reducing substances in the soil.