Naturally occurring organic compounds in soils have been studied for more than 200 years because agricultural scientists early recognized the importance of natural organics in enhancing soil fertility, soil structure, and water-holding capacity (Kononova, 1966). The most general term for the natural organic compounds in soils, sediments, and natural waters is natural organic matter (NOM); however, a number of other terms have been applied to different NOM fractions. The most commonly used of these are: humus, humic substances, humic acid, fulvic acid, and humin. The dissolved fraction of the NOM in natural waters is commonly called dissolved organic matter (DOM) or dissolved organic carbon (DOC). Historically, the term humus has been applied to the dark-colored, "rotted" organic matter in soils, and the terms humic acid, fulvic acid, and humin have been used to designate different fractions of humus (Kononova, 1966).