On sedimenting shorelines in temperate latitudes, large areas are often dominated by salt marshes with the vast stands of a smooth cordgrass Spartina alterniflora. These marshes have a very high rate of primary production, approximately 1400 g C m-2 y-1, and are stable over long time periods in spite of growing anthropogenic pressure. It is to assume that humic matter that represents a significant fraction of the natural organic matter contribute to the stability of the salt marsh ecosystem due to their interaction with a wide spectrum of inorganic and organic elements and compounds. Their formation, characteristics, and fate accomplish quite a completely the natural processes of biogeochemical cycling. Both fresh and dead biomass of S. alterniflora represents a natural source of humic substances, and different epiphytic microscopic fungi are capable of forming humic-like polymers by utilizing water extractable plant constituents. Also, natural populations of microorganisms autochthonous to the salt marsh sediment demonstrated capability of releasing humic matter from the plant biomass into sea water. Simultaneously, however, the sediment-related microflora showed capacity in employing water dispersed humic substances under aerobic and semi-anaerobic conditions as a supplemental source of nutrients and the sole sources of carbon and nitrogen. Due to the microbial activities, humic matter usually exhibit structural changes characteristic of diagenetic transformations. In addition, humic matter interacts with clay minerals by forming organic-mineral complexes (humins) that can resist degradation. Conclusively, the sedimentary humic matter serves an excellent example of important complex biogeochemical processes occurring in a natural ecosystem such as salt marsh estuary.