FLUVISOL

one of the 30 soil groups in the classification system of the Food and Agriculture Organization (FAO). Fluvisols are found typically on level topography that is flooded periodically by surface waters or rising groundwater, as in river floodplains and deltas and in coastal lowlands. They are cultivated for dryland crops or rice and are used for grazing in the dry season. They occupy about 2.8 percent of the continental land area on Earth, mainly in the great river basins and deltas of the world (e.g., the Amazon basin and the Nile delta). Fluvisols are technically defined by a weak or nonexistent surface horizon (uppermost layer) and by parent material derived from river, lake, or marine sediments that have been deposited at regular intervals or in the recent past. These soils exhibit a stratified profile that reflects their depositional history or an irregular layering of humus and mineral sediments in which the content of organic carbon decreases with depth. Wide variations in texture and mineral composition are observed. Fluvisols are related to the Inceptisol and Entisol orders of the U.S. Soil Taxonomy, wherever the latter occur on floodplains and deltas. Fluvisols are sometimes found in conjunction with Gleysols, a related FAO soil group formed under the influence of water.