SPODOSOL

one of the 12 soil orders in the U.S. Soil Taxonomy. Spodosols are ashy gray, acidic soils with a strongly leached surface layer. Their suitability for cultivation is limited to acid-tolerant crops and orchards, provided that sufficient lime and fertilizer are applied. Covering about 3.5 percent of the nonpolar continental land area on Earth, they are mainly found bordering the tundras in northern North America and Asia, but they also occur in southern South America (Tierra del Fuego), in subtropical North America (Florida), and in lowland regions of tropical South America (northern Brazil, extending into Colombia and Venezuela). The sandy-textured underlying layer, known as the spodic horizon, is found not more than two metres (about six feet) below the land surface and is overlain by a strongly bleached sandy layer that gives the soil profile a spectacular visual contrast. The defining characteristic of the spodic layer is the significant accumulation of iron- and aluminum-bearing minerals intermixed with humus. The horizon develops over centuries as percolating water from ample, but seasonal, precipitation facilitates the translocation (migration) of material from upper horizons. The presence of organic compounds derived from acid-tolerant vegetation, usually needle-leaved evergreen forest, contributes to the acidic humus content of the soil and to the mobilization of the iron and aluminum. Spodosols differ from Alfisols and Ultisols, both of which can exhibit bleached layers, by the absence of subsurface accumulations of translocated layer silicate clay; in addition, Spodosols generally develop under cooler climatic regimes.