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Formation of B Horizons



The subsoil in an AC soil consists of the C horizon and, perhaps, the upper part of the parent mate­rial. Under favorable conditions, this subsoil layer eventually develops a distinctive color and some other properties that distinguish it from the A hori­zon and underlying parent material, commonly at a depth of about 60 to 75 centimeters. This altered subsoil zone becomes a B horizon and develops as a layer sandwiched between the A and a new deeper C horizon. At this point in soil evolution, insufficient time has elapsed for the B horizon to have been significantly enriched with fine-sized (colloidal) particles, which have been translo­cated downward from the A horizon by percolat­ing water. Such a weakly developed B horizon is given the symbol w (as in Bw), to indicate its weakly developed character. A Bw horizon can be distinguished from A and C horizons primarily by color, arrangement of soil particles, and an inter­mediate content of organic matter.

During the early phases of soil evolution, the soil formation processes progressively transform parent material into soil, and the soil increases in thickness.

The Bt Horizon. Soil parent materials frequently contain calcium carbonate (CaCO3), or lime, and are alkaline. In the case of glacial parent materi­als, lime was incorporated into the ice when gla­ciers overrode limestone rocks. The subsequent melting of the ice left a sediment that contains limestone particles. In humid regions, the lime dissolves in percolating water and is removed from the soil, a process called leaching. Leaching effects are progressive from the surface down­ward. The surface soil first becomes acid, and subsequently leaching produces an acid subsoil.

An acid soil environment greatly stimulates mineral weathering or the dissolution of minerals with the formation of many ions.

Clay formation results mainly from chemical weathering. Time estimates for the formation of 1 percent clay inn rock parent material range from 500 to 10,000 years.

Many soil parent materials commonly contain some clay. Some of this clay, together with clay produced by weathering during soil formation, tends to be slowly translocated downward from the A horizon to the B horizon by percolating water. When a significant increase in the clay content of a Bw horizon occurs due to clay trans­location, a Bw horizon becomes a Bt horizon.

Thin layers or films of clay can usually be ob­served along cracks and in pore spaces with a 10-power hand lens. The process of accumulation of soil material into a horizon by movement out of some other horizon is illuviation. The t(as in Bt) refers to an illuvial accumulation of clay. The Bt horizon may be encountered when digging holes for posts or trenching for laying underground pipes.

Alternating periods of wetting and drying seem necessary for clay translocation. Some clay particles are believed to disperse when dry soil is wetted at the end of a dry season and the clay particles migrate downward in percolating water during the wet season. When the downward per­colating water encounters dry soil, water is with­drawn into the surrounding dry soil, resulting in the deposition of clay on the walls of pore spaces. Repeated cycles of wetting and drying build up layers of oriented clay particles, which are called day skins.

Many studies of clay illuviation have been made. The studies provide evidence that thou­sands of years are needed to produce a significant increase in the content of clay in B horizons. An example is the study of soils on the alluvial floodplain and adjacent alluvial fans in the Cen­tral Valley of California. Here, increasing eleva­tion of land surfaces is associated with increasing age. The soils studied varied in age from 1,000 to more than 100,000 years.

The Hanford soil developed on the floodplain is 1,000 years old; it shows no obvious evidence of illuviation of clay. The 10,000-year-old Greenfield soil has about 1.4 times more clay in the subsoil (Bt horizon) than in the A horizon. Snelling soils are 100,000 years old and contain 2.5 times more clay in the Bt horizon than in the A horizon. The San Joaquin soil is 140.000 years old, and has 3.4 times more clay in the horizon of maximum clay accumulation as compared to the A horizon.

The three youngest soils (Hanford, Greenfield, and Snelling) are best suited for agriculture be­cause the subsoil horizons are permeable to wa­ter and air, and plant roots penetrate through the B horizons and into the C horizons. Conversely, the impermeable subsoil horizon in San Joaquin soil causes shallow rooting. The root zone above the impermeable horizon becomes water saturated in the wet seasons. The soil is dry and droughty in the dry season.

Water aquifters underlie soils and varying thick­nesses of parent materials and rocks. Part of the precipitation in humid regions migrates com­pletely through the soil and recharges underlying aquifers. The development of water-impermeable claypans over an extensive region results in less water recharge and greater water runoff. This has occurred near Snittgart, Arkansas, where wells used for the irrigation of rice have run dry because of the limited recharge of the aquifer.

The Bhs Horizon Many sand parent materials contain very little clay, and almost no clay forms in them via weathering. As a consequence, clay illuviation is insignificant and Bt horizons do not evolve. Humus, however, reacts with oxides of aluminum and/or iron to form complexes in the upper part of the soil. Where much water for leaching (percolation) is present, as in humid regions, these complexes are translocated down­ward in percolating water to form illuvial accumu­lations in the B horizon. The illuvial accumulation of humus and oxides of aluminum and or iron in the B horizon produces Bhs horizons. The h indi­cates the presence of an illuvial accumulation of humus and the s indicates the presence of illuvial oxides of aluminum and /or iron. The symbol s is derived from sesquioxides (such as Fe2O3 and Al2O3). Bhs horizons are common in very sandy soils that are found in the forested areas of the eastern United States from Maine to Florida.

The high content of sand results in soils with low fertility and low water-retention capacity (droughtiness).







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