interaction. On the other hand, greater interaction appears to have reduced the
social distance separating racial groups in the United States since the 1950s; more
recently, the social distance between groups of different sexual orientation seems
to have diminished.Soils
Soils are at the interface between the lithosphere and theatmosphereand
cover a large majority ofEarth’s land surfaces. Incredibly variable over the
planet, they are natural and, most critically, capable of supporting life in ways
impossible in solid bedrock. They are able to store water and provide vital links
in thebiogeochemical cycles.
What makes up soil? About half the mass of most soils is composed of highly
weathered remains of rocks. Usually, soil contains a bit of decayed and semi-
decayed plant and animal material (about 5 percent of a good agricultural soil). The
remainder of the soil is pore space, which is occupied with soil air and soil moisture.
In an examination of a pit dug through the soil and known as a soil profile, there
are usually major differences in soil characteristics with depth. Most soils can be
conceived of as having horizontal layers called horizons. These horizons can vary
in color, fertility, texture depth, structure, and water percolation rates. Moreover,
these main horizons can be subdivided into subhorizons. The O horizon is com-
posed of organic material and is on top of the mineral soil. The A horizon is the
topmost layer of the mineral-dominated soil, and is known astopsoil. The B hori-
zon is below the A horizon and is where materials from above and, sometimes,
below have accumulated. Together, the A and B horizons are known as thesolum
(true soil). The C horizon resides beneath the solum and consists of unconsoli-
dated materials beneath the weathering and most of the organic material. Not all
soils have horizons (see Table 4) and not all soils with horizons have subhorizons.
The interaction of organic and inorganic features of the physical environment
vis-a`-vis soils is frequently illustrated by a pseudo-equation:This means that the soil observed in the field is the product of a combination of
mass and energy exchanges associated with these five factors.
Parent material is the matter forming the bulk of mineral soils. The parent
material can be rock, weathered remains of rocks (like clay, silt, or sand), organic
materials, previously existing soils, or organic matter. Parent material has a large310 Soils