Soil

The layers of weathered particles of earth material that contain organic matter and can support vegetation are defined as soil. Soil can be all or just part of the sedimentary material that covers the bedrock. For example, in Wisconsin the bedrock is covered by up to 120 meters (400 feet) of glacial gravel, sand, silt, and clay—yet only the upper few feet is considered to be soil. It can take hundreds of thousands of years to form soil. Soil contains mostly quartz and clay particles of varying sizes. The quartz sand grains help keep the soil porous, and the clay particles hold water and nutrients for plant growth. Loam contains sand, silt, and clay with abundant organic material. Topsoil is the upper part of a section of loam, has the highest organic content, and is considered to be the most fertile layer. The rocky subsoil underlies the topsoil and contains less organic material.

Residual soil and transported soil. When soil is developed from the weathering of the underlying bedrock it is called residual soil. Transported soil is deposited by agents such as ice and water and is not derived from the underlying bedrock. Examples include sand left by retreating glaciers and the mud that is left after a flood. Much of the rich black soil in the midwestern United States was deposited by melting glaciers about 10,000 years ago.

Soil horizons. A fully developed soil, or mature soil, consists of three layers, or soil horizons (Figure 1). The uppermost layer is called the A horizon. Meteoric water moves down through the horizon and typically leaches clay minerals, iron, and calcite from the soil. The top part of the A horizon consists of the dark, organic loamy topsoil (the thin layer of humus and plant litter on the surface that is usually considered part of the A horizon is sometimes referred to as the O horizon); the rest of the horizon is a pale yellowish‐tan.

Figure 1

Soil Horizons


The middle layer is called the B horizon or the zone of accumulation. The leached materials from the A horizon often precipitate in this more clay‐rich zone that is reddish‐brown from iron minerals. This horizon is also called the subsoil. The combined O‐A‐B section is sometimes called the solum. Known as the “true soil,” the solum is where most of the biological activity in soil is confined.

The C horizon is the lowest soil layer and lies directly above the bedrock. This layer is part soil and part decomposing bedrock fragments. It contains very little organic material. The interface between the bedrock surface and overlying sedimentary material is called the regolith and consists of broken, rubbly pieces of bedrock that are variably weathered and decomposed.

Immature soils do not have the complete O‐A‐B‐C profile and are commonly found on steep slopes, where the weathering products migrate down the slope instead of accumulating in place.

Soil types. The kind of soil that develops, and how quickly it develops, depends largely on the parent rock and the climate. Felsic rocks with high amounts of quartz weather to light‐colored soil consisting of sand and clay with quartz grains. A basalt, on the other hand, which has a high percentage of ferromagnesian minerals, weathers to a darker‐colored soil that is higher in clay content and lower in quartz.

Parent rocks form soils the most rapidly in wet, humid climates, where chemical weathering is enhanced. A pedalfer is a B horizon that is high in aluminum and iron. It develops in response to abundant rainfall, organic acids, and strong downward leaching. Pedocals are formed in arid climates by water drawn upward by subsurface evaporation and capillary action, which often crystallizes hard calcium salts in the soil. Pedocals are thin and poorly leached and have only a small amount of humus. Alkali soils, such as the white salt flats in the western United States, are pedocals that are toxic to plants because of the high salt content. Hardpan is a layer of soil, usually the B horizon, that is so hard (usually cemented by calcite or quartz) that it must sometimes be blasted loose. Common in the western United States, a hardpan formed by the precipitation of salt by evaporation is called caliche.

Laterites are highly leached, residual soils that form in tropical regions. The high temperature and rainfall result in deep and intense weathering. The laterite is typically bright red and composed of iron and aluminum oxides that are the most resistant to leaching. Valuable metals such as aluminum, copper, silver, gold, nickel, and iron are concentrated in laterites through secondary enrichment and can sometimes be mined economically. Secondary enrichment works in two ways: It can remove many of the other elements from the rock, which enriches the remaining valuable elements in place, or it can leach valuable elements such as copper and gold and deposit them at a particular level lower in the laterite.