Soil formation

A soil is formed when decomposed organic material is encompassed into weathered mineral material at the earth’s surface. The climate, the organisms living in the soil, the type of parent material, the local topography, and the amount of time the soil has been developing all influence the resulting soil characteristics. Changes in the soil arise from the main processes of soil formation: addition, removal, transfer, and transformation of parts of the soil.

Stages of Formation

Soil formation and development are continuing processes. A soil is a layer of unconsolidated weathered mineral matter on the earth’s surface that can support life. It is initially formed when the remains of plants are mixed into weathered mineral fragments. Soil characteristics can change, however, for soil develops over time as the environment dictates.

Many processes contribute to the two stages of soil formation, which may grade indistinctly into each other. The first stage is the accumulation of rock fragments known as parent material. These sediments may have formed in place from the physical and chemical weathering of hard rocks below or by the accumulation of sediments moved by wind, water, gravity, or glaciers. The second stage is the formation of soil horizons, which may either follow or occur simultaneously with the first stage. The near-surface horizon, where materials are removed, is called the A horizon. Some of the removed material accumulates in the subsurface, in a zone called the B horizon. If the soil has an organic-rich top layer, that horizon is called the O horizon. In some cases, the A horizon has so much iron removed that it looks bleached. A very light zone at the base of the A horizon is called the E horizon. Unmodified parent material is termed the C horizon.

The characteristics of the horizons and their degree of differentiation depend upon the relative strengths of four major processes in the soil: weathering, transfers, gains, and losses of the soil constituents. These processes have been observed directly in laboratory experiments but also are implied by relating physical, chemical, and morphological properties in different parts of the soil profile.

Transformations and Transfers

Weathering is the fundamental process of soil formation, as it transforms soil constituents into new chemicals. The primary minerals found in the initial sediments and rocks are decomposed by physical and chemical agents into salts that are soluble in water and new minerals. Clay is a very important new mineral formed from the breakdown of primary minerals by the agents of water and air. Calcite, or calcium carbonate, is another mineral formed through weathering, especially of rocks such as basalt. Iron and aluminum hydroxides form a third major group of weathering-produced minerals. Organic matter is added to the soil when roots die and vegetation residues fall onto the surface. These are transformed by microorganisms into a mixture of chemically stable, organic substances called humus.

The transfer of soil constituents is generally in a downward direction. Rainwater is the major transporting agent. As it moves through the soil, it picks up salts and humus in solution and clays and humus in suspension. These materials are usually deposited in another horizon when the water is withdrawn by roots or evaporation, or when the materials are precipitated as a result of differences in pH (degree of acidity) or salt concentration. The clays normally coat the exteriors of sediment grains and line the interiors of pores. This transfer of materials through solution is commonly called leaching and requires good drainage. The soil profile often becomes vertically zoned according to the solubility of the dissolved substances. In forest soils, leaching generally removes iron and aluminum, leaving behind a bleached-looking zone. This process is called podzolization. In arid soils, calcium minerals precipitate in the subsoil to form calcite or gypsum, a deposit called caliche.

Gains and Losses

Gains consist of additions of new materials to the soil. Wind can deposit fine sands and silts if there is a source nearby. Chemical reactions during weathering add oxygen and water to the soil minerals. Groundwater brings new minerals into the soil pores. Floods can add new sediment to the soils in a floodplain. The largest addition to soils, however, comes from organic matter from the decomposition of plant material. Annual additions of new organic material between 255 and 22,900 kilograms per hectare have been estimated for soils in different environments ranging from deserts to tropical rain forests.

Losses from the soil profile generally occur when there is so much water moving through the system that the dissolved and suspended materials exit at the bottom of the profile and move downslope and eventually enter a stream. Soil erosion also occurs on the surface by water runoff, especially where there is minimal vegetation to control the sediment. Soil erosion is one of the major environmental problems facing the world today. Oxidation of plant matter balances some of the additions of new organic material.

Climate

After describing many soil profiles around the world and classifying them, scientists concluded that this weathered material at the earth’s surface was quite variable and differed from environment to environment. In 1980, Hans Jenny summarized the work of early researchers and listed the five main soil-forming factors that created this variation: climate, organisms, parent material, topography, and time. The rates and degrees of these processes in different soils depend upon these five factors. These factors are interdependent, and, therefore, very different soils may be formed from the same parent material.

Of the five factors, climate has probably the greatest impact on soil development. This dominance is best shown on a large-scale geographic basis, in which the distribution of soils on soil maps is related to vegetation type, which is in turn related to climatic zones. Precipitation provides the water for the transfer processes in the soils. In dry climates, where little water is moving through the system, calcium carbonate and salts build up in the soils. In humid climates, the abundance of water moving through the systems allows leaching and clay movement processes to dominate in the soils. Temperature is the second important element of climate, as it controls the rates of chemical weathering reactions in the soil. In colder environments, soil weathering and development are slower because weathering reactions are not active all year round. Some extreme climatic conditions also can control soil development, such as permafrost, which limits soil depth and drainage. Overall, climate controls vegetation, which also controls soil development.

Organisms and Parent Materials

Living organisms, primarily vegetation, control the distribution and types of humus that are formed in soils. In turn, humus controls some of the important soil-forming processes. Grassland vegetation produces humus that is highest in mineral matter and not very acid, so resulting soils have thick A horizons that are high in organics. Conifer forests produce humus that is low in mineral constituents and highly acidic. These forest soils are dominated by leaching and the production of O, E, and B horizons (B horizons are rich in iron and aluminum). Deciduous forest soils are located somewhere between the conifer forest and the grassland soils in characteristics. Therefore, it is rare to find white, bleached E horizons under grassland vegetation, and vice versa—it is rare to find thick A horizons under conifer forest vegetation. Through its leaves and root systems, vegetation also protects the soil from erosion and, therefore, promotes soil development.

The parent material is the rock or sediment deposit from which the soil has developed. The composition of the original minerals controls the complexity of the soil processes by determining the weathering by-products, and controls the rate of these processes by porosity. A parent material of pure quartz sand would undergo minimal weathering, producing a simple soil profile. A parent material of wind-deposited silt, or loess, produces a more complex soil, as it has many minerals initially that can weather to by-products such as clays, salts, carbonates, and oxides. A soil produced on granite would yield a fairly deep, sandy soil, whereas a soil produced on basalt would yield a fairly thin, clay-rich soil. A soil developed in shale would be clay-rich, whereas one developed on sandstone would be sandy. Soils formed on rocks rich in iron tend to produce very red soils. Soils formed on limestone are difficult to acidify and podzolize because the parent material neutralizes those chemical processes. The porosity, or the amount of pore space, of the parent material gives ready access of gases and liquids to weathering. In porous, or permeable, soils, weathering and transfers of soil constituents are more rapid than in soils with low permeability.

Topography and Time

Topography is the geometric configuration of the soil surface and is defined by the slope gradient, orientation, and elevation. This factor essentially controls the microclimate. A slope modifies rainfall by allowing rapid runoff on a steep slope or ponding in low places. Slope orientation modifies soil temperatures by the differences at which the sun’s rays strike the soil. In the northern latitudes, southfacing slopes are warmer and drier than those on north-facing slopes. In mountain ranges, rainfall increases and temperature decreases with increases in elevation.

Time is the factor of the duration of the processes working in the soil. All the characteristics in the soil take time to form. The A horizon forms most rapidly, taking as little as ten years with the aid of humankind to more than one thousand years in a cold and dry arctic or alpine environment. B horizons take longer to form because minerals weather slowly. In the dry western parts of the United States, discoloration from the weathering of iron-rich minerals appears at about one thousand years, but the strongest red colors require 100,000 years of development in the same environment. In the same part of the United States, initial detection of clay movement into the B horizon in a sandy parent material occurs after about ten thousand years of development, with maximum development taking more than 100,000 years. Calcium carbonate deposition in arid regions of the world takes between 100,000 and 500,000 years to produce a strongly cemented horizon. It is estimated that it takes more than 1 million years to produce a highly weathered lateritic soil (decayed iron-rich rock that is red in color) in the tropics.

Soils change with time just as humans change as they grow. Young or poorly developed soils in a grassland environment have only A and C horizons. With time and the movement of iron oxides and clay into the lower parts of the profile, a faint B horizon develops between the A and C horizons. An old or well-developed soil in this environment would have a thick A horizon and a thick B- horizon with abundant clay moved into it, all overlying a C horizon.

Scientists are finding that soils can be useful tools in the interpretation of past environments of an area and also in the formulation of land-use plans for that region. First, the soil scientists must determine the exact soils that are in the area through a mapping program. During this project, they determine the regional developmental sequence of soils—that is, the poorly developed and the well-developed soils of the different places on the landscape. They assign ages to each soil in the sequence using radiocarbon-dated samples from the soils. As a result, when a particular soil is located in that region and compared with the sequence, an age can be assigned to the land surface upon which the soil developed. Geologists commonly use soils to estimate the ages of deposits, especially on glacial landscapes. The idea of multiple glaciations has been developed through the use of soils. Poorly developed soil on glacial sediments represents a recent glaciation compared with a site that features well-developed soil lying on sediment deposited by an older glacial advance.

The frequency of geological hazard events such as landslides, rockfalls, and flooding can be estimated by the study of buried soils in sediment deposits. An ancient, buried A horizon within a soil profile means that some geological event covered an old land surface with sediment. If the organic matter of that buried A horizon is dated using radiocarbon dating, an approximate time of burial can be determined. If a valley bottom has many buried soils, it probably means that the nearby stream floods quite frequently. These decisions are important for land-use planning, for one does not want to build in an area where hazards occur frequently. For example, nuclear power plants must not be located in areas where there are active faults (ground breaks). Soils are commonly used to determine if faults are active in an area and, if so, the frequency of movement on the fault.

The U.S. Department of Agriculture produces maps of soils across the country. The maps describe the productivity of crop growth and manageability related to land use of each soil. These data help farmers grow crops that are best suited for their soils; they tell the farmers which soils are most susceptible to erosion so that extra precautions can be taken with the fragile soils. The data also help land-use planners keep the most productive soils in agriculture instead of paving them. In addition, the maps help to predict how much fertilizer to put on the fields. Natural soil fertility is related to geological activity and weathering, as the oldest soils are leached of most of their nutrients and are least fertile. Young soils have had little weathering and leaching and are most fertile, especially those on floodplains, young glacial deposits, and volcanic deposits. Soil maps are also used to recognize where shrink-swell clays occur in the soils. The presence of shrink-swell clays in the soil is important for home owners. If present, these clays can crack the foundation of a house. To prevent these damages, special foundations must be constructed and the clays stabilized with the use of lime.

Principal Terms

horizon: a layer of soil material approximately parallel to the surface of the land that differs from adjacent related layers in physical, chemical, and biological properties

leaching: the dissolving out or removal of soluble materials from a soil horizon by percolating water

sediment: rock fragments such as clay, silt, sand, gravel, and cobbles

soil profile: a vertical section of a soil, extending through its horizons into the unweathered parent material

weathering: the mechanical disintegration and chemical decomposition of rocks and sediments

Bibliography

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