(World Geography) SOILS - Major Components, Characteristics, Soil Horizons, Soil Formation & Pedogenic Processes


Concepts of Geography, Natural Resources


Soils are the product of complex mixture of weathered and eroded rock material on the one hand and organic residues on the other. Soil is a dynamic body of natural materials that is a capable of supporting a vegetative cover.

Major Soil Components

  • Soils contain four major components: inorganic materials, soil water, soil air, and organic matter.

1) Inorganic Materials: The chemical constituents of a soil typically come from many sources-the weathering of rocks, deposits of loose sediments, and solution in water. Most soil minerals are composed of elements common in Earth’s surface rocks, such as silicon, aluminium, oxygen, and iron.

2) Soil Water: When precipitation falls on the land, the water that does not run downslope or evaporate is absorbed into the rock or soil, or by vegetation. Moving through a soil, water dissolves certain materials and carries them through the soil. The water in a soil is found in several different circumstances.

3) Soil Air: Much of a soil consists of spaces between soil particles and clumps. Voids that are not filled with water contain air or gases. For most microorganisms and plants that live in the ground, soil air supplies oxygen and carbon dioxide necessary for life.

4) Organic Matter: Most soils contain decayed plant and animal materials, collectively called humus. Humus rich soils have a good capacity for water retention. Humus supplies a soil with nutrients and minerals, and is important in chemical reaction that help plants extract nutrients. Humus also provides an abundant food an abundant food source for microscopic soil organisms.

Soil Characteristics

Knowing a soil’s water, mineral, and organic components and their proportions can help us determine its productivity and what the best use for that soil might be. The most important properties include colour, texture, structure, acidity or alkalinity, and capacity to hold and transmit water and air.

1) Colour: A soil’s colour is generally related to its physical and chemical characteristics. Decomposed organic matter is black or brown, so soils rich in humus tend to be dark. If the human content is low because of limited organic activity or loss of organics through leaching, soil colours typically are light brown or black soils are often referred to as rich. However, this is not always true because some dark soils have little or no humus, but are dark because of other soil forming factors.

Red or yellow soils typically indicate the presence of iron. In moist climates, a light gray or white soil indicates that iron has been leached out, leaving oxides of silicon and aluminium, but in dry climates, the same colour typically indicates an accumulation of calcium or salts.

2) Texture: Soil texture refers to the particle sizes (or distribution of sizes) in a soil. In clayey soils, the dominant size is clay, particles defined as having diameters of less than 0.002 millimetre (soil scientists use the metric system). In silt soils, the dominant silt particles are defined are defined as being between 0.002 and 0.05 millimetres. Sandy soils have mostly particles of sand size, with diameters between 0.05 and 2.0 millimetres. Rocks larger than 2.0 millimetres are regarded as pebbles, gravel, or rock fragments, and technically are not soil particles.

Soil Textural Triangle

Loam soils, which occupy the central areas of the triangular graph, are soils with a good mix of the three grades (sizes) of soil particles without any size being greatly dominant. A second soil sample that is 20% silt, 30% clay, and 50% sand would be a sandy clay loam. Loam soils are generally best suited for supporting vegetation growth.

3) Structure: Soil structure refers to their arrangement of soil particles. Structure can partially modify the effects of soil texture. Porosity is the amount of space that may contain fluids, and while permeability is the rate at which water can pass through. Permeability is usually greatest in sandy soils, and poor in clayey soils.

4) Acidity and Alkalinity: Low pH values indicate an acid soil, and high pH indicates alkaline condition. In arid and semiarid regions, soils tend to be alkaline and soils in humid regions tend to be acidic.

Factors affecting soil formation

Pedogenesis can be defined as the process of soil development. Soil development is a function of climate, organic matter ,relief ,parent material ,and time.

1 ) Parent Material: Parent material refers to the rock and mineral materials from which the soils develop .These materials can be derived from residual sediment due tot the weathering of bedrock (residual parent material) or from sediment transported into an area by way of the erosive forces of wind, water, or ice. Soils that develop from weathering-resistant rocks tends to have a high level of similarity to their parent materials .The influence of parent material on pedogenesis is usually related to soil texture ,soil chemistry ,and nutrient cycling. The particle sizes that result from the breakdown of parent material are a prime determinant of a soil’s texture and structure. Rock material such as a sandstone ,which contains little clay and weathers into relativity coarse fragments will produce a soil of coarse texture.

2) Climate: The two most important climatic variables ,influencing soil formation are temperature and moisture. Moisture conditions affect the development and character of soil directly than any other climatic factors .High moisture availability in a soil promoted the weathering of bed rock and sediments ,chemical reactions and plan growth. Precipitation amounts of affect plant growth, which directly influences a soils organic content and fertility. In wet equatorial climates high rainfall will cause leaching of nutrients and a relatively infertile soil. Extremely aridity may result in the absence of any soil development .Evaporation is also and important factor .Salt and gypsum deposits from the upward migration of capillary water are more extensive in hot, dry regions than in colder ,dry regions .The availability of moisture also has an influence of soil pH and the decomposition of organic matter.

Temperature has a direct influence on the weathering of bedrock to produce mineral particles. Rates of bedrock weathering generally increase with highest temperatures .Chemical activity increases and decreases directly with temperature, given equal availability of moisture.

3) Organic Activity: Living organisms have a role in a number pedogenic processes including organic matter accumulation, profile mixing and biogeochemical nutrient cycling. The Cycling of nitrogen and carbons in soil in almost completed controlled by the presence of animals and plants.

4) Topography: Topography generally modifies the development of soil on a local or regional scale the slope of the land, its relief, and its aspect all influence soil development. Soil developing on moderate to gentle slopes are often better drained than soils found at the bottom of valleys. Steep topographic gradients inhibit the development of soil because of erosion. Erosion can retard the development through the continued removal of surface sediments.

5) Time: Mature soils have well developed horizons that indicate the conditions under which they formed. Young or ‘Immature’ soils are in the early stages of the development process and the typically have poorly developed horizons. For example, Alluvial soil, loess etc.

Development of Soil Horizons

Soil development begins when plants and animals colonize rocks, or deposits rock fragments, the parent material on which soil will form. Once organic process begin among mineral particles or rock fragments, Chemical and physical difference begin to develop from the surface down through the parent material.

Soil Profile

Initially, Vertical differences result from surface accumulations of organic litter and the removal of line particles and dissolved minerals by percolating water that deposits these materials at a lower level. A vertical section of a soil from the surface down to the parent materials at a lower level. A vertical section of a soil from the surface down to the land surface affect the soil development, this vertical differentiation becomes increasingly apparent. Well developed soils typically exhibit distinct layers in their soil profiles called soil horizons that are distinguished by their physical and chemical properties. Soils are classified largely on the differences n their horizons and the processes responsible for those differences. Soil Horizons are designated by a set of letters that refer to their composition, dominant process ,or position in the soil profile.

At the surface, but only in locations where there is a cover of decomposed vegetation litter, there will be an O Horizon, The “O” designation refers to this horizon’s high content of organic debris and humus. The A Horizon, immediate below, is commonly referred to as “topsoil”. A horizon are dark because they contains decomposed organic matter. Beneath the A horizon, certain soils have a lighter colored E Horizon, named for the action of strong eluvial processes. Below this is the B horizon a zone of accumulation, where much of the materials removed from the A and E horizons are deposited. The C horizon, is he weathered parent material from which the soil has developed either bedrock or deposits of rock materials that were transported to the site by surface process such as running water, wind or glacial activity. The lowest layer, sometimes called the R horizon, consist of unchanged parent material
Certain horizons in some soils may not be as well developed as others, and some horizons may be absent. Variations in color and texture within a horizon are not unusual.


There are four classes of soil forming processes : Soil enrichment, removal, translocation and transformation.

In Soil enrichment, matter organic or inorganic is added to the soil

In Removal processes, material is removed from the soil body. This occurs when erosion carries soil particles into rivers and streams.

Leaching – The loss of soil compounds and minerals by solution in water flowing to lower levels is another important removal process.

Transloaction describes the movement of materials upward or downward within the soil.

In Transformation one type of mineral is converted to the other type, example is decomposition of organic matter to produce humus, a process termed humification. Micro organisms break down the raw organic matter, producing CO and water and leaving behind resistant organic compounds humus to decay more slowly.

There are some specialized pedogenic processes or soil formation regimes that are responsible for the formation of soils. However, a he macro scale we can suggest that there are five main principal pedogenic processes acting on soils. These processes are laterization, podzolization, calcification, salinization and gleization.

1) Laterization: Laterization is a pedogenic process which is a common to the soils found in the humid tropical and subtropical environments. High temperatures and heavy precipitation result in the rapid weathering of rocks and minerals. Movements of large amounts of water through the soil cause eluviation and leaching to occur. Almost all of the byproducts of weathering are translocated from A horizon to B horizon by leaching, if not taken up by plants for nutrition, the two exceptions are insoluble iron and aluminium compounds which are not leached. As a result, the topsoil is reddish, coarse textured and tends to be porous. The B horizon in a lateritic soil has a heavy concentration of illuviated materials. Heavy leaching also causes these soils to have an acidic pH because of the net loss of base cations. This soil type is known as Laterite which means “brick-like” as it is quarried in tropical areas for making bricks.

2) Podzolization: Podzolization is associated with humid cold mid latitude climates having short, cool summers and long, severe winters and coniferous vegetation. Decomposition of coniferous litter and heavy summer precipitation create a soil solution that is strongly acidic. This acidic soil solution enhances the processes of eluviation and leaching causes the removal of soluble base cations and aluminium and iron compounds from the A horizon. This process creates a sub layer in the A horizon comprising of silica sand and ash gray in color. The coniferous forests of these climate regions are an integral part of the podzolization process.

3) Calcification: Calcification occurs when evapotranspiration exceeds, precipitation causing the upward movement of dissolved alkaline salts from the groundwater. At the same time, the movement of rain water causes a downward movement of the salts. The net result is the deposition of the translocated cations in the B horizon. In some cases, these deposits can form a hard layer called Caliche. The most common substance involved in this process is calcium carbonate.

4) Salinization, the concentration of salts in the soil ( Na+, K++, C++, Mg++), is often detrimental to plant to growth. It functions in a similar way to calcification. It differs from calcification in that the salt deposits occur at or very near the soil surface. Salinization occurs in stream valleys, interior basins, and other low lying areas, particularly in arid regions with high groundwater tables. Salinization can also be consequence intensive irrigation under arid or semi arid conditions. Rapid evaporation leaved behind high concentration of soluble salts and may destroy a soil’s agricultural productivity. On the other hand alkalinisation involves accumulation of sodium salts only like Na2Co3 and Na2(CO3)2
5) Another localized soil regime, gleization occurs in poorly drained areas in cold, wet environments. This process involves the accumulations of organic matter in the upper layers of the soil. In lower horizons, Mineral layers are stained blue-gray because of the chemical reduction of iron, in water saturated clay. Gley soils, as they are called are typically associated with peat bogs.

Search Exam Books