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Essay on Soil Formation
Essay # 1. Introduction to Soil Formation:
Soils are formed by physical or chemical disintegration of rocks. Thus, the chief parent material of soil is the rock. Vegetation, including plants and trees, derives its food (nutrients) from the soil and, in the process, causes changes in the chemical composition and the structure of soil. Microorganisms living in the soils cause the chemical and biological decomposition of dead plants and animals and thus add to the soil composition referred to as organic matter.
The processes responsible for soil formation are a part of geological cycle involving weathering, denudation, erosion, transportation, deposition, and upheaval. The process of formation of soils is the same as the process of formation of sedimentary rocks.
Weathering and erosion break down rocks into particles of various sizes leading to formation of soil. Transportation is the displacement of loosened rock materials from the place of weathering by such agents such as water, wind, glaciers, etc., in the form of sediments. Deposition is the accumulation of sediments that are transported previously from a different area. This process leads to formation of transported soils.
Residual soils are formed when the surface rocks break down into smaller pieces by weathering and then mix with moss and organic matter. Over time, this creates a thin layer of soil. Plants help in the development of the soil as they attract animals, and when the animals die, their bodies decay. Decaying organic matter makes the soil thick and rich. This continues until the soil is fully formed.
New (secondary) minerals are formed from the primary rock minerals, depending on the temperature and rainfall. The aluminum and iron oxides present in the soil/rock combine with silica to form clay. The montmorillonite clay mineral, with three-layer structure, is formed in temperate regions. Kaolinite and other two-layer clay minerals are formed in hot, humid, and tropical regions.
As the soils are formed by weathering, their strength decreases considerably compared to that of rocks. This is because rocks are massive with individual grains bonded by primary bonds, whereas soils are particulate systems with individual grains held together by secondary valence bonds and weak cohesive forces.
Soil formation is a continuous and time-dependent process that may require hundreds and thousands of years for the soil to reach a stage of equilibrium with the surrounding environment. Soils are produced naturally at a rate of 1 mm in 200-400 years, averaging to about 0.05-0.1 ton/ha/year. A full soil profile develops in 2000-10000 years, a period that is long for humans but short in the geological time scale. This underlines the need for soil conservation.
Granite and rhyolite weather to form non-plastic and permeable sandy loam. In dry climates, the primary minerals of these rocks are decomposed to form clay minerals such as illite, vermiculite, and montmorillonite. Montmorillonite group of clay minerals are also formed by weathering of primary rock minerals such as andesite, diorite basalt gabbro, perdotite, and dunite. Further weathering of these minerals, under high temperature and rainfall conditions, leads to formation of kaolinite.
Sandstone, consisting mainly of quartz, is weathered to form sandy soils. Shales, made up of clay minerals weather to form impermeable clayey soils which consist mainly of illite and montmorillonite. Limestone and dolstone containing calcite and dolomite, respectively, are weathered to form clayey soils.
Weathering of gneiss, in a cool dry climate, results in the formation of soils containing primarily montmorillonite and illite clay minerals. With increase in temperature and rainfall, these minerals further decompose to form kaolinite. Weathering of schist containing mica results in the formation of silty soils containing illite and vermiculite, whereas schist containing hornblende forms clays containing montmorillonite.
Essay # 2. Weathering of Rocks:
Weathering is in situ breakdown and disintegration of rocks and minerals at or near the Earth’s surface due to the action of weather and atmospheric agencies.
The process of removal of weathered product from the place of weathering by transporting agents is called erosion. Erosion is the process during which the sediment that was loosened by weathering is transported and deposited elsewhere.
When weathered rock fragments are transported from the site, a fresh rock surface is exposed to the atmosphere and this process is known as denudation. Denudation results in the weathering of the newly exposed rock surface followed by erosion, transportation, and again by denudation and thus the geological cycle occurs indefinitely.
Igneous and sedimentary rocks metamorphose into new harder rock forms like gneiss, schist greywacke and marble, when subjected to extreme pressure or temperature. Physical weathering is the mechanical disintegration of rocks into finer particles, without a change in the chemical composition and mineralogy. Chemical weathering is the decomposition of rocks by a change in the chemical composition and mineralogy, through a combination of several chemical processes. Physical and chemical weathering usually takes place simultaneously in a rock mass.
Common minerals present in the rocks are quartz, feldspars, muscovite, and ferromagnesium minerals Quartz undergoes physical weathering, producing sands, which may be deposited in the form of sandstone. Potassium feldspars and muscovite undergo chemical weathering by carbonation or hydrolysis forming clay minerals and silica, which are finally deposited in the form of shale or chert.
Essay # 3. Erosion of Rock Fragments:
The weathered rock fragments are dislodged and eroded by various transporting agents such as wind, water, glaciers, waves, and gravity. Erosion loosens and carries away rock debris formed by weathering. Without erosive agents, rock debris would accumulate where it is formed.
While weathering breaks down rocks to large particles of a few centimeters to several meters size, erosion by transporting agents causes thorough breakdown of these large particles into smaller particles, ranging in size from a few millimeters to micrometers (0.001 mm) and below.
I. Erosion by Wind:
Moving air, or wind, is an important transporter of sediments, especially in dry regions. Action of wind is more pronounced in seashores, desert regions, and regions of high altitude and those subjected to cyclones or depressions.
The effect of wind during transportation results in the following processes:
Abrasion occurs when sand-sized particles, eroded and carried by wind, strike against exposed surface of a softer rock, causing further erosion of the rock surface. Abrasion will be more effective in erosion when the wind velocity is high as well as by repeated abrasive action on a relatively softer rock.
Attrition is the breakdown of particles carried by wind and occurs due to –
a. Impact when they strike the exposed surface of a relatively harder rock.
b. Collisions among heterogeneous particles, which occur because of their differential weights and velocities.
Weathering by attrition is intensified when –
a. The velocity of wind is high.
b. The exposed rock surface is harder than the wind-carried particles.
c. The particles are carried by wind for longer distances.
Deflation is the process in which a powerful blast of wind blows and scours the entire loose weathered rock fragments. The underlying rock surface is exposed to the atmosphere initiating further weathering.
Water, carrying suspended rock fragments, has a scouring action on surfaces. Examples are the grinding action of glaciers, gravel, pebbles, and boulders moved along and constantly abraded by fast-flowing streams.
Moving water is the most potent erosive force on Earth. A river is a significant transporting agent causing weathering by erosion during transportation. Powered by the force of gravity, the world’s rivers deliver about 20 billion tons of loose rock fragments, or sediment, to the oceans each year.
Runoff, which is part of rainfall flowing on the ground surface, also causes erosion.
The intensity of erosion by runoff is increased:
a. When the intensity of rainfall increases.
b. When the duration of the rainfall increases.
c. When the frequency of rainfall increases.
The repeated impact action of waves on the seashore weathers the rocks into fine sand. In the sea, the coarser particles accumulate close to shore, whereas the finest particles settle out farther away from the shore. As thick layers are formed, water is squeezed out and the sediment is compacted to form new sedimentary rocks such as conglomerate, sandstone, and limestone.
Following are the processes involved in erosion by the river:
i. Hydraulic Action:
The kinetic energy due to the velocity of flowing water causes physical breakdown of rocks and weathered rock fragments into smaller particles.
The hydraulic action of rivers causes more intense weathering:
a. When the rivers are in their initial and youth stages of their formation, flowing through steep slopes with high velocity.
b. When the rocks, subjected to hydraulic action of rivers, are un-cemented and porous or contain soluble materials resulting in cracks and fractures.
c. When repeated action of flowing water aggravates erosion and weathering.
It is the disintegration of rocks due to impact by large-sized hard particles carried as sediments by the flowing river water. The effectiveness of abrasion by water to cause erosion and weathering of rocks depend on same factors as in abrasion by wind.
When the weathered rock fragments are lifted and carried by the flowing water, the heterogeneous fragments collide with one another or with the rock surface, through which river flows, causing their physical disintegration into smaller-sized particles. The process is similar to the attrition by wind.
It is one of the processes of chemical decomposition of rocks, in which the water dissolves and removes soluble cementing materials such as calcium carbonate. Most of the rock minerals are affected by this process when they are constantly exposed to water for longer durations. Limestone, marl, calcareous shale, dolomite, and quartz are some of the rocks that are subject to chemical decomposition by solution.
Weathering and transportation by glaciers is maximum in glacial regions, where the temperature is sub-zero with annual rainfall < 1000 mm. Glacier weathering is mostly mechanical.
When large bodies of water or rock, such as waterfalls, avalanches, or landslides, fall from a height, they strike the rock surface due to potential energy gained by gravity, resulting in physical disintegration of rock due to abrasion, attrition, and impact.
The rate of weathering, whether physical, chemical, or biological, is influenced by the type of rock, climate, topography, and vegetation. Soil formation and development is a dynamic, rather than static, process and thus it is a continuous, time-dependent process.
Following are the important factors that influence the rate and intensity of weathering and soil formation:
3. Parent material.
We will discuss each of these factors in the following subsections:
Climate influences the rate of weathering. It also controls which type of weathering processes will predominate. Climate also determines the amount of water available for weathering and transporting the minerals and releasing the weathered elements. For example, frost heaving occurs in cold climates. Chemical weathering is more intense in moist climate than in a dry climate. Chemical weathering rates are three and a half times higher in tropical environment, where temperature and moisture are at their maximum, than those in temperate environments.
The atmospheric agencies causing weathering include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), water vapor (H2O), gravity, etc.
Warm, moist climates encourage rapid plant growth and thus high organic matter production. Mild temperature in moist regions provides favorable conditions for the growth of organisms that cause biological weathering. Bacterial growth in warm humid regions consumes decayed vegetable matter, reducing the organic content of the soil. Under the control of climate, freezing, thawing, wetting, and drying cause weathering of rocks.
In cold regions, the decaying vegetation not only adds to the organic content of the soil but also leads to formation of organic acid, resulting in biological weathering. Freezing temperatures in humid regions prevent biological activity. These regions, however, are favorable for frost wedging and frost heaving resulting in physical weathering.
Wind redistributes sand and other particles especially in arid regions. Seasonal and daily changes in temperature affect moisture effectiveness, biological activity, rates of chemical reactions, and kinds of vegetation. Earthquake causes breaking of the Earth’s crust. An earthquake can be one of the most destructive agents of weathering.
The amount, intensity, timing, and kind of precipitation influence soil formation. Rain dissolves some minerals, such as carbonates, and transports them deeper into the soil. Some acid soils have developed from parent materials that originally contained limestone. Rainfall can also be acidic, especially on the downwind side of industrial areas.
When rainfall exceeds evaporation, soils are acidified by the trees’ resins, causing downward leaching of nutrients and clays, where most clay is found in B-horizon, leaving A-horizon sandy.
In the case of rainforest soils, the water table is very close to the ground surface because of extensive rainfall for most of the year. Deep soils cannot develop in these areas and all minerals and nutrients are stored in the vegetation above and in a rich, deep organic O-horizon. In the mid-latitudinal regions, called the temperate zone, evaporation roughly equals rainfall and deeper soils are formed in such areas.
As the water table descends further, soils deepen, whereas warm, dry conditions are favorable for savannah and prairies. These are populated by deep-rooting grasses that produce just enough acidity to retain clay but not enough to leach its nutrients. Under these conditions, fertile black soils are formed, rich in humus.
As the water table descends still further, and evaporation far exceeds rainfall, soils become only a step away from the desert. Seasonally, these soils dry so thoroughly that instead of clays, oxides of iron are formed. These soils contain very little clay and are very poor in humus and organic material.
The rate of flow of water in a soil influences the movement of calcium and other chemical compounds in the soil. Ultimately, if more chemicals are removed, the soils will be deeper and more developed.
Chemical weathering processes such as hydration and hydrolysis require water in the chemical reactions. Rainfall itself may be responsible for the removal of silica through leaching and desilication.
Biological weathering processes are controlled by organisms and plants that survive on precipitation. Deficiency of rainfall results in evaporation and consequent precipitation of carbonates and sulfates in the top layers of soil that participate in weathering.
Precipitation influences vegetation and, therefore, greatly determines the organic content of soils.
The type of rock, including the mineralogy, determines the intensity of weathering. Seventy-five percent of the rocks up to a depth of 2 km from the surface of Earth’s crust are sedimentary or metamorphic rocks. The remaining 25% of the rocks are igneous rocks.
The susceptibility of parent material to weathering depends on the following factors:
a. Types of minerals present in the parent rock.
b. Surface area of rock exposed.
c. Porosity of rocks.
Basic igneous rocks weather rapidly compared to acidic igneous rocks. Carbonate rocks are more susceptible to weathering. Shales decompose faster by slaking in the presence of water. Schist and phyllites, among the metamorphic rocks, weather faster compared to quartzite, which is most stable.
Granites undergo hydration, hydrolysis, and carbonation in hot humid climates resulting in the formation of different types of clays. In cold climates, granites may just undergo physical disintegration, resulting in the formation of coarse-grained soils. Sandstone and quartzite weather into sands and gravels by physical weathering, without undergoing any chemical decomposition even in hot humid regions. Fine-textured soils develop from fine-grained rocks such as basalt, limestone, and shale, whereas coarse-grained soils result from granites, sandstone, and cherty limestones.
Weathering also depends on lithological structures such as number and size of fractures or bedding planes, both of which can be sites for focused weathering activity.
Porosity of rock is another important property that influences weathering. Higher porosity permits intrusion of water with other chemicals into the interior of the rock, resulting in both physical weathering, such as frost wedging or salt crystallization, and chemical weathering. Soil development may take place quicker in materials that are more permeable to water. Dense, massive, clayey materials can be resistant to soil formation processes.
Rocks consisting of coarse fragments (e.g., granite) weather physically fast but do not easily weather chemically. The smaller the particle size of the parent material, the larger is the surface area and the more intensive is the chemical weathering. In rocks consisting of fine fragments (e.g., basalt), chemical weathering is higher than physical weathering. The weathering of stratified sedimentary rocks is dependent on the orientation of stratification and cementation.
In general, the resistance of a primary mineral to weathering increases with the degree of sharing of oxygen between the adjacent Silica tetrahedra in the crystal lattice and the bond energy. The Si-O bond has the highest energy of formation, followed by the Al-O bond, and even weaker bonds formed between oxygen (O) and the metal cations (e.g., Na+, Ca2+). Olivine weathers rapidly because the silica tetrahedra are only held together by oxygen-metal cations. In contrast, quartz is very resistant because it consists entirely of linked silica tetrahedra.
In the chain (amphiboles and pyroxenes) and sheet (phyllosilicates) structures, the weakest points are the oxygen-metal cation structures. Isomorphous substitution of Al3+ for Si4+ also contributes to instability because the proportion of Al-O td Si-O bonds increases and more oxygen-metal cation bonds are necessary. This accounts for the decrease in stability of the calcium feldspars when compared with the sodium and potassium feldspars.
The most abundant living organism in the soil is vegetation. Plants affect soil development by supplying upper layers with organic matter, recycling nutrients from the lower to upper layers, and helping to prevent erosion. Plants act as conduits for dispersion of calcium, potassium, sodium, etc., from lower to upper layers of soil, changing the chemistry of the soils. Vegetation root systems, in addition to physically weathering rock, also produce two chemical weathering agents, carbon dioxide and humic acid.
In general, deep-rooted plants contribute more to the soil development than shallow rooted plants because the passages they create allow greater water movement, which in turn aids in leaching. The presence or absence of vegetation results in the formation of organic or inorganic soils, respectively.
Leaves, twigs, and barks from large plants fall onto the soil and are broken down by fungi, bacteria, insects, earthworms, and burrowing animals. These organisms eat and break down organic matter, releasing plant nutrients. Decayed vegetation causes formation of humus. Bacteria act to consume humus or fix nitrogen, affecting the composition and structure of soils.
Soils formed under trees are greatly different from soils formed under grass, even though other soil-forming factors are similar. Trees and grass vary considerably in their search for food and water and in the amount of various chemicals taken up by roots and deposited in or on top of the soil when tree leaves and grass blades die.
Soils formed under grass are much higher in organic matter than those formed under forests because of their massive fibrous root structure and annual senescence (becoming old) of above ground vegetation. Grassland soils tend to be darker, particularly to greater depths, and have a more stable structure than forest soils. Grass roots are “fibrous” near the soil surface and easily decompose, adding organic matter. Taproots open pathways through dense layers.
There are a multitude of organisms living in the soil, such as mites, snails, beetles, millipedes, springtails, worms, ground squirrels, gophers, grubs, nematodes, and microorganisms, such as bacteria, fungi, actinomycetes, and algae.
Microorganisms are the most abundant organisms in the soil, apart from vegetation. They affect chemical exchanges between roots and soil. The remains of dead plants and animals on the soil surface are worked upon by microorganisms and are converted into organic matter, which enriches the soil. Microorganisms produce humus that acts as a kind of glue to hold soil particles together in aggregates.
Certain living organisms, such as lichens, produce corrosive acids that eat away the surface of the rock on which they grow. Lichens and mosses can squeeze into cracks and crevices, where they grow. As they grow, so do the cracks, breaking the rock into pieces. Big and small critters trample, crush, and plow rocks as they scurry across the surface and burrow underground.
Topography determines the extent to which the rock is exposed to the atmosphere. It also influences the amount of vegetation that may take hold. Weathering may be faster in steep slopes as it prevents accumulation of thick layers of weathered product on its surface due to gravity or flowing waters, frequently exposing the rock surface to weathering. Thus, the soils are of shallow thickness on steep slopes. Areas with good drainage may suffer leaching and washing of the major components of soil, while poorly drained soils are rich in original residues and soluble minerals.
Runoff has a high velocity on a sloping ground, and therefore infiltrates less into the soil. Plants, therefore, tend to have shallower root systems and less organic matter is produced, as compared to a level land. Steep slopes are also subjected to more erosion, which removes soil as fast, if not faster, as it forms. Water tends to pond on the surface of a level land. This may lead to abundant plant growth resulting in the production of large amounts of organic matter.
Generally, an increase in slope is associated with a reduction in:
i. Solum (soil) thickness.
ii. Organic matter content.
iii. Mineral weathering.
v. Clay translocation.
vi. Horizon differentiation.
Soils at the bottom of a hill get more water than soils on the slopes. Soils on the slopes that directly face the sun are drier than those that do not face the sun. Aspect (facing direction) affects soil temperature. Generally, soils on north-facing slopes tend to be cooler and wetter than soils on south-facing slopes. Soils on north-facing slopes tend to have thicker A and B horizons and tend to be less droughty Slopes with a southern exposure are warmer and drier than slopes with a northern exposure.
Sediments along rivers have different textures, depending on whether the stream moves quickly or slowly. Fast-moving water leaves gravel, rocks, and sand. Slow-moving water and lakes leave fine-textured material (clay and silt) when sediments in the water settle out.
The Earth is believed to be about 300-crores of years old. The duration for which rocks and minerals have been exposed at the Earth’s surface will influence the degree to which they have weathered. The duration and sequence of various agencies of weathering and transportation strongly decide the weathering and soil formation.
The longer a parent material has been exposed to an adverse environment, the greater the degree of weathering and the more developed the soil. The weathering processes often are slow, extending hundreds to thousands of years Depending on the climatic, weathering and transporting agencies of an area, weathering and soil formation may proceed rapidly over a decade or slowly over millions of years.
It takes 20-200 years for weathering of 1 mm layer of a rock in different climates. The formation of soil happens over a very long period of 1000 years or more. Soil profiles continually change from weakly developed to well developed over the time.
Soils may not reach the stage of equilibrium at any time and are subjected to weathering perennially for indefinite time. The rate of weathering, however, slows down considerably as the soil reaches equilibrium with its environment. The environment may change again with time and the cycle of weathering and soil formation continues A mature soil is in equilibrium with its environment at a given instant of time and shows full development of layers or horizons in its profile.
Soils on older, stable surfaces generally have well-defined horizons when the rate of soil formation exceeds the rate of geologic erosion or deposition. As soils age, many original minerals are destroyed and many new ones are formed. Soils become more leached, more acidic, and more clayey. In many well-drained soils, B horizons tend to become reddish with time.
Essay # 5. Residual Soils:
Soils are classified into two types, residual soils and transported soils, depending on the absence or presence of transporting agents in the soil-formation process. Soil that is formed by the weathering of the underlying bed rock and remains at the place of its origin without undergoing significant transport is called residual soil. Residual soils are sometimes called in situ soil or sedentary soil.
If the residual soils are formed by physical weathering, their composition and mineralogy will be the same as those of the underlying bed rock. However, if these soils are formed by chemical and/or biological weathering, their composition and mineralogy may be different from those of the underlying bed rock.
Residual soils consist of very fine material like clay and silt at top. The particle size gradually increases with depth, with stones and boulders of same properties as that of bed rock present at the bottom. The thickness of residual soil is generally limited from a few decimeters to a few meters. The engineering properties of residual soils vary considerably with depth from top to bottom because the influence of weathering is maximum at the surface and goes on decreasing with depth.
Essay # 6. Transported Soils:
The soil which is formed by the weathering of the bed rock and undergoes significant transport by wind, water, glacier, or gravity and which is deposited at a new place away from the place of weathering is called transported soil.
Depending on the type of transporting agent, transported soils are named as alluvial (water), aeolian (wind), glacial (glacier), and talus (gravity) deposits. Transported soils are also known as drifted soils, as they drift from the place of weathering by various geological agents.
As the transported soils are eroded and transported from the place of weathering and deposited elsewhere, their composition and mineralogy may not have any relation to those of the underlying bed rock, except by coincidence. Transported soil deposits are usually very thick and consist of several layers of soil. Each layer is more or less of uniform thickness and consists of particles size, such as gravel, sand, silt, or clay. The properties of soil in each layer are usually uniform. Most of the foundation and construction problems associated with geotechnical engineering are usually related to transported soils.
Soils that are transported and deposited by water are known as alluvial soils. As the areas occupied by alluvial soils are subject to continual deposition of loose sediments, these soils are very thick, extending up to 100 m or more, occurring in several layers.
Soils transported and deposited by water undergo decrease in particle size by solution. Abrasion is low for particles carried in suspension and moderate abrasion takes places for particles carried by traction. The shape of particles changes from angular to round and the surface becomes smooth and polished. Silt particles, which have very small size, do not undergo any significant change in shape and surface texture. Soils are highly sorted as the velocity of water in early youthful stretches of the river is high and decreases gradually toward the later reaches.
As the moving water slows down towards the later reaches of a river, the coarse materials, such as the cobbles, shingles, and gravels settle down to the river bed. When the velocity decreases further in last reaches, sand, silt, and, finally, clay settle. In the flood plains of a river, silt and sand are deposited with some mud (clay), creating some of the most fertile and workable (plastic) soils of the world.
Alluvial soils cover about 24% of land surface in India and are basically fluvial-transported soils. They consist of sand and gravel interlayered with clay and silt. The soils have low density and are susceptible to liquefaction when subjected to vibrations due to earthquakes or construction machinery. Loam is a mixture of sand, silt, and clay occasionally having organic matter. Alluvial soils occur in Gangetic plains, Narmada and Tapti valleys, Godavari and Krishna deltas, etc. They occur in major parts of Punjab, Haryana, U.P., West Bengal, Assam, and northern Bihar and to a small extent of deltaic areas in other states.
Soils that are transported by rivers and deposited along the shores in the sea are known as marine soils. These soils essentially consist of very soft clay with high sensitivity, low density, and shear strength. The marine deposits are very thick extending to several tens of meters. They are usually underlain by sand layers of varying thickness.
These soils cover the narrow strip along the coastal belt of south and central India, covering the states of Orissa, Andhra Pradesh, Tamil Nadu, Kerala, Karnataka, Maharashtra, and Gujarat. Marl is a marine deposit of clay or silt with lime.
Lacustrine soils are formed from the materials transported by water and are deposited at the bottom of lakes, when the lake dries up. Bog lime is a white fine-grained calcareous deposit precipitated by plants in ponds. It is also called Lake marl.
Soils eroded and transported by wind undergo considerable reduction in particle size. Particles become rounded but the surface may be rough with sharp edges due to impact. Particles are highly sorted. The wind-transported soils are called aeolian soils and consist mainly of silt and clay. The loose deposit of wind-blown silt that has been weakly cemented with calcium carbonate and montmorillonite clay mineral is known as loess.
This soil is formed in arid and semi-arid regions and is yellowish brown in color. These soils have uniform grain size, high void ratio, low density, and high compressibility. Because of light cementation, these soils can stand deep vertical cuts without any lateral support.
The permeability of these soils is high in vertical direction, unlike the usual soils for which permeability in horizontal direction is many times that in vertical direction. Saturation with water makes such soils collapse suddenly. The bearing capacity of these soils is also very low.
The wind-blown soil whose particles are strongly cemented by large quantity of calcium carbonate is called caliche. This soil contains more calcium carbonate than what loess has. Caliche is also known as hardpan. Hardpan is a relatively hard, cemented, cohesive soil, which does not soften when wet and which cannot be normally drilled with ordinary Earth-boring tools.
Dune sands of deserts are wind-transported soils and are uniform or poorly graded. Tuff is a fine-grained slightly cemented volcanic ash that has been transported by wind or water. Shale is an unstable rock which rapidly decomposes, when exposed to air or water. Adobe is a clay transported by wind and deposited in shallow water.
Gumbo is a sticky, plastic, and dark-colored clay. Diatomaceous earth is a deposit of fine siliceous powder composed chiefly of the remains of diatoms, which are a group of microscopic unicellular marine or fresh water algae.
Soils eroded and transported by glaciers undergo considerable grinding and impact. Particles are generally angular with striated surfaces. Soils deposited by glaciers contain particles of several sizes, as there is very little sorting during transportation.
Deposits formed directly by melting of glaciers are called glacial till. Glaciers carry large quantity of rock fragments and boulders, and when these are deposited due to melting of glaciers at the end of their reach, such soils are called terminal moraine. The land which was covered by glaciers in the past and on which till is deposited by melting of glaciers is called ground moraine. The soil carried by water from melting of glacier is called out wash.
Boulder clays or glacial tills are also sometimes called hard-pans. Varved clay consists of alternate thin layers of silt and clay, deposited in fresh water glacial lakes. One layer of silt and clay is deposited every year. Glacial deposits are well-graded soils and possess high dry density and shear strength when compacted.
Colluvial soil is the rock debris deposited at the foot of steep hills by gravity and is also known as talus. It consists of boulders and stones without any fine material. Talus is a source of gravel and sand.
The soil cover above the bed rock, originated from weathering at the same site or weathered and transported from a different site, is called regolith.
Depending on the extent of formation of distinct layers, soils may be classified into the following three types:
i. Zonal Soils:
They are mature, have distinct profiles and clear horizons, and have climate as the major determining factor.
ii. Azonal Soils:
They are far more recent, where soil-forming processes have not been in operation for long. Horizons are unclear and they are not linked with surrounding climate and vegetation. Their immaturity is a result of high altitudes, low temperatures, and slow decay of organic matter. Examples include scree, till, and volcanic soil.
iii. Intrazonal Soils:
These are soils found within the climate belt but different from normal. They are formed as a result of a dominant local factor, for example, parent rock.
Intra-zonal soils are of three types:
i. Calcomorphic/Calcareous soils, which develop on limestone.
ii. Hydromorphic soils, where water content is always high.
iii. Halomorphic soils, which are saline due to high salt level.
Although soil is formed from the rock below, it is eroded away from the top. A cover of plant life slows down erosion, allowing the soil layer to build up. If soils are formed in a region where there is active or past leaching by water with P/E ratio ≫ 1 (P, precipitation; E, evaporation), the differential removal of elements and components from the soil leads to the formation of very distinct layers in the soil called “soil horizons.” Residual soils generally show distinct horizons of a soil profile and do not show any stratification.
There are eight layers of soil called Horizons O, P, A, B, E, C, D, and R. All horizons may not be usually present at one place. These horizons collectively are known as a soil profile. The thickness varies with location, and all horizons will not be present when the profile is disturbed by heavy agriculture, construction, or severe erosion.
A brief description of various types of soil horizons is as follows:
This layer contains organic content known as humus. Humus is decomposed organic litter, like fallen leaves and twigs, which in turn helps in preventing erosion and holds moisture. O horizon is distinct from soil itself because it contains the layer of leaf litter and no weathered mineral particles. This layer is dark because of the decomposition that is occurring. This layer is not present in cultivated fields.
This horizon is also rich in organic content. It is formed under waterlogged conditions. The “P” designation comes from their common name, peats.
It is the top soil in a soil profile and consists of fine-grained mineral material mixed with organic matter, humus, and decayed vegetation. Its thickness varies from a few centimeters up to a few meters.
A-horizon may be darker or lighter in color depending upon the quantity of humus and clay present in it. Usually, it is darker than lower layers. This is the zone where most of the biological activities occur. Soil organisms such as earthworms, potworms, arthropods, nematodes, fungi, bacteria, and archaebacteria are present in this horizon. A-horizon is therefore also referred to as biomantle.
This layer provides nourishment to the plants. In cultivated fields, the plowed layer is top soil. This is generally the most productive layer of the soil. This is the layer where soil conservation efforts are focused. As water moves down through the top soil, many soluble minerals and nutrients dissolve. The dissolved materials leach downward into lower horizons.
The letter “E” in E-horizon stands for eluviated. The horizon is pale in color due to leaching of its minerals and/or organic content and is largely composed of silicates. These are present only in older, well-developed soils and generally occur between the A- and B-horizons.
This layer is below the A-horizon and contains actual soil that is derived from weathering of rocks and more or less free from organic matter. The soil in this region may be fine grained with pebbles at the bottom. B-horizon is commonly referred to as “subsoil.” Subsoils are usually dense and are lighter in color. Most of the materials leached from A-horizon stop in this zone.
B-horizon is the zone of accumulation, illuvial horizon, characterized by concentrations of clay and iron. Some lime may accumulate, but if the accumulation is excessive, the horizon is named K-horizon. Thus, K-horizon contains appreciable carbonate, mainly calcium carbonate. A- and B-horizons together are called solum. These horizons can be further subdivided into A1, A2, A3, …; B1, B2, B3, …; etc., based on chemical or physical properties.
C-horizon is the layer of soil next to B horizon. It is a zone of partly weathered rock and contains material more or less identical to the underlying bed rock. The soil is coarse grained with large size pebbles and boulders and little organic material. It is a transition area between soil and parent material. Leaching also carries some minerals from B-horizon down to C-horizon.
7. G- and R-Horizons:
G-horizon is the gleyed horizon, which forms under reducing (anoxic) conditions with impeded aeration, reflected in bluish, greenish, or grayish color. R-horizon means regolith and consists of the unconsolidated, un-weathered bedrock or parent material.