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What is soil?
Soils are the loose mineral or organic materials found on the earth's surface, usually (or averagely)
made up of about 25% air, 25% water, 45% mineral and 5% organic matter (humus, tiny living
organisms and sometimes plant residue).
Soil develops slowly over time and is composed of many different materials. Inorganic materials,
or those materials that are not living, include weathered rocks and minerals. Weathering is the
mechanical or chemical process by which rocks are broken down into smaller pieces. As rocks are
broken down, they mix with organic materials, which are those materials that originate from living
organisms. For example, plants and animals die and decompose, releasing nutrients back into the
soil.
Importance (Functions) of soils
Soils are essential for life, in the sense that they provide
the medium for plant growth
habitat for many insects and other organisms
act as a filtration system for surface water
carbon store and maintenance of atmospheric gases
Types of Soil
Soil is a natural resource that can be categorised into different soil types, each with distinct
characteristics that provide growing benefits and limitations. The soil types determines how and
what we can grow and are the basis for all farming. The United States Department of Agriculture
(USDA) provides an elaborate classification of soil based on the origin, composition, and other
features.
Gelisols: Frozen
Histosols: Organic, wet
Spodosols: Sandy, acidic
Andisols: Volcanic ash
Oxisols: Very weathered
Vertisols: Shrink and swell
Aridisols: Very dry
Ultisols: Weathered
Mollisols: Deep, fertile
Alfisols: Moderately weathered
Inceptisols: Slightly developed (young)
Entisols: Newly formed
Gelisols (from the Latin gelare to freeze) are soils that are permanently frozen (contain
“permafrost”) or contain evidence of permafrost near the soil surface.
Gelisols are found in the Arctic and Antarctic, as well as at extremely high elevations.
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Permafrost influences land use through its effect on the downward movement of water and
freeze-thaw activity (cryoturbation) such as frost heaves. Permafrost can also restrict the rooting
depth of plants.
Gelisols make up about 9% of the world’s glacier-free land surface.
Histosols (from the Greek histos tissue) are mainly composed of organic material in their
upper portion. The Histosol order mostly contains soils commonly called bogs, moors, peatlands,
muskegs, fens, or peats and mucks.
These soils form when organic matter, such as leaves, mosses, or grasses, decomposes more
slowly than it accumulates due to a decrease in microbial decay rates. This most often occurs in
extremely wet areas or underwater; thus, most of these soils are saturated year-round.
Histosols can be highly productive farmland when drained; however, draining these soils can
cause them to decompose rapidly and subside dramatically. They are also not stable for
foundations or roadways, and may be highly acidic.
Histosols make up about 1% of the world’s glacier-free land surface.
Spodosols (from the Greek spodos wood ash) are among the most attractive soils. They often
have a dark surface underlain by an ashy, gray layer, which is subsequently underlain by a
reddish, rusty, coffee-colored, or black subsoil horizon.
These soils form as rainfall interacts with acidic vegetative litter, such as the needles of conifers,
to form organic acids. These acids dissolve iron, aluminum, and organic matter in the topsoil and
ashy gray (eluvial) horizons. The dissolved materials then move (illuviate) to the colorful subsoil
horizons.
Spodosols most often develop in coarsely textured soils (sands and loamy sands) under
coniferous vegetation in humid regions of the world. They tend to be acidic, and have low
fertility and low clay content.
Spodosols occupy about 4% of the world’s glacier-free land surface.
Andisols (from the Japanese ando black soil) typically form from the weathering of volcanic materials
such as ash, resulting in minerals in the soil with poor crystal structure. These minerals have an unusually
high capacity to hold both nutrients and water, making these soils very productive and fertile.
Andisols include weakly weathered soils with much volcanic glass, as well as more strongly weathered
soils.
They typically occur in areas with moderate to high rainfall and cool temperatures. They also tend to be
highly erodible when on slopes.
These soils make up about 1% of the glacier-free land surface.
Oxisols (from the French oxide oxide) are soils of tropical and subtropical regions, which are
dominated by iron oxides, quartz, and highly weathered clay minerals such as kaolinite.
These soils are typically found on gently sloping land surfaces of great age that have been stable for a
long time. For the most part, they are nearly featureless soils without clearly marked layers, or horizons.
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Because they are highly weathered, they have low natural fertility, but can be made productive through
wise use of fertilizers and lime.
Oxisols are found over about 8% of the glacier-free land surface.
Vertisols (from the Latin verto turn) are clay-rich soils that contain a type of “expansive” clay that
shrinks and swells dramatically. These soils therefore shrink as they dry and swell when they become
wet.
When dry, vertisols form large cracks that may be more than one meter (three feet) deep and several
centimeters, or inches, wide. The movement of these soils can crack building foundations and buckle
roads.
Vertisols are highly fertile due to their high clay content; however, water tends to pool on their surfaces
when they become wet.
Vertisols are located in areas where the underlying parent materials allow for the formation of expansive
clay minerals. They occupy about 2% of the glacier-free land surface.
Aridisols (from the Latin aridus dry) are soils that occur in climates that are too dry for “mesophytic”
plantsplants adapted to neither a too wet nor too dry environmentsto survive.
The climate in which Aridisols occur also restricts soil weathering processes. Aridisols often contain
accumulations of salt, gypsum, or carbonates, and are found in hot and cold deserts worldwide.
They occupy about 12% of the Earth’s glacier-free land area, including some of the dry valleys of
Antarctica
Ultisols (from the Latin ultimus last) are soils that have formed in humid areas and are intensely
weathered.
They typically contain a subsoil horizon that has an appreciable amount of translocated clay, and are
relatively acidic.
Most nutrients are held in the upper centimeters of Ultisol soils, and these soils are generally of low
fertility although they can become productive with additions of fertilizer and lime.
Ultisols make up about 8% of the glacier-free land surface.
Mollisols (from the Latin mollis soft) are prairie or grassland soils that have a dark-colored surface
horizon. They are highly fertile and rich in chemical “bases” such as calcium and magnesium.
The dark surface horizon comes from the yearly addition of organic matter to the soil from the deep roots
of prairie plants.
Mollisols are often found in climates with pronounced dry seasons. They make up approximately 7% of
the glacier-free land surface.
Alfisols (from the soil science term Pedalfer aluminum and iron) are similar to Ultisols but are less
intensively weathered and less acidic.
They tend to be more inherently fertile than Ultisols and are located in similar climatic regions, typically
under forest vegetation.
They are also more common than Ultisols, occupying about 10% of the glacier-free land surface.
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Inceptisols (from the Latin inceptum beginning) exhibit a moderate degree of soil development and
lack significant clay accumulation in the subsoil.
They occur over a wide range of parent materials and climatic conditions, and thus have a wide range of
characteristics.
They are extensive, occupying approximately 17% of the earth’s glacier-free surface.
Entisols (from recent new) are the last order in soil taxonomy and exhibit little to no soil development
other than the presence of an identifiable topsoil horizon.
These soils occur in areas of recently deposited sediments, often in places where deposition is faster
than the rate of soil development.
Some typical landforms where Entisols are located include: active flood plains, dunes, landslide areas,
and behind retreating glaciers. They are common in all environments.
Entisols make up the second largest group of soils after Inceptisols, occupying about 16% of the Earth’s
surface.

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What is soil? Soils are the loose mineral or organic materials found on the earth's surface, usually (or averagely) made up of about 25% air, 25% water, 45% mineral and 5% organic matter (humus, tiny living organisms and sometimes plant residue). Soil develops slowly over time and is composed of many different materials. Inorganic materials, or those materials that are not living, include weathered rocks and minerals. Weathering is the mechanical or chemical process by which rocks are broken down into smaller pieces. As rocks are broken down, they mix with organic materials, which are those materials that originate from living organisms. For example, plants and animals die and decompose, releasing nutrients back into the soil. Importance (Functions) of soils Soils are essential for life, in the sense that they provide • • • • the medium for plant growth habitat for many insects and other organisms act as a filtration system for surface water carbon store and maintenance of atmospheric gases Types of Soil Soil is a natural resource that can be categorised into different soil types, each with distinct characteristics that provide growing benefits and limitations. The soil types determines how and what we can grow and are the basis for all farming. The United States Department of Agriculture (USDA) provides an elaborate classification of soil based on the origin, composition, and other features. • • • • • • • • • • • • Gelisols: Frozen Histosols: Organic, wet Spodosols: Sandy, acidic Andisols: Volcanic ash Oxisols: Very weathered Vertisols: Shrink and swell Aridisols: Very dry Ultisols: Weathered Mollisols: Deep, fertile Alfisols: Moderately weathered Inceptisols: Slightly developed (young) Entisols: Newly formed Gelisols (from the Latin gelare – to freeze) are soils that are permanently frozen (contain “permafrost”) or contain evidence of permafrost near the soil surface. Gelisols are found in the Arctic and Antarctic, as well as at extremely high elevations. Permafrost influences land use through its effect on the downward movement of water and freeze-thaw activity (cryoturbation) such as frost heaves. Permafrost can also restrict the rooting depth of plants. Gelisols make up about 9% of the world’s glacier-free land surface. Histosols (from the Greek histos – tissue) are mainly composed of organic material in their upper portion. The Histosol order mostly contains soils commonly called bogs, moors, peatlands, muskegs, fens, or peats and mucks. These soils form when organic matter, such as leaves, mosses, or grasses, decomposes more slowly than it accumulates due to a decrease in microbial decay rates. This most often occurs in extremely wet areas or underwater; thus, most of these soils are saturated year-round. Histosols can be highly productive farmland when drained; however, draining these soils can cause them to decompose rapidly and subside dramatically. They are also not stable for foundations or roadways, and may be highly acidic. Histosols make up about 1% of the world’s glacier-free land surface. Spodosols (from the Greek spodos – wood ash) are among the most attractive soils. They often have a dark surface underlain by an ashy, gray layer, which is subsequently underlain by a reddish, rusty, coffee-colored, or black subsoil horizon. These soils form as rainfall interacts with acidic vegetative litter, such as the needles of conifers, to form organic acids. These acids dissolve iron, aluminum, and organic matter in the topsoil and ashy gray (eluvial) horizons. The dissolved materials then move (illuviate) to the colorful subsoil horizons. Spodosols most often develop in coarsely textured soils (sands and loamy sands) under coniferous vegetation in humid regions of the world. They tend to be acidic, and have low fertility and low clay content. Spodosols occupy about 4% of the world’s glacier-free land surface. Andisols (from the Japanese ando – black soil) typically form from the weathering of volcanic materials such as ash, resulting in minerals in the soil with poor crystal structure. These minerals have an unusually high capacity to hold both nutrients and water, making these soils very productive and fertile. Andisols include weakly weathered soils with much volcanic glass, as well as more strongly weathered soils. They typically occur in areas with moderate to high rainfall and cool temperatures. They also tend to be highly erodible when on slopes. These soils make up about 1% of the glacier-free land surface. Oxisols (from the French oxide – oxide) are soils of tropical and subtropical regions, which are dominated by iron oxides, quartz, and highly weathered clay minerals such as kaolinite. These soils are typically found on gently sloping land surfaces of great age that have been stable for a long time. For the most part, they are nearly featureless soils without clearly marked layers, or horizons. Because they are highly weathered, they have low natural fertility, but can be made productive through wise use of fertilizers and lime. Oxisols are found over about 8% of the glacier-free land surface. Vertisols (from the Latin verto – turn) are clay-rich soils that contain a type of “expansive” clay that shrinks and swells dramatically. These soils therefore shrink as they dry and swell when they become wet. When dry, vertisols form large cracks that may be more than one meter (three feet) deep and several centimeters, or inches, wide. The movement of these soils can crack building foundations and buckle roads. Vertisols are highly fertile due to their high clay content; however, water tends to pool on their surfaces when they become wet. Vertisols are located in areas where the underlying parent materials allow for the formation of expansive clay minerals. They occupy about 2% of the glacier-free land surface. Aridisols (from the Latin aridus – dry) are soils that occur in climates that are too dry for “mesophytic” plants—plants adapted to neither a too wet nor too dry environments—to survive. The climate in which Aridisols occur also restricts soil weathering processes. Aridisols often contain accumulations of salt, gypsum, or carbonates, and are found in hot and cold deserts worldwide. They occupy about 12% of the Earth’s glacier-free land area, including some of the dry valleys of Antarctica Ultisols (from the Latin ultimus – last) are soils that have formed in humid areas and are intensely weathered. They typically contain a subsoil horizon that has an appreciable amount of translocated clay, and are relatively acidic. Most nutrients are held in the upper centimeters of Ultisol soils, and these soils are generally of low fertility although they can become productive with additions of fertilizer and lime. Ultisols make up about 8% of the glacier-free land surface. Mollisols (from the Latin mollis – soft) are prairie or grassland soils that have a dark-colored surface horizon. They are highly fertile and rich in chemical “bases” such as calcium and magnesium. The dark surface horizon comes from the yearly addition of organic matter to the soil from the deep roots of prairie plants. Mollisols are often found in climates with pronounced dry seasons. They make up approximately 7% of the glacier-free land surface. Alfisols (from the soil science term Pedalfer – aluminum and iron) are similar to Ultisols but are less intensively weathered and less acidic. They tend to be more inherently fertile than Ultisols and are located in similar climatic regions, typically under forest vegetation. They are also more common than Ultisols, occupying about 10% of the glacier-free land surface. Inceptisols (from the Latin inceptum – beginning) exhibit a moderate degree of soil development and lack significant clay accumulation in the subsoil. They occur over a wide range of parent materials and climatic conditions, and thus have a wide range of characteristics. They are extensive, occupying approximately 17% of the earth’s glacier-free surface. Entisols (from recent – new) are the last order in soil taxonomy and exhibit little to no soil development other than the presence of an identifiable topsoil horizon. These soils occur in areas of recently deposited sediments, often in places where deposition is faster than the rate of soil development. Some typical landforms where Entisols are located include: active flood plains, dunes, landslide areas, and behind retreating glaciers. They are common in all environments. Entisols make up the second largest group of soils after Inceptisols, occupying about 16% of the Earth’s surface. Name: Description: ...
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