essay on soil horizon

Soil Horizons – 7 Soil Layers and Profile Explained

There are 7 soil horizons beneath the surface of the Earth. Each has a unique mineral content and variation in texture, but all contribute to the health of the soil of an area and how well plants grow overtop.

What Are Soil Horizons?

Soil horizons, according to Britannica , are layers of soil that are underground, which “develop from the combined actions of living organisms and percolating water.” This definition basically means that many living creatures and other forces of nature shape the soil in a way that creates identifiable layers. You can take a vertical piece of soil from the ground and see noticeable changes as you look down the sample – these are the soil horizons.

Many factors in the environment can contribute to the formation of soil horizons, such as the topography ( physical features in the land), its parent material, nearby climate, and the length of time the soil’s components have spent in the ground. Anything that moves, compacts, or erodes the soil can cause a soil horizon to form, or it will at least affect the overall mineral content.

After a long time, when these soil horizons begin to form, they will start to have distinctive differences in their characteristics. The factors that played a part in the horizon’s formation are what give it its color and texture. The color of the soil horizons helps identify the different layers when analyzing the soil profile.

Each soil horizon is at a unique range of depth, but the thickness of the horizon can increase or decrease depending on the area. The soil horizons under mountain ranges and hilly slopes are much different than the soil horizons under a flat field or plateau. Whether the horizons are thick does not necessarily depend on the size of the hills.

Soil horizon depths can range from a few centimeters to several meters , but will always stay in the same order – horizon O, A, E, B, C, and R. The soil horizon, O, starts at the topsoil and the other horizons are below it. Horizon R is the deepest horizon, which represents the bedrock at the bottom of the soil, but it is not actually a component of the soil and consists mainly of rock.

The 7 Soil Horizons

There are 7 soil horizons in total. These include horizon Oi, Oa, A, E, B, C, and R. As you may have noticed, horizon O is split into two types – Oi and Oa. We will discuss both, but it is important to recognize that they are much different in their composition and their effect on soil type .

To learn about the different uses of soil other than gardening and agriculture click here.

Horizon Oi and horizon Oa are both located in horizon O.

The Oi horizon is the part of horizon O that contains the uppermost materials in the soil . Slightly decomposed organic matter, such as the remains of plants and animals, are what create this layer, and they will slowly push down into the deeper horizons over time.

The organic material in the shallowest part of this horizon is saturated with moisture due to its exposure to the weather and other environmental forces that do not usually reach the other layers of the soil .

The Oa horizon is just below horizon Oi, and also has decomposed organic matter. However, the organic matter in this section of the O horizon is slightly more decomposed and has been sitting in the soil longer. Typically, the Oa layer of the soil is less saturated than the Oi layer.

Once decomposing organic matter goes through the first horizon, it moves into Horizon A. When this movement happens, the soil is then referred to as eluviated soil.

By this point, the organic matter has turned into humus , which is the dark material that forms when organic matter finishes decomposing. The humus provides vital nutrients to the soil and acts as a natural type of fertilizer for the plants that are still alive in the topsoil. Once organic matter reaches this stage, it looks less like leaves, twigs, and bones, and more like soil.

Although Horizon O is the top horizon in most soils, some have only a horizon A; in which case, it would the surface horizon .

In horizon E, the humus that formed in the previous horizon is now lacking nutrients and various minerals, such as iron and aluminum. When plant roots pull these nutrients out of the soil, it starts to become lighter in color and pushes farther into the soil to become this horizon.

The texture of horizon E is primarily sand and silt particles. Typically, soils have a balance of sand, silt, and clay. However, when the soil reaches this horizon, it no longer contains the same amount of clay. This is because of the loss of organic matter and minerals.

Horizon E is typically light brown and sandy.

Ironically, horizon B tends to have more content in the soil than horizon E. The B horizon is older and also has more structure, which has built up over many cycles of the soil.

The B horizon has a higher concentration of silicate clay compared to the E horizon, and it also contains an increased amount of minerals, such as iron, aluminum, gypsum, and silica.

According to ScienceDirect , horizon B can also show signs of the following:

• Evidence of the removal of carbonates
• Residual concentration of sesquioxides – coatings of these sesquioxides make the horizon lower in value and more colorful
• A granular, prismatic structure

Overall, horizon B acts as a buffer horizon between the upper layers and the deeper horizons that have more rocks and stone.

Horizon C is the bottom layer of the soil, which is also called the substratum. This layer has unconsolidated earth material.

Horizon C is substantially different from the other horizons since it has not undergone the same soil-forming factors that effected the upper layers of the soil.

Horizon R is not part of the soil, but it is important to recognize because it acts as the foundation of all the other horizons in the ground. Horizon R is the bedrock, which consists of hard, consolidated rocks and stone that are practically impenetrable.

Importance of Soil Horizons Profile

Soil scientists (pedologists), agricultural experts, gardeners, archeologists, and anyone else who researches and handles soil must use a soil profile to find out vital information about its contents.

Extracted sections of soil (the soil profile) show the soil horizons and how they compare to each other. Anyone analyzing the soil layers and the materials within can learn about the origin of the soil, including its parent material, and well as any other useful information about the mineral contents.

In agriculture, a farmer can use this type of information to adjust aspects of the soil like the pH of the soil or its nutrient content.

Certain crops need specific minerals and nutrients to produce a fully mature harvest. Without this information, the person taking care of the crops could miss out on an entire harvest season due to stunted plant growth.

For example, when it comes to planting sweet corn, they will need a soil pH between 5.8 and 6.5 , 70-80% moisture when planting, and a constant supply of nitrogen and phosphorus throughout the growing season . There are many more requirements for planting and growing sweet corn, but these specific requirements are elements that can be checked through an analysis of the soil profile.

To detrmine the pH of your soil with three simple methods click here.

If the topsoil (or horizon at the planting depth) has the nutrients and minerals a plant crop needs, then there will be less maintenance throughout the season. However, if the soil is lacking but has other concentrations of minerals and nutrients, it may be a good idea to rotate the crops or combine nutrient-rich additives to the soil.

Farmers are more likely than gardeners to need a thorough soil analysis. Typically, for home gardening soil , you do not have to view the lower soil horizons because of the fact that most garden plants remain in the topsoil. Soil nutrients and pH are common factors that gardeners check and adjust before planting their seeds.

How to Extract A Soil Profile

A soil profile, if extracted correctly, should show multiple of the soil horizons in one piece, or adjoining pieces.

Soil profiles often come from one area of the ground and will not give accurate results if pulled from more than one location. The goal of extracting a soil profile is to find out the mineral content of a specific area of the soil.

To create a soil profile, you must dig a hole. Find a spot in the soil where it will be the least difficult to dig up a decent-sized hole. Pay attention to nearby plants to avoid breaking any roots and dig a hole that is at least a foot in diameter.

Gardeners may only need a small piece of soil, but the larger the soil profile is, the more it will tell you in the analysis.

Quickly dig a hole that is a few feet deep – 2 to 3 feet will do.

Try to look for a noticeable difference in the soil at the bottom.

Once the hole is deep enough, take a shovel or a gardening tool and scrape one side of the hole to make one long, flat piece. This method will help with comparing the different soil horizons and measuring their depths, but you can also pull a small amount of soil from each horizon as you dig the hole and place them in separate contains for examination.

Soil Test Kits

Some soil test kits allow for a quick analysis of certain aspects of the soil. For example, there are numerous test kits for testing soil pH, but not as many for testing nutrients.

Here are a few soil test kits that you should consider using for your soil profile analysis. Be sure to write down your observations before using the soil in a test kit, so that you do not have to extract a second soil profile.

Frequently Asked Questions About Soil Horizons

Q: What soil horizons are impermeable?

A: Horizon R, which is at the very bottom of soil (the bedrock) is impermeable because of the compact rock that forms the horizon. However, other soil horizons can be impermeable as well, if there is a dense structure of soil, rock, and no cracks or gaps through which water could seep.

Q: What soil horizon is subsoil?

A: Horizon B is the subsoil. It is rich in minerals due to contents that have moved further down into the soil from the upper layers. Horizon B can contain high levels of iron, aluminum, gypsum, and silica clay.

Q: Why do soil horizons form?

A: Soil horizons form because of the effects of nature. If all the water, wind, and animals remained completely still on Earth, all the time, soil horizons would not form because there would be no means for movement of the soil. Plants and animals of all sizes are equally important to the formation of the soil .

The elements in soil horizons can provide knowledge to anyone who is looking to learn more about their soil. Gardeners can use the information they gain from horizons to grow taller plants, and farmers can use it to grow healthier crops.

The analysis of soil is easy, and it is possible to extract and examine a soil profile right at home. However, the deeper layers of soil are harder to reach and are not necessary for the soil analysis of small areas of land. The most useful information comes from the top soil layers, such as horizon O and horizon A.

Soil horizons are informative because they are a collection of everything that has gathered in a specific area of soil since its original formation . Of course, minerals that were present in the original, parent material may not exist so much now, but if they do, you could find them at the deepest point in the soil. Soil formation takes hundreds of years – it is an untapped database until someone digs it up.

• Best potting soil and plant food for fiddle leaf fig tree

4 thoughts on “Soil Horizons – 7 Soil Layers and Profile Explained”

Thank you very much for sharing the knowledge! This is very helpful and clear to know the soil layers and how to extract soil profile. And I am very curious about the soil test kits, because I haven’t seen one before. I am doing the research about heavy metal contamination and soil microbial diversity, so I do need to determine the TN (TP, TK), AP, AK, AN by some laboratory methods and facilities. I would like to know the principle of the N, P, K determination of soil test kits. Have you compared the results from soil test kits and laboratory methods? Thank you very much!

each horizon and color, composition and the presence or absences of soil organisms(both plants and animals)

Nice and concise, everyone should learn this since soil loss has always been a huge danger. Only our planet has soil, elsewhere it is regolith.

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Soil Horizons

The soil is the topmost layer of the earth’s crust consisting of air, water, inorganic minerals (rock, sand, clay, and slit), and organic matter (dead plants and animals). It forms the source of food for plants. It provides shelter for many animals such as insects, centipedes, burrowing animals, microorganisms, and many others. It is thus also called the ‘skin of the earth.’

There are different soil types, each having unique characteristics like color, texture, structure, thickness, mineral content, and organic matter.

What is Soil Horizon?

During its formation, the soil is arranged in different layers. Each of these layers is called a soil horizon, and when these layers are arranged sequentially one above the other, it forms the soil profile. In other words, the soil profile is the vertical section of the soil exposed by a soil pit.

There is the significant importance of soil horizon in soil science. It allows one to understand the several processes that play a role in soil development and determine the different soil types. It also forms the basis for soil classification.

How Many Horizons are there in Soil?

There are six different layers or horizons that make up a mature soil profile. These layers or horizons are represented by alphabets O, A, E, C, B, and R. Immature soils lack some of these layers.

1) O Horizon – (Organic Layer)

‘O’ is for organic. This layer is the uppermost layer of the soil rich in organic matter, such as the remains of plants and dead animals. Due to high organic content, this layer is typically black brown or dark brown. The O horizon is thin in some soil, thick in some others, or absent in the rest.

2) A Horizon – (Topsoil)

Found below the O horizon, it has a dark brown color as it contains the maximum organic matter of the soil. The A horizon or topsoil is thus also called the humus layer. The topsoil is the region of intense biological activity and has the most nutrients. Insects, earthworms, centipedes, bacteria , fungi, and other animals are found inside this layer.

The humus makes the topsoil highly porous, allowing it to hold air and moisture necessary for seed germination. Here, the plants stretch their roots deep down, allowing it to hold the topsoil together. In this layer, minerals and clay particles may dissolve in the fresh water and get carried to lower layers as water percolates down the soil.

3) E Horizon – (Eluviation Layer)

This layer consists of nutrients leached from O and A horizons and is thus called the eluviations layer. Leaching of clay, minerals, and organic matter leaves this layer with a high concentration of sand, slit particles, quartz, and other resistant materials. E horizon is absent in most soils but is more common in forested areas. 

4) B Horizon – (Subsoil)

Mostly found below the topsoil is another layer called the subsoil or horizon B. It is lighter in color than the topsoil due to lower humus content. However, it is comparatively more rigid and compact than the topsoil. This layer has less organic content but is rich in minerals that are leached down from the topsoil. The subsoil is the region of deposition of certain minerals and salts of certain metals such as iron oxides, aluminum oxides, and calcium carbonate in large proportion.

This layer holds enough water due to its clayey nature. Farmers often mix topsoil and subsoil while plowing their fields. 

5) C Horizon – (Parent Rock)

Also known as regolith or saprolite, it lies just below the subsoil. It is called the parent rock because all the upper layers developed from this layer. C horizon is devoid of any organic matter and is made of broken-up bedrocks, making it hard. Plant roots do not penetrate this layer. This layer is a transition between the inner layer of earth and the upper A and B horizons. 

6) R Horizon – (Bedrock)

Found beneath all the layers, it consists of un-weathered igneous, sedimentary, and metamorphic rocks . It is highly compact. Granite, basalt, quartzite, sandstone, and limestone make up the bedrock.

How do the Different Soil Horizons Develop?

The formation of soil is a continuous process occurring still today from the time of the earth’s inception.

• The process starts when big rocks are broken down into smaller ones by wind and rain. This process is known as weathering. The two types of weathering processes are physical and chemical weathering. Several natural forces such as wind, water, sunlight act as physical agents. In contrast, water, oxygen, and carbon dioxide act as chemical agents of weathering.
• These rocks get further broken down into finer particles such as sand, silt, and gravel, and the process continues.
• This process continues for thousands of years to form just a 1 cm layer of soil. These fine particles ultimately form the topmost layer of the soil.

Ans . Five factors that cause soils and their horizons to differ from one another are parent material, weather or climate, topography, biological factors such as the type of plants and animals living on the soil, and time.

Ans . The decomposition of dead organic remains of plants and animals over time by microorganisms such as bacteria and fungi help soils become dark.

Ans . The A Horizon makes up the topsoil.

Ans . The A Horizon or the topsoil is best for growing plants.

Ans . The O horizon contains the most humus among all other layers of soil.

• Soil Layers – Enchantedlearning.com
• Soil Horizons – Soils4teachers.org
• What is a Soil – Soils4kids.org
• Soil and Soil Profile – Toppr.com

Article was last reviewed on Friday, February 17, 2023

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One response to “Soil Horizons”

Very useful for one of my school assignments, thank you!

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4.3 - Soil Profiles and Horizons

Through the interactions of these four soil processes, the soil constituents are reorganized into visibly, chemically, and/or physically distinct layers, referred to as horizons.  There are five soil horizons:  O ,  A ,  E ,  B , and  C . ( R  is used to denote bedrock.)

There is no set order for these horizons within a soil.  Some soil profiles have an A-C combination, some have an O-E-B, an O-A-B, or just an O.  Some profiles may have all the horizons, O-A-E-B-C-R.  And some profiles may have multiple varieties of one horizon, such as an A-B-E-B.  There are some generalized concepts of how soil layers develop with time; these are expressed below, but due to the variability of natural processes over geologic time, generalized concepts are sometimes overly general.  Knowing something about the geomorphic history of the area being investigated helps unlock the landscape history the soils show.

• A:  An A horizon is a  mineral horizon.  This horizon always forms at the surface and is what many people refer to as topsoil.  Natural events, such as flooding, volcanic eruptions, landslides, and dust deposition can bury an A horizon so that it is no longer found at the surface.  A buried A horizon is a clear indication that soil and landscape processes have changed some time in the past.  Compared to other mineral horizons (E, B, or C) in the soil profile, they are rich in organic matter, giving them a darker color.  The A horizon, over time, is also a zone of loss – clays and easily dissolved compounds being leached out – and A horizons are typically more coarse (less clay) compared to underlying horizons (with the exception of an E horizon).  Additions  and  losses  are the dominant processes of A horizons.

• B : A B horizon is typically a mineral subsurface horizon and is a zone of accumulation, called illuviation. Materials that commonly accumulate are clay, soluble salts, and/or iron. Minerals in the B horizon may be undergoing transformations such as chemical alteration of clay structure. In human modified landscapes, processes such as erosion can sometimes strip away overlying horizons and leave a B horizon at the surface. Such erosion is common in sloping, agricultural landscapes. A bulldozer preparing land for a new subdivision can also leave a B horizon at the surface. The dominant processes in a B horizon are transformations and additions.

• C:  A C horizon consists of  parent material , such as glacial till or lake sediments that have little to no alteration due to the soil forming processes.  Low intensity processes, such as movement of soluble salts or oxidazation and reduction of iron may occur.  There are no dominant processes in the C horizon; minimal additions and losses of highly soluble material (e.g., salts) may occur.

• O : An O horizon has at least 20%   organic matter  by mass. Two main scenarios result in the formation of an O horizon: saturated,  anaerobic  conditions (wetlands) or high production of leaf litter in forested areas. Anaerobic conditions slow the  decomposition  process and allow organic material to accumulate. An O horizon can have various stages of decomposed organic matter: highly decomposed, sapric; moderately decomposed, hemic; and minimally decomposed, fibric. In a fibric O layer, plant matter is recognizable (e.g., it is possible to identify a leaf). Sapric material is broken down into much finer matter and is unrecognizable as a plant part. Hemic is in between sapric and fibric, with some barely recognizable plant material present. It is possible to have multiple O horizons stacked upon one another exhibiting different decomposition stages. Because of their organic content, these horizons are typically black or dark brown in color. The dominant processes of the O horizon are additions of organic matter, and transformations from fibric to sapric.

• E:  The E horizon appears lighter in color than an associated A horizon (above) or B horizon (below). An E horizon has a lower clay content than an underlying B horizon, and often has a lower clay content than an overlying A horizon, if an A is present.  E horizons are more common in forested areas because forests are in regions with higher precipitation and forest litter is acidic.  However, landscape hydrology, such as perched water tables, can result in the formation of an E horizon in the lower precipitation grasslands, as seen in the profile below. The dominant processes of an E horizon are losses.

• R:   An R layer is bedrock.  When a soil has direct contact with bedrock, especially close to the soil surface, the bedrock becomes a variable when developing land use management plans and its presence is noted in the soil profile description.

*This animation has no audio.*

The layer at 60-90 cm (2-3 feet; measuring tape is in feet) in this image is darker because it is wicking water up from below, much like a sponge set in a pan of water. The layers above it are drier. Wetting a soil up tends to make it darker. The texture of the indicated layer is sand and there is very little organic matter or soil activity (no clay accumulation, no leaching) going on. What is the likely horizon designation for this layer? 

The soil seen here is very deep formed in alluvial and wind-blown deposits. The area between 30-60 cm (1-2 feet; measuring tape is in feet) has strong prismatic structure due to an accumulation over time of clay from overlying horizons. This accumulation of clay suggests what horizon type? 

This moist, dark slice of soil came from a pasture and shows the upper 30 cm (12 inches; measuring tape is in inches) of the soil profile. There is strong granular structure and lots of grass roots. There is even an earthworm (at yellow arrow). This layer of soil is mainly sand, silt, and clay, but there is also much more organic matter in this layer than the ones deeper in this profile. The likely soil horizon is:   

This slice of soil was pulled from a wetland. The first 10 cm (4 inches; in the yellow box) is all decomposing organic matter. The dark gray layer underneath is mostly mineral with some organic matter mixed in. The horizon indicated in the yellow box is a(n) ____ horizon.  

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Soil horizons

Pedons and polypedons.

• Grain size and porosity
• Water runoff
• Mineral content
• Organic content
• Biological phenomena
• Parent material
• U.S. Soil Taxonomy
• FAO soil groups
• Erosive processes
• Rates of soil erosion
• Resistance to erosion
• Carbon and nitrogen cycles
• Soils and global warming
• Xenobiotic chemicals
• Pathways of detoxification

What is soil?

What are the grain sizes in soil, what are the layers of soil.

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• Table Of Contents

Soil is the biologically active and porous medium that has developed in the uppermost layer of Earth’s crust. It serves as the reservoir of water and nutrients and a medium for the filtration and breakdown of injurious wastes. It also helps in the cycling of carbon and other elements through the global ecosystem.

The grain size of soil particles are categorized into three groups: clay, silt, and sand. Clay measures less than 0.002 mm (0.0008 inch) in diameter, silt is between 0.002 mm (0.0008 inch) and 0.05 mm (0.002 inch), and sand is between 0.05 mm (0.002 inch) and 2 mm (0.08 inch).

What are the five factors of soil formation?

The evolution of soils and their properties is called soil formation, and according to pedologists, five fundamental soil formation processes influence soil properties. These five “state factors” are parent material, topography, climate, organisms, and time.

Soils have a unique structural characteristic that distinguishes them from mere earth materials: a vertical sequence of layers produced by the combined actions of percolating waters and living organisms. These layers are called horizons and are designated A horizon, B horizon, C horizon, E horizon, O horizon, and R horizon.

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soil , the biologically active, porous medium that has developed in the uppermost layer of Earth’s crust. Soil is one of the principal substrata of life on Earth, serving as a reservoir of water and nutrients, as a medium for the filtration and breakdown of injurious wastes, and as a participant in the cycling of carbon and other elements through the global ecosystem . It has evolved through weathering processes driven by biological, climatic, geologic, and topographic influences.

Since the rise of agriculture and forestry in the 8th millennium bce , there has also arisen by necessity a practical awareness of soils and their management. In the 18th and 19th centuries the Industrial Revolution brought increasing pressure on soil to produce raw materials demanded by commerce, while the development of quantitative science offered new opportunities for improved soil management. The study of soil as a separate scientific discipline began about the same time with systematic investigations of substances that enhance plant growth. This initial inquiry has expanded to an understanding of soils as complex, dynamic , biogeochemical systems that are vital to the life cycles of terrestrial vegetation and soil-inhabiting organisms—and by extension to the human race as well.

This article covers the structure, composition , and classification of soils and how these factors affect soil’s role in the global ecosystem. In addition, the two most important phenomena that degrade soils, erosion and pollution, are discussed. For a cartographic guide to the distribution of the world’s major soils, featuring links to short descriptive entries on each soil type, see the interactive world map .

The soil profile

Soils differ widely in their properties because of geologic and climatic variation over distance and time. Even a simple property, such as the soil thickness, can range from a few centimetres to many metres, depending on the intensity and duration of weathering , episodes of soil deposition and erosion , and the patterns of landscape evolution. Nevertheless, in spite of this variability, soils have a unique structural characteristic that distinguishes them from mere earth materials and serves as a basis for their classification: a vertical sequence of layers produced by the combined actions of percolating waters and living organisms.

These layers are called horizons , and the full vertical sequence of horizons constitutes the soil profile (see the figure ). Soil horizons are defined by features that reflect soil-forming processes. For instance, the uppermost soil layer (not including surface litter) is termed the A horizon . This is a weathered layer that contains an accumulation of humus (decomposed, dark-coloured, carbon-rich matter) and microbial biomass that is mixed with small-grained minerals to form aggregate structures.

Below A lies the B horizon . In mature soils this layer is characterized by an accumulation of clay (small particles less than 0.002 mm [0.00008 inch] in diameter) that has either been deposited out of percolating waters or precipitated by chemical processes involving dissolved products of weathering. Clay endows B horizons with an array of diverse structural features (blocks, columns, and prisms) formed from small clay particles that can be linked together in various configurations as the horizon evolves.

Below the A and B horizons is the C horizon , a zone of little or no humus accumulation or soil structure development. The C horizon often is composed of unconsolidated parent material from which the A and B horizons have formed. It lacks the characteristic features of the A and B horizons and may be either relatively unweathered or deeply weathered. At some depth below the A, B, and C horizons lies consolidated rock , which makes up the R horizon.

These simple letter designations are supplemented in two ways (see the table of soil horizon letter designations). First, two additional horizons are defined. Litter and decomposed organic matter (for example, plant and animal remains) that typically lie exposed on the land surface above the A horizon are given the designation O horizon , whereas the layer immediately below an A horizon that has been extensively leached (that is, slowly washed of certain contents by the action of percolating water) is given the separate designation E horizon , or zone of eluviation (from Latin ex , “out,” and lavere , “to wash”). The development of E horizons is favoured by high rainfall and sandy parent material, two factors that help to ensure extensive water percolation. The solid particles lost through leaching are deposited in the B horizon, which then can be regarded as a zone of illuviation (from Latin il , “in,” and lavere ).

The combined A, E, B horizon sequence is called the solum (Latin: “floor”). The solum is the true seat of soil-forming processes and is the principal habitat for soil organisms. (Transitional layers, having intermediate properties, are designated with the two letters of the adjacent horizons.)

The second enhancement to soil horizon nomenclature (also shown in the table) is the use of lowercase suffixes to designate special features that are important to soil development. The most common of these suffixes are applied to B horizons: g to denote mottling caused by waterlogging, h to denote the illuvial accumulation of humus, k to denote carbonate mineral precipitates, o to denote residual metal oxides, s to denote the illuvial accumulation of metal oxides and humus, and t to denote the accumulation of clay.

Soils are natural elements of weathered landscapes whose properties may vary spatially. For scientific study, however, it is useful to think of soils as unions of modules known as pedons. A pedon is the smallest element of landscape that can be called soil. Its depth limit is the somewhat arbitrary boundary between soil and “not soil” (e.g., bedrock). Its lateral dimensions must be large enough to permit a study of any horizons present—in general, an area from 1 to 10 square metres (10 to 100 square feet), taking into account that a horizon may be variable in thickness or even discontinuous. Wherever horizons are cyclic and recur at intervals of 2 to 7 metres (7 to 23 feet), the pedon includes one-half the cycle. Thus, each pedon includes the range of horizon variability that occurs within small areas. Wherever the cycle is less than 2 metres, or wherever all horizons are continuous and of uniform thickness, the pedon has an area of 1 square metre.

Soils are encountered on the landscape as groups of similar pedons, called polypedons, that contain sufficient area to qualify as a taxonomic unit. Polypedons are bounded from below by “not soil” and laterally by pedons of dissimilar characteristics.

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What is Soil Horizon?

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Soil horizon can be defined as the parallel layer of the soil surface. Each layer has its own composition of physical, chemical and biological characteristics, they quite differ from each of the layers above and beneath each layer. Horizons have definite physical features such as the colour and texture of each layer of the soil. These soil horizons are described both in absolute terms like in terms of the particle size distribution for texture, and in terms, they are relatively ‘coarser’ or ‘sandier’ than all the soil horizons above and below.

In our prevailing section, we will plunge deeper into the Soil Horizon Definition in detail, know about each layer of soil and its benefits. 

Horizons of Soil

In this section, we will continue to explain the soil horizons layers. 

As studied vividly on the subject ‘the types of the soil’, we know there are varied types of soil, with each one having distinct characteristics. If anyone would dig down deep into any of the soils, one can see the soil is made of layers, or the horizons (O, A, E, B, C, R). Putting the horizons together, they will structure into a soil profile. Each of the profiles informs about the nature of the particular soil which has been dug deep. Majorly these soils have three major horizons that are - A, B, C and some have an organic horizon as well denoted by O. The horizons are:

1. O Horizon Which Contains Hummus or any Organic Matter: 

This layer is mostly filled with organic content like the decomposing leaves. The O horizon is particularly thinner in most soils, while thicker in others, also the horizon may not be present at all in other types of soil.

2. A Which is the Topsoil: 

Minerals are present in this layer which are generated from the parent material with the organic matter that is being incorporated. This layer serves as a good material for the plants and other organisms to live.

3. E is Known as the Eluviated Layer: 

Here clay, minerals, and other organic matter, with a concentration of sand and silt particles of quartz are present. They are mainly found in older soils and forest soils.

4. B the Subsoil Layer:

They are quite rich in minerals, the mineral seeps down from the A or E horizons and gets accumulated in this layer.

5. C the Parent Material: 

The deposit is present at the Earth’s surface, this is the place from which actually the soil is originated. 

6. R, the Bedrock Layer: 

This layer has a mass of rock like granite, basalt, quartzite, limestone and even sandstone, this layer forms the parent material for some soil.

Thus, clearly, the Soil Horizons are explained. In the next section, we will take up each of the layers in the soil profile and determine the quality of the soil that influenced such a layer. 

Soil Profile Horizons 

The Different Layers of Soil have different functions to do. While the soil profile is quite defined as the vertical section of the soil which forms the ground surface, this seeps downwards to where the soil meets the underlying bedrocks.

The soil is arranged in layers or in horizons. They are arranged during their formation. The layers or the horizons are basically known as the soil profile. The vertical section of the soil which is exposed by a soil pit is called the soil horizon profile. The layers of soil are easily identified by their colour and texture. In presence, there are soil particles as well. The different layers of soil are as follows:

The Topsoil

The middle is the Subsoil

Then, comes the Parent rock

Each of the soil layers has its respective characteristics.

Layers of Soil

The soil profile consists of the series of the horizon of soil which is layered on to one another. The horizons are represented by the letters O, A, E, C, B and R, also refer to the diagram displayed above.

The O-Horizon

The uppermost layer of the topsoil is composed of organic materials like dried leaves, grasses, and other decomposed organic matter. This layer of soil is blackish brown or dark brown in colour, the colour is for the content of organic material.  

The A-Horizon is also known as the Topsoil

This layer is also rich in organic material and commonly known as the humus layer. This majorly consists of both organic matter and other decomposed type materials. The topsoil is very soft and is thus porous in nature to hold enough air and water.

The E-Horizon

This layer has nutrients seeped down from the O and A horizons. This layer is very common in forested areas which have low clayey content.

The B-Horizon or Subsoil

This horizon is present just below the topsoil, while above the bedrock. This is comparatively much harder and more compact than the topsoil. This contains less humus, soluble minerals, and organic matter. Rather this is a site of deposition of certain minerals and metal salts like iron oxide.

The C-Horizon, Known as the Saprolite

This layer has an absence of any organic matter and is made up of broken bedrock. 

The R-Horizon

They are the compacted and cemented layer with different types of rocks like granite, basalt and limestone.

FAQs on Soil Horizon

1. Which Layer of Soil is the Best For the Cultivation of Crops?

Ans. Topsoil is best for crop cultivation. Topsoil that is the A horizon is the layer of soil that is located just below the O Horizon. This layer consists of minerals and decomposed organic matter. Due to this, the soil is very dark in colour. This layer helps many plants to grow their roots firmly. The topsoil is the topmost layer of the soil which is usually 5 to 10 inches (that is 13–25 cm). This layer has the highest concentration of organic matter and microorganisms and thus it is most suitable for crops to grow. This soil layer is where most of the Earth's biological soil activity takes place. Topsoil is made up of mineral particles and organic matter, water, and air.

2. How is the Soil Horizon Formed?

Ans. Soils Developed as a result of the interactions between the climate, living organisms, and the landscape position. The soil horizon was influenced by the parent material that is the decomposition over time. The four major soil formation processes change the parent material into soil form and develop the soil horizons. 

3. What are the Basic Components Which Make Up the Soil?

Ans. The basic components which make up the soil are its minerals, organic matter, water and also air. Typically, the soil consists of moreover 45% of mineral, 5% of organic matter, 20-30% water, and the rest 20-30% of air content in it.

• Biology Article
• Soil Profile

Soil Profile Definition

“Soil profile is defined as the vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock.”

Table of Contents

Layers of Soil

• Soil Moisture
• Types of Soil Moisture
• Importance Of Soil Moisture
• Measurements Of Soil Moisture
• What is Soil Profile?

The soil is the topmost layer of the earth’s crust mainly composed of organic minerals and rock particles that support life. A soil profile is a vertical cross-section of the soil, made of layers running parallel to the surface. These layers are known as soil horizons.

Also Read:  Soil Teeming

The soil is arranged in layers or horizons during its formation. These layers or horizons are known as the soil profile. It is the vertical section of the soil that is exposed by a soil pit. The layers of soil can easily be identified by the soil colour and size of soil particles. The different layers of soil are:

• Parent rock

Each layer of soil has distinct characteristics.

Soil profile helps in determining the role of the soil as well. It helps one to differentiate the given sample of soil from other soil samples based on factors like its colour, texture, structure, and thickness, as well as its chemical composition.

Also refer :  Minerals In The Soil And Soil Pollution

Read on to explore what is soil profile and the different layers of soil that make up the soil profile.

The soil profile is composed of a series of horizons or layers of soil stacked one on top of the other. These layers or horizons are represented by letters O, A, E, C, B and R.

The O-Horizon

The O horizon is the upper layer of the topsoil which is mainly composed of organic materials such as dried leaves, grasses, dead leaves, small rocks, twigs, surface organisms, fallen trees, and other decomposed organic matter. This horizon of soil is often black brown or dark brown in colour and this is mainly because of the presence of organic content.

The A-Horizon or Topsoil

This layer is rich in organic material and is known as the humus layer. This layer consists of both organic matter and other decomposed materials. The topsoil is soft and porous to hold enough air and water.

In this layer, the seed germination  takes place and new roots are produced which grows into a new plant. This layer consists of microorganisms such as earthworms, fungi, bacteria, etc.

The E-Horizon

This layer is composed of nutrients leached from the O and A horizons. This layer is more common in forested areas and has lower clay content.

The B-Horizon or Subsoil

It is the subsurface horizon, present just below the topsoil and above the bedrock. It is comparatively harder and more compact than topsoil. It contains less humus, soluble minerals, and organic matter. It is a site of deposition of certain minerals and metal salts such as iron oxide.

This layer holds more water than the topsoil and is lighter brown due to the presence of clay soil. The soil of horizon-A and horizon-B is often mixed while ploughing the fields.

The C-Horizon or Saprolite

This layer is devoid of any organic matter and is made up of broken bedrock. This layer is also known as saprolite. The geological material present in this zone is cemented.

The R-Horizon

It is a compacted and cemented layer. Different types of rocks such as granite, basalt and limestone are found here.

Explore more about:   Preparation of Soil for Agriculture

Apart from the rocks, minerals, and layers, soil profile also consists of a water content, which is referred to as soil moisture.

What Is Soil Moisture?

Water in the soil is referred to as soil moisture.  Water absorption in the soil  is determined by various factors. It plays a major role in soil formation. As a result of precipitation, water arrives at the surface. The particle size distribution of soil determines its porous nature and causes downward movement of water vertically which is known as infiltration. This penetration continues deep in the layers of soil until it reaches saturation.

Water, on reaching this barrier, cannot seep vertically further, hence it moves sideways. Formation of puddles as a result of saturation is called surface ponding which can be long-lasting. Water that is available to plants is called Root zone moisture while surface soil moisture is the water available in the immediate upper region of soil.

Moisture content in the soil can be measured using a device known as Tensiometer. They are water-filled tubes which are sealed with a porous ceramic tip towards the bottom and a gauge at the top which is devoid of air molecules. They are penetrated into the soil till the root level. Water passes between the tip of the device and the ambient soil until it reaches an equilibrium and hence, tension is recorded on the gauge. Readings thus obtained give a measure of soil moisture in that region.

Also Refer : What Is Soil?

Types Of Soil Moisture

The different types of water present in the soil include:

• Gravitational Water

The water that reaches the water table of the soil due to the gravitational force is referred to as gravitational water. This is not available to the plants.

• Hygroscopic Water

This water is also not available to the plants. It is a thin film of water tightly held by the soil particles.

• Chemically Combined Water

The chemical compounds present in the soil particles contain water. This is known as chemically combined water. This is also not available to the plants.

• Capillary Water

This water is available to the plants for absorption. This water exists between soil particles in small capillaries.

• Atmospheric Humidity

The hanging roots of the epiphytes absorb the moisture in the air due to the presence of hygroscopic hairs and spongy velamen tissues.

Also Read: Types of Soil

Recommended Video:

Importance Of Soil Moisture Content

Soil water carries food nutrients for the growth of plants

Soil moisture content determines the yield of the crop in a region

Crucial in maintaining soil’s temperature

Soil moisture acts as nutrients

Important for soil formation

Moist soil is ideal for the growth of many plants that demand a huge supply of water (Ex: Rice)

Soil moisture catalyses biological activities of microbes in the soil

Water is a primary need for photosynthesis in plants

Measuring Soil Moisture

The soil moisture can be measured by various tools mentioned below:

Tensiometers

These tools measure the tension of soil moisture. They are water-filled tubes, with a porous ceramic tip at the bottom. These are sealed and have a vacuum gauge at the top. They are inserted in the soil to the depth of the plant root zone. The readings obtained in the tensiometers indicate the availability of water in the soil.

Electrical Resistance Blocks

These consist of two electrodes connected to lead wires extending to the soil surface. The electrodes are embedded in the blocks of porous material. It is used to measure soil water tension.

Time Domain Reflectometry (TDR)

TDR – Time Domain Reflectometry is used to determine the soil moisture content. Steel rods are placed in the soil and electrical signals are sent through them. The returned signals are measured to determine soil water content.

To know more about soil profile, layers of soil, soil moisture or any other related topics, visit us at  BYJU’S Biology.

Important Questions for Soil Profile

• What is Soil?

Soil is one of the most important naturally occurring resources. It is the natural habitat of plants and many microorganisms. It nourishes plants with water and essential nutrients hence enabling their growth. Soil is the most important raw material for agriculture. Agriculture provides food, clothing and shelter to all entities either directly or indirectly. Hence, soil is an inseparable part of our living.

The soil profile is a vertical section of the soil that depicts all of its horizons. The soil profile extends from the soil surface to the rock material.

• How is Soil Formed?

Soil is mainly formed by the breakdown of bigger rocks into smaller and fine particles with the continuous action of wind, rain and other agents of natural force. It takes hundreds to thousands of years for the formation of soil.

• What are the basic components of Soil?

Air, water, minerals and other organic matter are the basic components of soil.

• What is the importance of Soil Profile?

The soil profile plays an important role in maintaining the fertility of the soil and the nutrition content in the soil.

• What are the horizons of soil?

The soil profiles are composed of a series of horizons or layers of soil, which are stacked one above the other. The 4 horizons of soil are:

• The O-Horizon.
• The A-Horizon.
• The B-Horizon.
• The C-Horizon.
• What is Topsoil?

The topsoil is the topmost layer of the soil. It is dark brown coloured soil which mainly consists of organic matter, decomposed material and many living organisms including some microbes, earthworms and other worms.

• List out the different types of Soil Moisture.
• Capillary Water.
• Hygroscopic Water.
• Gravitational Water.
• Atmospheric Humidity.
• Chemically Combined Water.

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Essay on Soil: Introduction and Formation

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In this essay we will discuss about soil. After reading this essay you will learn about: 1. Introduction to Soil 2. Components of Soil 3. Functions 4. Microclimate 5. Formation of Soil 6. Types of Pollutants 7. Fertiliser Contamination 8. Greenhouse Effect and Soil Fertility 9. Weathering 10. The Soil Profile 11. Textural Classes 12. Cation Exchange Capacity 13. Soil Reaction 14. Economy of Essential Elements.

Essay on Soil

Essay Contents:

• Essay on the Economy of Essential Elements of Soil

ADVERTISEMENTS: (adsbygoogle = window.adsbygoogle || []).push({}); Essay # 1. Introduction to Soil : (500 Words)

Soils form a narrow interface between the atmo­sphere and the lithosphere and possess elements of both: water, a gaseous phase and mineral mat­ter, together with a diverse range of organisms and materials of biological origin.

They continu­ally interact with the atmosphere above and the lithosphere beneath. Soils are that part of the earth’s thin surface ‘rind’ within which organic ma­terials are broken down to form stable humic compounds thereby releasing their contained nu­trient elements for uptake by organisms and dissi­pating their contained energy.

This veneer of ma­terials supports the growth of higher plants and therefore the primary production on which the human population directly depends. Soils provide other important services including the stabilisation of waste materials and part of the excess CO 2 re­leased to the atmosphere by human activities.

The thin veneer of soil is readily damaged or lost by misuse. Following such effects, it does not reform in any major sense within the time frames of human existence and must be considered a non-renewable resource.

Many current systems of agricultural management are not sustainable in the longer term because of the pressures they place on the soil. Production levels may frequently be set on the basis of economic goals rather than the capacity of the soil to withstand particular stresses.

Most cropping systems, for example, require sub­stantial regular inputs of energy and nutrient ele­ments and the sustainability of their use of this purpose is contingent upon continuing inputs. Similar principles apply in other situations. Con­tinued overgrazing in pastoral environments situ­ations soon leads to soil degradation and loss and stresses imposed by chemical contamination may eventually result in impaired functioning.

Faced with such pressures, soils are clearly a threatened resource. From this, one of the emerg­ing challenges facing soil ecologists is the mainte­nance or amelioration of soil fertility in the devel­opment of long-term sustainable agricultural sys­tems. This requires the integration of biological process knowledge into general models of soil functioning and the design of land management systems based on such models.

An understanding of soil functioning and the definition of appropriate management options demand a knowledge of the processes operating in both the above and below ground subsystems and identification and quantification of fluxes of energy and materials between and within them. The maintenance of the physical integrity and fer­tility of soils depends largely on these transfers.

The soil is a unique environment that com­bines solid, liquid and gaseous phases to form a three-dimensional matrix. The organisms that in­habit this porous, humid and amphibious environ­ment face quite specific constraints due to:

(i) The predominance of poor-quality food resources for species at the lower trophic levels;

(ii) Spatial con­straints in an environment where most organism live within pores that differ broadly in size, shape and inter-connectedness; and

(iii) The rapid alter­nation, in both space and time, of air and water- filled porosities. The adaptive strategies evolved under such conditions are certainly different from those of their above ground counterparts.

Essay # 2. Components of Soil: (500 Words)

Soil components may be classified in many ways, depending on the intended purpose of the classi­fication. Some common ways of classifying such materials are on their sizes, shapes and origins, the phases they belong to, their chemical or physical characteristics, their mineralogical compositions or on combinations of these.

The classification of soil components employed here is arbitrary and hierarchical and attempts to characterize the indi­vidual components in ways that reflect their eco­logical interrelationships (Fig. 10.1).

The division of soil materials into separate components in no way implies that associations and interactions between the various components do not occur, or are unimportant. Indeed, nearly all-solid and liquid phase entities that occur within soils include both organic and inorganic compo­nents, or are influenced by them.

The formation and functioning of soil depends on the myriad interactions that occur between the organic and inorganic, and the living and non-living components of soils. An enormous variety of organic and in-organic components are available within soils to interact in the synthesis and development of the unique substance which is soil.

Table 10.1 presents the indicative value of concentrations of selected major and minor ele­ments in surface soils together with the ranges that may be expected to occur in areas where mineralization has occurred or which have been polluted by human activities.

The most important of the inorganic com­ponents in terms of soil behaviour is undoubt­edly the highly-diverse group of substances characterised generally as clays. The term clay may refer to three distinct entities. In the textural sense, clay refers to the important colloidal particles less than 0.002 mm (2 µm) in diameter.

More loosely, it may also refer to a class of soils with a high proportion of such particles. It may also be ap­plied to the phyllosilicate clay minerals discussed below. Here, the term clay will refer to clay-sized particles unless otherwise qualified.

The phyllosilicate and other mineral particles in the clay size range have large surface areas relative to their masses and, in soils with appreciable clay contents, they control many reactions important to biologi­cal processes. Surface area is also closely corre­lated with a range of other properties that regu­late the physical and chemical characteristics of the soil mass and influence plant growth.

Organic materials found in soils may be di­vided into the living organisms and non-living materials of biological origin. The later comprise a diversity of materials including roots and other plant and animal remains in all stages of subdivision and decomposition.

In addition, dead fungal hyphae, spores, bacteria and other larger constructs of microbial (e.g., the sporocarps of mycorrhizal fungi) and faunal (e.g., termitaria) origin are frequently present. During the later stages of decomposition of plant materials, less resistant materials are lost leaving only cell wall outlines and fragments which, in the terminal stages of decomposition, may appear amorphous under microscopic examination.

Other directly-derived biological materials include such diverse products as plant root lysates and exudates, the faeces, excreta and secreta of animals, the cutane­ous mucus secreted by earthworms and gels pro­duced by microorganisms.

Essay # 3. Functions of Soil: (400 Words)

i. Mechanical Support:

The role of the soil in providing a mechanical support for plants is clear.

All the rooted plants get such support from soil in varying degree to resist the wind pressure on aerial parts of plant.

ii. Habitat Provision:

The vast majority of soil organisms are too small and too weak to cre­ate their own habitats and these latter must exist within the soil to sustain the organisms essential to its functioning. Small organisms must live within existing voids formed through physical processes or through the activities of such larger organisms as plant roots and soil-dwelling invertebrates and vertebrates.

In par­ticular, decomposing roots comprise an im­portant habitat for many soil organisms.

iii. Storage of Organic Matter:

The soil is an important store of dead organic materials present in all stages of decomposition from freshly-fallen litter and recently-dead plant roots to highly humified materials of great age and chemical complexity.

Dead biological materials are important energy sources for many soil organisms and the more decom­posed fractions interact with inorganic mate­rials to form organo-mineral structures cen­tral to the organisation and stability of the soil matrix.

iv. Element Release:

In addition to such ele­ments as silicon and aluminium which nor­mally dominate the soil mass, the soil also con­tains stores of elements that are of biological and pedological importance. Certain forms of such elements as iron, aluminium and silicon are important in both these respects.

These and other elements may he held within the organic materials (living and dead) considered above but are also present in the soil solution, retained at and near the surfaces of the or­ganic and inorganic soil colloids and, in less accessible forms, within the mineral soil par­ticles.

Decomposition of organic materials lib­erates the contained elements in inorganic forms (mineralization) in a controlled or ‘slow-release’ way for uptake by plant roots and other soils organisms, or for involvement in ped­ological processes.

v. Water Storage:

Soils possess a store of water that supports the growth of plants and other organisms. The magnitude of this store dif­fers substantially between soils depending on soil depth, the size distribution and organisation of the soil particles and location in the landscape.

Within soils, plants may have access to stores of different sizes, depending on their individual capacities to extract water from the smaller pores and on such factors as their rooting depths, mycorrhizal associations and salt tolerances.

Other soil organisms also have their own characteristic physiological tol­erance ranges beyond which they become in­active, or die. By and large, soil harbour a variety of organ­isms which can be categorized as producer, con­sumer and decomposer.

Essay # 4. Microclimate of Soil : (300 Words)

The microclimate of the soil is defined in terms of its internal temperature and hydrological re­gimes and is therefore generally determined by the external climate. Since this varies broadly with lati­tude, elevation, rainfall distribution and, to a lesser degree, with vegetation type and cover, aspect and a range of other factors, it is clear that soil micro­climates are likely to differ almost as widely as those of the surface, albeit increasingly buffered from rapid change with greater depth in the profile.

The soil properties that determine water en­try and movement within the soil are clearly im­portant in defining the microclimate. Soil tempera­ture regimes and the factors governing their varia­tion in time and space form the remaining part of its definition.

The temperature regimes that per­tain in soils influence many processes that occur therein and play a part in controlling the rates and processes of soil development and the composi­tion and activities of the biota. Agricultural pro­ductivity is frequently limited by either low or ex­cessively high temperatures, although both effects are often related to moisture status.

In the pedogenetic sense, water is a major agent of physi­cal weathering through expansion: contraction processes and particularly the frequency with which the 0°C boundary is crossed. The rates of many biological and chemical weathering processes are also temperature dependent.

All species have required minimum and maxi­mum temperatures for growth and survival and, in the arthropods, these may differ between de­velopment stages. However, most species usually have a somewhat narrower preferred range.

The species present in extreme environments usually possess specific adaptations to permit their survival therein. However, a minimum requirement for the persistence of most species is that tem­perature and moisture regimes be regularly within a favourable range for a sufficient period to per­mit successful reproduction and development.

Essay # 5. Formation of Soil : (300 Words)

The formation and evolution of soils involve a series of physical, chemical and biological pro­cesses which act progressively over time, are con­trolled by climatic variables and are greatly influ­enced by topography. Simonson (1978) divides such processes into additions from without the soil system, removals or losses, translocations (or transfers) within the system and transformations of Contained materials.

The original parent material is transformed by in situ weathering into a mixture of stable mineral components which blend intimately with organic materials to form the soil. The parent material is first broken down into its primary minerals whose decomposition products may be partially trans­formed into secondary or neo-formed minerals (Table 10.2).

From this early stage, the nutrients necessary for plant production an such other essential com­ponents as A1 and Fe accumulate progressively in the upper parts of the incipient soils. Clay frac­tions are formed firstly through alteroplasmation i.e., transformation of primary minerals into clays with no subsequent modification of rock struc­ture.

Pedoplasmation is a subsequent transforma­tion whereby clay minerals acquired a pedological structure and such specific properties as swelling and shrinkage.

The initiation of biological activities within the developing substrate leads to the accumula­tion of organic matter. This organic matter mixes with the weathering mineral components to form an A horizon that becomes an active source of further physicochemical changes in the underly­ing parent material to develop a C horizon. CO 2 evolved from decomposing organic matter also participates in the process.

With further develop­ment, weathering and downward transport of materials progressively modify the deeper strata of the parent material and, depending on the pro­cesses operating, E and B horizons may form. At this stage, translocation, biological transport and erosion become the dominant processes in the evolution and differentiation of the soil profile.

Nutrient and other elements (e.g., Si and Al) and organic materials are continually lost in solu­tion and suspension through erosion and by trans­port in subsurface water flows. Such losses are expected to be greatest in incipient soils with ju­venile ecosystems and to diminish with ecosystem and soil development.

Essay # 6. Types of Pollutants of Soil: (700 Words)

i. Pesticides:

Whether pesticides are applied to plant foliage, to the soil surface, or are incorporated into the soil, a high proportion of the chemicals eventually moves into the soil. These chemicals then move in one or more of six major directions (Fig. 10.4).

a. They may vaporize into the atmosphere with­out chemical change.

b. They may be absorbed by humus and clay par­ticles.

c. They may move downward through the soil in liquid or solution form and be lost from the soil by leaching.

d. They may undergo chemical reactions within or on the surface of the soil.

e. The may be broken down by soil microorgan­isms.

f. They may be absorbed by plants and detoxi­fied within the plants.

ii. Toxic Inorganic Compound Sources and Accumulation:

There are many sources for the inorganic chemi­cal contaminants that can accumulate in soils. The burning of fossil fuels, smelting, and other pro­cessing techniques release into the atmosphere tons of these elements, which can be carried for miles and later deposited on the vegetation and soil.

Lead, nickel, and boron are gasoline additives that are released into the atmosphere and carried to the soil through rain and snow. Borax is used as a detergent and in fertilizer, both of which com­monly reach the soil. Superphosphate and lime­stone usually contain small quantities of cadmium, copper, manganese, nickel, and zinc.

Cadmium and chromium are used in plating metals, and cadmium in the manufacture of batteries. Arsenic, for many years used as an insecticide on cotton, tobacco, and fruit crops, is still being used as a defoliant or vine killer and on lawns. Some of these elements are found as constituents in specific organic pesti­cides and in domestic and industrial sewage sludge.

The quantities of most of the products in which these inorganic contaminants are used have increased notably in recent years, enhancing the opportunity for contamination. They are present in the environment in increasing amounts and are daily ingested by people either through the air or through food and water.

The domestic and industrial sewage sludge are considered to be the major sources of potentially toxic chemicals, and at least one-third of these wastes in the United States is being applied to the soil. Sewage sludge commonly carry significant quantities of inorganic as well as organic chemi­cals that can have harmful environmental effects.

iii. Organic Waste Contamination:

Soils have long been used as disposal “sinks” for municipal refuse. “Sanitary landfills” are widely employed to dispose of a variety of wastes from our towns and cities. These wastes include paper products, garbage, and non-biodegradable mate­rials such as glass and metals.

The sites are often located in swampy lowland areas that eventually become built up by the dumping to create upland areas for such uses as city parks and other facili­ties.

iv. Soil Salinity and Irrigation:

Contamination of soils with salts is one form of soil pollution primarily agricultural in origin. Fur­thermore, it is not a new problem. Ancient civilizations in both the New and Old Worlds crumbled because salts built up in their irrigated soils. The same principles govern the management of irri­gated soils today and the same dangers exist of salt build up and concomitant soil deterioration.

Salts accumulate in soils because more salts move into the plant rooting zone than move out. This may be due to application of salt-laden irri­gation waters or it may be caused by irrigating poorly drained soils. Salts move up from the lower horizons and concentrate in the surface soil lay­ers.

v. Acid Rain Impact on Soil:

Effects of acid rain are more pronounced on the acidity of water than on soil acidity. Soils gener­ally are sufficiently buffered to accommodate acid rain with little or no increase in soil acidity on an annual basis. But continued inputs of acid rain at pHs of 4.0-4.5 would have significant effects on the pH of soils, especially those that are weakly buffered.

This is also serious for soils that are al­ready quite acid, since increased acidity could well make them even less fertile.

Essay # 7. Fertiliser Contamination: (200 Words)

Fertiliser applications that supply nutrients in quan­tities far in excess of those taken up by plants can result in contamination of both surface and drain­age waters. Nitrates and phosphates are the chemi­cals most often involved. Nitrate contamination can occur in both surface runoff and drainage waters, while excessive levels of phosphates gen­erally occur only in surface runoff.

The loss of nitrogen and phosphorus from the soil has adverse effects on soil fertility, but the effect on water quality is even more serious. Ni­trate levels in drinking water above about 10 mg per liter are considered a human health hazard. In some heavily fertilised areas, the drainage waters are sufficiently high in nitrates to be a problem. Some rural wells have been found to contain ni­trates significantly above this safe limit.

A second problem stemming from high nu­trient-bearing waters coming from soils is the “over fertilisation” of lakes. Nitrogen and phosphorus in lake waters stimulate the growth of algae and other water-loving plants in the lakes.

Algal growth depletes the water of oxygen, which is essential for fish. Other aquatic plants (weeds) are stimu­lated and produce heavy mats near the shoreline interfering with recreational uses of the lakes.

Essay # 8. Greenhouse Effects and Soil Fertility: (400 Words)

Widespread concern has been expressed that the Earth is warming up. Furthermore, it is predicted that this warming trend will continue and even accelerate in the future. The cause of this warm­ing is thought to be the so-called greenhouse ef­fect.

When certain gases are emitted from the earth, they move into the upper atmosphere, capture, and return to the earth radiant heat that would ordi­narily escape into space. In so doing the gases serve the same purpose as the glass in a greenhouse.

With the advent of modern industrialisation the content of these greenhouse gases in the at­mosphere has increased markedly. For example, the carbon dioxide content of the upper atmosphere is thought to have been about 280 parts per mil­lion (ppm) in pre-industrial times.

It has increased to about 350 ppm in the past 30 years. Significant increases have also occurred in nitrogen and sulphur-bearing compounds and in organic synthetics such as the chlorofluorocarbons (CFCs) used as aerosol propellants, refrigerants, and solvents.

The soil is a source of several of the gases involved in the greenhouse phenomenon includ­ing carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ). The breakdown of soil or­ganic matter as land is cleared and put under culti­vation has been a major source of released CO 2 . This loss is small however, in comparison with the much larger releases from the combustion of fos­sil fuels (petroleum and coal) and the burning of tropical forests.

Nitrous oxide (N 2 O) content of the atmo­sphere has increased about 25% in this century. About two thirds of this increase is thought to be due to the combustion of coal and oil, the remain­der to agricultural practices. The N 2 O is released during the process of de-nitrification, which is fuelled by the presence of nitrates in the soil and in the organic matter cover.

Although heavy nitro­gen fertilisation may be a source of part of the nitrates that undergo de-nitrification, much nitrate reduction occurs in un-fertilised areas. For example, tropical forests have been found to be a signifi­cant year-round source of N 2 O.

The methane (CH 4 ) content of the upper at­mosphere has essentially doubled in the past hun­dred years, and there is no consensus as to the reasons for this increase. However, the soil is a known source of methane. It is released under anaerobic conditions such as are common in swamps or in a rice paddy.

Also, termites can and do produce methane, and some investigators have suggested that 20-40% of the methane reaching the atmosphere may come from this source.

Essay # 9. Weathering of Soil : (300 Words)

Weathering is the sum of the processes involved in the alteration of materials and not the surface through complex interactions between the lithosphere, the atmosphere, the hydrosphere and the biosphere that occur over time.

It may extend far below the surface and includes all the physical and chemical processes responsible for soil fragmen­tation and the production of dissolving ions.

Weathering in the upper part of the parent mate­rials has been considered separately as pedochemical weathering because of the consid­erable influence of the biomass in the production of, inter alia, complexing agents, substances that attack clay minerals or form organo-mineral com­plexes.

However, biological weathering influences may extend well into the underlying regolith al­though increasingly attenuated with depth.

While physical weathering is often considered separately from chemical weathering, in reality they operate together, often in a synergistic way. Weath­ering involves the simultaneous activities of a range of processes including physical fragmentation, inorganic chemical processes (hydrolysis, oxida­tion, hydration and dissolution) and biologically- mediated processes (e.g., acidolysis and acidocomplexolysis).

The weathering processes predominating at a site are determined by climatic, biological and lithological factors and the degree of evolution of the soil.

In all weathering systems, water plays domi­nant physical and chemical role. In the solid phase it is a major agent of landscape sculpting and trans­port while as a liquid, it is also an important agent for the diffusion and transport of materials.

It is a potent medium of physical disruption through volume change, both as a consequence of phase change and through involvement in hydration and related reactions. Chemically, it is an effective sol­vent, a component of many reactions and of neo-formed products, and an important buffering agent. Little chemical weathering occurs in very dry and frozen environments.

Thus, studies of soil throughout the world have shown that the kinds of soil that develop are largely determined by major factors:

1. Climate (particularly temperature and precipi­tation)

2. Living organism (especially native vegetation, microbes, soil animals and human beings)

3. Nature of parent material

4. Topography of the site

5. Time that parent materials are subjected to soil formation

Essay # 10. The Soil Profile : (400 Words)

The layering or horizon development described in the previous section gradually gives rise to natu­ral bodies called soils. Each soil is characterised by a given sequence of these horizons. A vertical ex­posure of the horizon sequence is termed a soil profile. Attention now will be given to the major horizons making up soil profiles and the termi­nology used to describe them.

For convenience in study and description, five master soil horizons are recognised. These are des­ignated using the capital letters O, A, E, B, and C. Subordinate layers or distinctions within these master horizon are designated by lowercase let­ters.

O Horizon (Organic):

The O group is com­prised of organic horizons that form above the mineral soil. They result from litter derived from dead plants and animals. O horizons usually occur in forested areas and are generally absent in grass­land regions.

The A horizons are the topmost minerals horizons. They contain a strong mixture of partially decomposed (humified) organic mat­ter, which tends to impart a darker color than that of the lower horizons.

E horizons are those of maxi­mum leaching or eluviation’s of clay, iron and aluminium oxides which leaves a concentration of resistant mineral, such as quartz, in sand and silt sizes. E horizon is generally lighter in color and is found under the A horizon.

The subsurface B horizon include layers in which evolution of materials has taken place from above and even from below. In humid regions the B horizons are the layers of maximum accumulation of materials such as iron and aluminium oxides and silicate clays. In arid and semi- arid regions, Calcium carbonate, calcium sulfate and other salts may accumulate in the B horizons.

The C horizon is the unconsoli­dated material underlying the solum (A and B) it may be or may not be the same as the parent ma­terials from which the solum form. The C hori­zon is outside the zone of major biological activi­ties and is generally little affected by the processes that formed in horizons above it. Its upper-layers may be with time become apart of the solum as weathering and erosion continues.

Underlying consolidated rock, with little evidence of weathering.

Transition Horizons:

These horizons are transitional between the master horizons (O, A, E, B and C). They may be dominated by proper­ties of one horizon but have prominent charac­teristics of another. Both capital letters are used.

Essay # 11. Textural Classes of Soil : (100 Words)

To convey an idea of the textural make-up of soils and to give an indication of their physical proper­ties, soil textural class names are used. Three broad groups of these classes are recognised-sands, loams and clays (Table 10.3, Fig. 10.2).

The silt group includes soils with at least 80% silt and 12% or less clay. Naturally the properties of this group are dominated by those of silt. The sand group includes all soils in which the sand separates make up at least 70% and the clay separate 15% or less of the material by weight. The properties of such soils are therefore charac­teristically those of sand in contrast to the stickier nature of clays included.

To be designated a clay, a soil must contain at least 35% of the clay separate and, in most cases, not less than 40%. In such soils the characteristics of the clay are separated.

Essay # 12. Cation Exchange Capacity of Soil : (100 Words)

The cation exchange capacity (CEC) of a given soil is determined by the relative amounts of dif­ferent colloids in that soil and by the CEC of each of these colloids. Thus, sandy soils have lower CECs than clay soils because the coarse-textured soils are commonly lower in both clay and humus content.

Likewise, a clay soil dominated by 1: I- type silicate clays and Fe, Al oxides will have a much lower CEC than will one with similar humus content dominated by smectite clays.

ADVERTISEMENTS: (adsbygoogle = window.adsbygoogle || []).push({}); Essay # 13. Soil Reaction: Acidity and Alkalinity : (100 Words)

Perhaps the most outstanding characteristic of the soil solution is its reaction—that is, whether it is acidic, alkaline, or neutral. Microorganisms and higher plants respond markedly to soil reaction because it tends to control so much of their chemi­cal environment.

Soil acidity is common in all regions where precipitation is high enough to leach appreciable quantities of exchangeable base-forming cations (Ca 2+ , Mg 2+ , K + , and Na + ) from the surface lay­ers of soils. The condition is so widespread and its influence on plants is so pronounced that acid­ity has become one of the most discussed properties of soils.

Essay # 14. Economy of Essential Elements of Soils: (200 Words)

i. Nitrogen and Sulphur Economy of Soils:

Of the various essential elements, nitrogen prob­ably has been subjected to the most study, and for many good reasons still receives much attention. The amount of this element in available forms in the soil is small, while the quantity withdrawn an­nually by crops is comparatively large.

When there is too much nitrogen in readily soluble forms, it is lost in drainage and may become a water pollut­ant. Nitrogen can be added to the soil by some microbes that “fix” it from the atmosphere, and can then be released back to the atmosphere by still other organisms. Nitrogen can acidify the soil as it is oxidised. Most soil nitrogen is unavailable to higher plants. All in all, nitrogen is an impor­tant nutrient element that must be conserved and carefully managed.

ii. Phosphorous and Potassium Economy of Soils :

Next to nitrogen, phosphorus and potassium are most critical essential elements in influencing plant growth and production throughout the world. Un­like nitrogen, these elements are not supplied through biochemical fixation but must come from other sources to meet plant requirements.

The sources include:

(a) Commercial fertiliser;

(b) Animal manures;

(c) Plant residues, including green manures;

(d) Human, industrial, and domestic wastes; and

(e) Native compounds of potassium and phospho­rus, both organic and inorganic, already present in the soil.

Related Articles:

• Essay on Soil: Meaning, Composition and Layers
• Fundamental Soil-Forming Processes

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Soil horizon

Soil horizon A distinct layer of soil , more or less parallel with the soil surface, having similar properties such as colour, texture and permeability , the soil profile is subdivided into the following major horizons: A-horizon, characterised by an accumulation of organic material. B-horizon, characterised by relative accumulation of clay iron, organic matter or aluminum. C-horizon, the undisturbed and unaltered parent material. (Some soils have an E-horizon, characterised by leaching of organic and other material.)

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2.2: Soil Classification and Mapping

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• Page ID 14718

• Colby Moorberg & David Crouse
• Kansas State University via Prairie Press

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Learning Objectives

• Become familiar with the 12 soil orders.
• Understand the structure of Soil Taxonomy .
• Become familiar with the contents of a county soil survey report.
• Use a soil survey report for land use evaluation.

The word, “taxonomy” is based on the Greek words “taxis”, meaning arrangement; and “nomia”, meaning method. In biology, taxonomy refers to a hierarchical system in which organisms are grouped based on shared characteristics, with domains and kingdoms at the top of the hierarchy, and genus and species at the lowest levels. Similarly, Soil Taxonomy is a hierarchical system used to group soils based on observable or measurable characteristics. A common application of soil classification (the act of identifying the taxonomic classification for a given soil) is to develop models of how soils of different classifications associate with one another within a landscape, which can eventually be used in soil mapping. The primary concepts of soil classification using Soil Taxonomy will be reviewed in this lab, followed by an overview of the Web Soil Survey (United States Department of Agriculture Natural Resources Conservation Service, 2016).

• Four soil monoliths (Crete, Clark, Morrill, and Chase soil series)
• Printed county soil survey reports
• Computer with internet access and a projector

For this lab, you will need to bring a laptop or tablet to use. If you do not have a laptop or tablet, please share with a partner.

Recommended Reading

• Illustrated Guide to Soil Taxonomy (Soil Survey Staff, 2015)
• From the Surface Down: An Introduction to Soil Surveys for Agronomic Use (USDA NRCS, 2010)

Prelab Assignment

Using the assigned readings and the introduction to this lab, consider the questions listed below. These definitions/questions will provide a concise summary of the major concepts addressed in the lab. They are also relevant to the soil survey report and are useful study notes for exams.

• Explain the difference between a profile, a pedon, and a polypedon.
• List the soil moisture regimes from dries to wettest, and note the criteria for each.
• Explain the difference between a soil phase and a soil consociation.
• What are the 12 soil orders? Describe the key properties or diagnostic features of each.
• What are the six formal categories of S oil T axonomy ?

Introduction

Soils can vary widely in their properties, and each has a unique arrangement of layers or horizons. The soil profile description provides the information that distinguishes one soil from another. Review the following example of a profile description, and note the explanation of terms in Table 4.1.

Harney silt loam [adapted from the National Cooperative Soil Survey (1997)]

• Ap – 0 to 9 in; dark grayish brown (10YR 4/2) silt loam, very dark grayish brown (10YR 3/2) moist; moderate medium granular structure; slightly hard, very friable; many fine roots; slightly acid; clear smooth boundary. (4 to 14 in thick)
• AB – 9 to 12 in; dark grayish brown (10YR 4/2) silt loam, very dark grayish brown (10YR 3/2) moist; moderate fine subangular blocky structure; hard, friable; many fine roots; neutral; clear smooth boundary. (0 to 10 in thick)
• Bt1 – 12 to 18 in; grayish brown (10YR 5/2) silty clay loam, dark grayish brown (10YR 4/2) moist; moderate medium subangular blocky structure; very hard, very firm; few fine roots; moderately alkaline; clear smooth boundary.
• Bt2 – 18 to 28 in; grayish brown (10YR 5/2) silty clay loam, dark grayish brown (10YR 4/2) moist; strong medium subangular blocky structure; very hard, very firm; few fine roots; moderately alkaline; gradual smooth boundary. (Combined thickness of the Bt horizon is 10 to 26 in)
• BCk2 – 8 to 35 in; brown (10YR 5/3) silty clay loam, brown (10YR 4/3) moist; moderate medium subangular blocky structure; hard, firm; few fine roots; many soft accumulations of carbonates; strong effervescence; moderately alkaline; gradual smooth boundary. (0 to 16 in thick)
• Ck3 – 5 to 47 in; pale brown (10YR 6/3) silt loam, brown (10YR 5/3) moist; massive; slightly hard, friable; common soft accumulations of carbonates; strong effervescence; moderately alkaline; gradual smooth boundary. (0 to 20 in thick)
• C – 47 to 60 in; pale brown (10YR 6/3) silt loam, brown (10YR 5/3) moist; massive; slightly hard, friable; strong effervescence; moderately alkaline.

Table adapted from King et al. (2003).

Completing a soil profile description involves a systematic approach:

• Observing the landscape setting.
• Examining the morphological features like texture, structure, color, consistence, etc. of the soil to distinguish any layers or horizons
• Describing in detail the texture, structure, color, consistence, and other important features of each horizon.
• Assigning horizon designations to each layer.
• Classifying the soil on the basis of its morphology and horizonation.

Soil Morphology and Land Use

Criteria that rate soils for a particular use are important to land use planning and land management decisions. Guidelines based on these criteria facilitate uniform and consistent land evaluations. Soil-based criteria can be developed for nearly any land use. To prepare a soil rating scheme, the following are required:

• Precise definition of the land use
• A list of soil properties affecting the use
• Limits for each soil property that would be favorable or unfavorable for the land use.

A comprehensive classification system is important for any science: soil science, plant science, biology, geology, among many others. Effective taxonomy allows us to organize knowledge and learn new relationships. Soil T axonomy helps in extrapolating soil management research among similar soils around the world. Soil T axonomy is a quantitative system based on soil properties that can be observed or measured, organized in a hierarchy based on six categories beginning with 12 broad soil orders and narrowing in specificity to more than 23,000 series. The following diagram illustrates the organization of a taxonomic name by category.

Table 4.2 Simplified key to the 12 soil orders

The formative elements are used in the names of suborders and lower taxonomic levels. (Table courtesy of R. Weil)

Many other formative elements can specify unique soil properties at each taxonomic level. Each formative element has a connotation for a given soil. These connotations of the formative elements used for suborders and great groups are listed in Table 4.3 and Table 4.4.

Table 4.3. Formative elements used to identify various suborders in Soil Taxonomy.

Table from King et al. (2003)

Table 4.4 Formative elements for names of great groups and their connotations

Table adapted from King et al. (2003)

A complete taxonomic name communicates a great deal of information about the soil if we understand each part of the name. As an example of the quantitative information revealed in a taxonomic name, the following classification name will be dissected by category. Consider, for example, the Harney soil, with a taxonomic classification of fine, smectitic, mesic Typic Argiustoll.

Table 4.5.Translation of the taxonomic classification of the Harney Series.

Table courtesy of C. J. Moorberg, adapted from King et al. (2003)

Activity 1: Practice Key to Soil Orders

Activity 2: Structure of Soil Taxonomy

Taxonomic Name: Fine-silty, mixed, superactive, calcareous, mesic Aridic Ustorthents

Taxonomic Name: Fine, smectitic, mesic Typic Haplusterts

Taxonomic Name: Fine, smectitic, mesic Aquertic Argiudolls

Activity 3: Interpreting Taxonomy

As a further exercise in understanding taxonomic names, complete the following questions. Use the list of taxonomic names of soils representative of Mollisols from the prairie pothole region of Iowa below to answer these questions.

The Des Moines lobe of the Wisconsin glaciation covered north-central Iowa with a deep layer of glacial deposits, and provides a good example of how taxonomic names depict important soil properties. The Clarion-Nicollet-Webster-Glenco topo-sequence, or “catena” (Figure 4.4), illustrates how Soil Taxonomy reflects wetness, or depth to a water table.

• Clarion series: Fine-loamy, mixed, superactive, mesic Typic Hapludolls
• Nicollet series: Fine-loamy, mixed, superactive, mesic Aquic Hapludolls
• Webster series: Fine-loamy, mixed, superactive, mesic Typic Endoaquolls
• Glencoe series: Fine-loamy, mixed, superactive, mesic Cumulic Endoaquolls

Table courtesy of C. J. Moorberg

Notice that the wetter the drainage class (that is, the shallower the depth to the seasonal high water table), the higher the “aqu” formative element becomes in the overall classification. That is because Soil Taxonomy prioritizes soil management considerations. The depth to the seasonal high water table would be a management concern for most land uses for the Nicollet, Webster, and Glencoe series; it would be of less concern for the Clarion series, and thus “aqu” is not included in the classification.

Also note that for the Glencoe series, in addition to having the “aqu” formative element as part of the suborder, the “cumulic” formative element has been designated in the subgroup. That formative element alludes to a “thickened epipedon” caused by the accumulation of organic matter. Because the water table is so shallow, little oxygen is available at the surface for a significant portion of the growing season. This slows decomposition, allowing organic matter to build, thus creating a thickened epipedon with lots of organic matter.

Activity 4: Practicing Soil Taxonomy Interpretations with State Soils of the US

State soils have been selected for all 50 states and three territories in the U.S. The group of soils represents a diverse sample of soil conditions and classifications. It serves as an interesting focus for a little practice at deciphering and understanding Soil Taxonomy . Use the attached list of state soils in Table 4.7 along with Table 4.2, Table 4.3, and Table 4.4 to answer the following questions:

Table 4.7. Soil Taxonomy classifications of state soils of the U.S.

Table courtesy of J. Kleiss and D. Lindbo

Activity 5: Soil Survey Reports

As an introduction to soil reports, look through a typical printed county soil survey report; take note of the manual’s organization and the extensive content. The report begins with some background information on the county, along with an overview of how the survey was conducted. The county soil conditions are described, and the soil mapping units are discussed in detail. A colored map displays these general soil units.

Following the brief overview are detailed soil map unit descriptions. These show the symbol that is on the soil map, the dominant soil series, topsoil texture and range of slope found in the unit. The descriptions of each map unit details the landscape setting, general properties, and major use and management considerations. If other soils are present in the map unit, this is an important part of map unit description. The next section of the soil report offers specific use and management suggestions and discusses how specific types of land use ratings were formulated. This is followed with an overview of what specific kinds of soil data are included.

Specific information on soil classification and detailed profile descriptions for each soil are followed by a glossary of terms used in the report. A sequence of tables provides detailed ratings on a wide range of land uses. This interpretative information is offered for each soil map unit. The last section of the report shows the soil maps on an aerial photograph base.

To become familiar with county soil survey reports select one provided and review the table of contents and the summary list of tables. Leaf through the report and note the following sections:

• Map Unit descriptions
• Use and management of soils
• Classification and profile descriptions
• Interpretive tables
• General soil map
• Soil legend

The United States Department of Agriculture Natural Resource Conservation Service (USDA NRCS) today provides these soil surveys in a digital format through the Web Soil Survey (United States Department of Agriculture Natural Resources Conservation Service, 2016). The Web Soil Survey provides all the information previously contained in the county soil survey reports. It also contains additional tools and information that has not been available in printed versions of the soil surveys. Another advantage of the Web Soil Survey is that the information contained in it can be updated as needed, instead of being updated following new surveys of the same county, which take 30 to 60 years! Your instructor will walk you through some of the main features of the Web Soil Survey and show you how to request a PDF copy of a soil survey report for a designated area. You will use these skills for your Soil Survey Report assignment.

Assignment: Soil Survey Report

For this lab, you will be preparing a soil survey report. The report assignment will be provided to you at the beginning of the lab. Your instructor will go over what to include in the report and where to collect the necessary information from the Web Soil Survey.

Subsequent Lab Set-Up

Some activities require preparation beyond the lab period and must be set up ahead of time. The soil texture by hydrometer activity in the Soil Texture and Structure lab involves dispersing soil particles chemically, which requires time for the reactions to take place. We will do this now, so the samples are ready next week.

For each of the three soils provided, do the following:

Weigh out 30.0 g of dry soil (assume oven-dry) into a 250-ml Erlenmeyer flask.

Wash sides of flask with distilled water from a wash bottle.

Add 100 ml of distilled water using a graduated cylinder. Then add 10 ml of sodium hexametaphosphate solution (500 g/L) from the dispenser on the sodium hexametaphosphate bottle.

Swirl to mix.

Cover the flask with Parafilm and label the flask with your lab section, table number, and soil type. Store the flasks in the location specified by your instructor for the next laboratory period.

How deep is the soil studied – an analysis of four soil science journals

• Review Article
• Published: 26 May 2020
• Volume 452 , pages 5–18, ( 2020 )

Cite this article

• Jenifer L. Yost 1 , 2 &
• Alfred E. Hartemink   ORCID: orcid.org/0000-0002-5797-6798 2  

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Background and aims

Soil depth is a critical attribute of any soil, and determines rooting, moisture and nutrient storage, mineral reserve, anchorage, and a range of conditions that affect plant growth. We reviewed papers from four primary soil science journals and extracted how deep the soils were studied in those papers.

Soil depth was obtained over a 30-years period (1989–2019) from papers in: European Journal of Soil Science , Geoderma, Plant and Soil , and Soil Biology and Biochemistry . In total, 1146 papers were reviewed, and 37% (420 papers) included information on how deep the soil was studied.

The number of papers that included soil depth increased from 31% in 1989 to 47% in 2019. The average soil depth studied was 27 cm, but it was 53 cm between 1989 and 1999, and 24 cm between 2004 and 2019. Most of the studies were from Europe, and 41% of the papers contained soil classification. Research that focused on soil mineralogy and technology tended to study soils to a greater depth (average 74 cm), whereas the depth in soil biology research was on average 18 cm. Over 80% of the soils were sampled by fixed depth and not by soil horizon.

Conclusions

Soil depth is lacking from about half of the papers in these four journals. The depth of the soil studied has halved in the past 30 years. Soil processes, soil properties, and microbial communities are depth-dependent, and for a more complete understanding, soils should be studied to a greater depth.

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Yost, J.L., Hartemink, A.E. How deep is the soil studied – an analysis of four soil science journals. Plant Soil 452 , 5–18 (2020). https://doi.org/10.1007/s11104-020-04550-z

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Short Essay on Soil Profile (844 Words)

Here is your essay on Soil Profile!

Soil profile is the term used for the vertical section of mature soil generally upto the depth of 2 meters or upto the parent material to show different layers or horizons of soil for the study of soil in its undisturbed state. It is made up of a succession of horizontal layers or horizons, each of which varies in thickness, colour, tex­ture, structure, consistency, porosity, acidity and composition.

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In general, soils have following four horizons: an organic or O-horizon and three mineral (A, B, C) horizons. Some workers recognized a D-horizon, in which rocks are in active weathering state, in between C and R-horizons. R-horizon is the consolidated bed rock on which a soil profile rests.

A and B-horizons form the true soil or solum. Each horizon of soil profile is further sub­divided. Horizon subdivisions are indicated by a series of letters with Arabic numbers as subscripts, e.g., O 1 , 0 2 , A 1 , A 2 , etc., (Fig. 9 3). Different layers of soil profile have following charac­teristics:

O Horizon :

The uppermost horizon of soil profile is called O horizon or litter zone. It is present in soils of forests but absent in the soils of deserts, grasslands and cultivated fields. It includes following two sublayers:

O 1 horizon (Aoo or L horizon):

It is the top layer of soil consisting of freshly fallen litter (i.e., dead leaves, twigs, bark, flowers, fruits and animal excreta and remains). In O 1 horizon, original form of plant and animal residues can be recognized with the naked eye and in it, the decomposition has not yet started. The A, level may be seasonal in nature: it is thickest in a deciduous forest immediately after leaf fall, when the forest floor is covered with fresh leaves and is virtually gone by the end of the following summer, when the leaves have largely decomposed.

Fig. 9-3. A generalized profile of soil. O 1 : Loose leaves and organic debris; O 2 : organic debris partly decomposed or matted; A 2 : A dark coloured horizon with a high content of organic matter mixed with mineral matter; A 2 : A light coloured horizon of maximum leaching; A 3 : Transitional to B but more like A than B; B 1 : Transitional to B but more like A than B; B 2 : A deeper coloured horizon of maximum accumulation of clay minerals or of iron and organic matter; B 3 : Transitional to C; C: Weathered material (regolith); R: Consolidated bedrock (Smith, 1974).

O 2 horizon (Ao or H horizon):

O 2 horizon underlies the O 1 or litter horizon and contains blackened unrecognizable decomposed litter. The upper portion of O 2 horizon contains partially decomposed detritus, the duff, so is called duff layer. Its lower part contains completely decomposed, light and amorphous organic matter, the humus and is called humus or H layer. Insects and other small animals are abundant in this layer.

Underlying the litter zone is the A horizon or topsoil. It is the zone of eluviation (leaching) or the horizon in which materials are brought into aqueous suspension or solution and move down­ward through the soil. The amount of material that is actually leached out of this zone is a function of the amount of percolating gravitational water. The topsoil or A horizon includes following three subzones:

A 1 horizon:

The A 1 horizon is the zone of humus incorporation with minerals of soil. It is almost always dark coloured and relatively rich in organic materials thoroughly mixed with the mineral soil. Micro-organisms like bacteria and fungi are present in huge numbers in A 1 layer.

A 2 horizon:

The A 2 horizon underlies A 1 horizon and is the zone of maximum leaching (eluviation). It contains less humus and is a light-coloured horizon from which materials like silicates, clays, oxides of iron (Fe) and aluminium (A1), etc., are being removed at the greatest rate.

A 3 horizon:

It is transitional to the subjacent B horizon.

B Horizon :

B horizon or subsoil underlies A horizon and is the zone of illuviation (collection of materials) in which much of the material leached out of the zone of eluviation (i.e., A horizon) is precipitated and enriched. It is coarse textured and deep coloured with alu­minium, iron and organic colloids and it is rich in clay. B horizon can also be divided into three zones of which the B 1 and B 3 are transitional to the A and the C horizons, respectively and B 2 is the zone of maximum precipitation of transported material. The roots of shrubs and trees usually reach upto this horizon.

C Horizon :

Underlying the B horizon is the weathered rock or sediment that serves as the parent material for the mineral fraction of the soil. It is called C horizon or regolith. It is a light-coloured and is virtually lacking in organic materials.

R Horizon :

C horizon is underlain by unweathered bedrock which is called R horizon.

The relative thickness and importance of the major horizons are highly variable. However, the concept of the soil profile is of great value because it provides a single genetic model by which all zonal soils can be compared.

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August 23, 2024

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Public debt ratios in Europe increased significantly in response to the pandemic and energy shocks and have remained higher than before the pandemic in most countries. Going forward, the projected public debt trajectories are broadly flat overall in advanced Europe but have a rising profile in emerging Europe. Government financing needs are still elevated, and the unwinding of quantitative easing by major central banks adds to financing pressures. Moreover, there are important medium- to long-term spending pressures from defense, climate transition, and aging, which are not fully reflected in the projected baseline trajectories. Against this backdrop, the risk that debts will not stabilize in the medium term has increased. Debt stabilization will hinge critically on achieving ambitious fiscal consolidation and sustained growth. Facing these elevated risks, policymakers need to implement carefully-calibrated fiscal adjustments that ensure debt sustainability while supporting growth. They could target debt stabilization over a longer, 10-year, horizon—while adhering to credible fiscal rules such as the reformed EU Economic Governance Framework—but with a high probability to reassure markets that debts will indeed be tamed.

Working Paper No. 2024/181

Fiscal consolidation Fiscal policy Fiscal stance Public debt

9798400285806/1018-5941

WPIEA2024181

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