essay on soil horizon

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AP®︎/College Environmental science

Course: ap®︎/college environmental science   >   unit 3, soil horizons and erosion.

• Soil texture triangle
• Earth's soil
• Soil is the loose surface material that covers most land. Soil is a mixture of minerals, organic matter, living organisms, gases, and water.
• Soil is produced from rocks, or parent material , as a result of weathering. Weathering describes the breakdown of rocks by physical, chemical, or biological processes. Particles that break away during weathering are transported and deposited as layers of soil on land, or layers of sediment underwater.
• Soils have layers called horizons . Soil horizons are distinguished by various properties, including color, texture, mineral content, and organic content.
• The vertical arrangement of horizons is known as a soil profile . Soil profiles can help distinguish soil types, and can also be used to predict soil fertility. The generalized soil profile below includes four major soil horizons: O, A, B, and C:
• The O horizon , or organic horizon, is made up mostly of organic matter such as leaf litter and decomposed plant material. This layer can be thin, thick, or not present at all, depending on how a soil forms.
• The A horizon , or topsoil, is the upper layer of soil in which plants have most of their roots. It has a high concentration of organic matter and microorganisms. So, this layer and the O horizon are often the most nutrient-rich and productive layers in a soil profile.
• The B horizon , or subsoil, is made up mostly of minerals from weathered parent material. It is usually lighter in color, ranging from yellow to reddish brown. The B horizon is less fertile than the A and O horizons, and is not capable of producing abundant plant growth.
• The C horizon is a layer of poorly weathered or unweathered rock. It contains a high concentration of parent material and is generally infertile.
• Soil erosion is the removal of the fertile top layers of soil. Soils can be eroded naturally by wind and flowing water. Erosion can be slowed by plants, whose roots help anchor the top layers of soil.
• Soils can also be eroded as a result of human activities. Deforestation, agriculture, and urbanization have greatly increased the rate of soil erosion in many places around the globe.
• Intact soils filter and clean the water that moves through them. In this way, soils provide humans with an ecosystem service—they help provide clean water for drinking and other purposes. So, protecting soils from erosion can benefit humans and society.

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Home » Soil » Living soils » The importance of deep soil horizons

The importance of deep soil horizons

Soils are not restricted to surface humus-bearing horizons. They also include so-called deep horizons that perform essential functions for the development of vegetation. Sometimes absent in very thin soils, deep horizons are the layers of soil between the upper layer more or less rich in organic matter and the underlying unweathered rock. In our climates, they start at a depth of 5 cm (some forms of forest humus) to 30-35 cm (deeply ploughed soils) and range from 10 to 180 cm (sometimes even more).

1. Reminder on soil horizons

2.1 deep rooting, 2.2 obstacles to root penetration, 2.3 mechanical anchoring of trees, 3. sufficient aeration to allow for biological activity and root development, 4.1. water reservoirs and water reserve in the soil, 4.2. water circulation by capillary rise, 4.3. water circulation by hydraulic lift, 4.4. stagnation and waterlogging, 5. the natural biogeochemical cycle (soil → plants → soil), 6. filtration and purification of rainwater and our liquid effluents, 7. trafficability (possibility of passage of farm equipment and large livestock), 8. anecic earthworm habitat, 9. carbon storage, 10. messages to remember.

It should be remembered that horizons are the superimposed layers of a soil, showing different aspects and properties from each other. This differentiation along a vertical axis is the result of natural phenomena that have followed one another over time over thousands or even hundreds of thousands of years (see  Soil formation in temperate climates & Six factors of pedogenesis ).

Due to their position (above 30 cm), deep horizons generally remain protected from disturbances related to agricultural tools and are less affected by the summer desiccation phases. Let’s see what roles they play in relation to all the functions we attribute to soils. Schematically, we can count nine of them:

• Agricultural production;
• Forest production;
• Support of vegetation for non-directly productive purposes;
• Balance of local and planetary terrestrial ecosystems (biodiversity, carbon storage);
• Provision of useful materials;
• Geotechnical function (support for various constructions);
• Major role in the water cycle (quality and quantity of surface and groundwater);
• Spreading of wastewaters and other wastes;
• Preservation of the memory of the past (archaeological remains).

Some of these roles are also played by surface horizons, but deep horizons, given their often much larger volume, play a quantitatively more important role.

2. Rooting possibilities of annual and perennial plants, cultivated or spontaneous

Annual plants such as spring wheat or corn have little time to establish deep roots, and they take root mainly in the ploughed horizon, where most of the nutrients (fertiliser) are found. However, they can easily reach a depth of 60 cm or even a meter if the soil is suitable. In certain circumstances, such as an exceptional drought, they develop their root system even deeper, in particular to find water (see Focus Water balance of a wheat crop).

On the other hand, it is well known that a perennial crop such as the vine can root up to two meters deep, or even much deeper. The same is true of some forest trees, which are also capable of exploiting cracks in hard rocks over several dozens of metres. But trees can also root laterally, making it possible to explore considerable volumes (see Focus Rooting 150-year-old oaks in thick soil). (See The root system of plants: from the shadows to the light & The tireless-quest for water by plants ).

• Video showing the roots of a 15-year-old oak tree in Kent

In the case of forest tree species, a very small number of fine roots present at depth can ensure the exploration of water reserves, which is essential during periods of drought, when the upper horizons are dry. Hénin et al. [1] also point out that a root (fine and healthy) can use water within a radius of 1 to 10 cm. It is therefore important never to neglect the description of deep rooting, even if the roots appear to be few in number.

Root penetration is favoured in deep horizons by the existence of a “macroscopic structure” composed of aggregates, which generates all types of connected voids (Figure 1, [2] ; see also Part 3). On the other hand, the existence of natural obstacles (of pedological origin) or those caused by human actions are unfavourable to any rooting. These obstacles constitute constraints that are all the more damaging for plants (and therefore for crops) the closer they appear to the surface.

• Physical obstacles : horizons that are naturally very dense ( fragipans Horizon that are not located at the surface, medium in texture and low in organic matter, very compact in dry state and with a high bulk density. ) or impermeable (unstructured or swollen clay layers), calcareous crusts or ferruginous accumulations.
• Chemical obstacles : calcium carbonate in the form of very fine particles (“active” lime) is an absolute obstacle to the rooting of strictly calcifuge plants. The same applies to so-called “free” aluminium (i.e. all forms that are neither well-crystallised oxides nor included in silicates) when the soil pH is below 5.5, which can cause a sharp reduction in root growth. The roots become thick and poorly branched and are no longer able to supply the plants with minerals and water. This is known as aluminium toxicity.
• Water barriers : horizons that are even temporarily waterlogged are a major constraint to rooting because of the hypoxic conditions that prevail (Figure 2). This explains the use of drainage techniques in agriculture (drainage using buried pipes, digging ditches). Only certain plant species, such as alders, are adapted to these hypoxic conditions.
• Lastly, there are obstacles created by agricultural practices , such as “plough pans”, thin, compacted levels that sometimes form at the base of the ploughed layer and which young roots will find difficult to penetrate.

In all horizons, even the deepest, the presence of oxygen is necessary for the respiration of all aerobic organisms and roots. Aeration, which depends on the macroscopic structure of each horizon, is necessary for microbial life, without which there would be no mineralization of organic matter, and above all no absorption of nutrients by the roots. In particular, mycorrhizae play a fundamental role in the hydromineral nutrition of most plants.

In deep horizons that are temporarily or almost permanently waterlogged ( i.e. poorly aerated), the little residual oxygen is quickly consumed by the microflora and the environment becomes hypoxic and then anoxic. Only anaerobic microorganisms can survive and thrive. The roots of most plants cannot grow there.

4. Water storage and circulation

In the soil, water can be stored and remain accessible to plants within a certain range of void dimensions. Through the largest voids, larger than 10 micrometres (spaces between aggregates, earthworm channels, root holes), rapid spontaneous draining by gravity takes place. In the very small pores and interstices (< 0.2 micrometres), the water is held back too tightly and is inaccessible to the roots. The deep horizons are the main place for storing water useful to plants, both because their volume is at least equal to, and more often than not much greater than, the volume of the surface horizon, and because they are protected from direct evaporation.

The water reservoir is a volume of porosity that can contain water on a long-term basis and is therefore a relatively permanent feature of a horizon or soil. But only part of this reservoir is accessible to plants – it is the “available reservoir”. The available water reserve, on the other hand, is the quantity of available  water actually in the reservoir at any given time. The analogy with your car is obvious: its petrol tank may hold up to 60 litres, but it may be half or completely empty.

In addition to meteoric origin, the water present in deep horizons can come from capillary rise. These originate from a deep water table. In the most favourable cases (horizons with a fine granulometry) they could reach one metre and, in a dry year, reach 100 mm, which would constitute a significant supplement. However, as the water rises through the finest pores, it would not cover the instantaneous needs of the plants and would not supply them sufficiently. This is not to be confused with the special case of chalky soils, which are often very thin and benefit from capillary rise in the mass of chalk that has remained in place. This explains the astonishing fertility of these soils, even though they are thin and highly calcareous.

The circulation of water in the deep horizons, in addition to the phenomena already described, could imply a hydraulic (or hydric) lift phenomenon. This notion, developed in agroforestry, but not proven with certainty for the moment, corresponds to a nocturnal redistribution of water in the soil, from bottom to top, through the roots of trees. Roots in the dry upper horizons would exude water from roots located in the still moist lower horizons [3] .

Mineral elements present in dissolved forms in the soil solution, essential for the development of plants (such as phosphorus, potassium, calcium, trace elements) or useless or even potentially toxic (such as lead, cadmium, mercury), are absorbed by plant roots from all the horizons actually explored by the roots. Once absorbed by plants, these elements are transferred to their various organs (stems, trunks, branches, leaves, fruits, seeds).

When the plant dies, these elements eventually return to the soil, either directly (in situ decomposition of dead roots) or by falling to the soil surface (debris from aerial parts, forest litter). It is here that fresh organic matter is decomposed more or less rapidly by biological activity (microarthropods including springtails, nematodes, earthworms, fungal filaments, algae, bacteria) until, in the most ‘active’ forms of humus, complete mineralisation is achieved, and chemical elements are released in forms that are fairly mobile and more easily bioavailable.

In agrosystems or production forests, the cycle is broken (Figure 4, arrow 9): [5] humans take some or all of the plants (wheat ears, lettuce, carrots, potatoes, fodder, tree trunks), whether cultivated or not, and take them elsewhere. A greater or lesser proportion of the biomass is therefore exported with the elements it contains. Very generally, food products (flour, vegetables, fruit), wood and their waste (wheat husks, peelings, human or animal excrement, sawdust, fire ashes) are not returned to the place where they were taken. This results in a gradual impoverishment of the environment as the harvests progress. In forest areas, this can lead to progressive soil acidification. The “poorest” ecosystems quickly become unbalanced and wither away. In agriculture (even organic) it is therefore essential to return to the soils all the elements (nitrogen, phosphorus, potassium, calcium, trace elements, organic matter) that have been removed from them, this is the reason for the addition of fertilizers and soil improvers (organic, calcareous). Under intensive farming, where exports are massive, this need for restitution is even more imperative.

Soil is a porous medium that acts as a mechanical filter and chemical and biological reactor for nitrates, phosphates, and certain pesticides in relation to the water that flows through it and the substances that are dissolved or suspended in it. In addition to purely mechanical filtering that blocks solid particles, surface reactions with clay minerals, organic matter, certain oxyhydroxides and biochemical reactions by the action of microorganisms take place.

This is why, in the natural environment, the chemical and biological quality of deep waters depends on the mineralogical properties and pH of the soils through which they pass. They are loaded with organic substances or calcium, magnesium, etc.

This “purifying power” has long been used by humans to purify their wastewater. But this purification capacity is not unlimited, and there is a risk that spreading wastewater can pollute the receiving soil. A famous example is that of the untreated water of the Paris conurbation, spread massively on the plain of Pierrelaye and other sewage spreading fields (Triel-sur-Seine, Gennevilliers) for market gardening [6] !

Considered at the end of the 19 th century as a hygienic solution to the disposal of water from Parisian sewers, this practice of massive irrigation proved to be heavily polluting. It is true that during the twentieth century the quality of wastewater had completely deteriorated as a result of the industrialization of the suburbs. In addition to organic matter of human origin, many trace metals have been added, including lead, cadmium, mercury, etc. in considerable doses, which led the authorities to ban agriculture.

It is therefore important to maintain satisfactory macroporosity in all horizons in order to maintain the soil’s natural ability to drain vertically. If necessary, excess water should be removed by appropriate techniques(drainage by pipes, ditches).

In forests, they are essential in the formation of mull-type humus as they ensure the rapid and intimate association of organic matter with mineral matter, in particular clay (clay-humic complex), in surface horizons. On the contrary, they are absent in acidic forest soils.

Peatlands are a special case of constant accumulation of plant organic matter with little or no decomposition in a waterlogged environment. As soon as the water level is lowered to allow the plot to be cultivated, organic matter mineralizes spontaneously and carbon destocking is observed and, eventually, the disappearance of peat. Soil scientists refer to these entirely organic soils as histosols . (See  Peatlands and marshes, remarkable wetlands ).

• Soils are not limited to humus-rich surface horizons. They also include so-called deep horizons that perform essential functions, in particular for the development of vegetation.
• The macroscopic structure of these horizons (existence, sizes and shapes of aggregates; dimensions, connectivity and arrangement of voids) remains fundamental for everything related to the circulation and storage of water, aeration and therefore the biological activity on which the development and life of plant roots depend.
• The primary interest of a deep, well-distributed and healthy root system lies not only in good anchoring and nutrition of the plant, but above all in the use of the soil’s water reserve, proportional to the volume prospected. This depends on the lateral distribution of the roots and not only on the depths reached.
• This is why many authors insist on the practical importance of the macroscopic structure of soils: “ The development and maintenance of a desirable and optimal soil structure for plant growth is an eternal requirement in agriculture “. [10]
• And this is true for the entire thickness of the soil, not just for the surface horizon reworked by agricultural tools.

Notes and references

Cover image. Source Photo Jacques Roque, reproduite avec l’autorisation de l’auteur.

[1] Hénin, S., Gras, R. et Monnier, G. (1969). Le profil cultural . Paris, Masson, 332 p., (in french).

[2] Rowell, D.L. (1994). Soil Science: methods and applications . Longman Scientific & Technical. 350 p., (in french).

[3] Dupraz, C. (2009). L’ascenseur hydraulique ou comment les arbres redistribueraient l’eau du sol. Agroforesteries , n° 02, pp. 13-18, (in french).

[4] Concaret, J. (coord.) (1981). Drainage agricole. Théorie et pratique . Chambre régionale d’agriculture de Bourgogne, Dijon. 509 p., (in french).

[5] Baize, D. (2007). Les Éléments Traces Métalliques (ETM) dans les sols . Cours à l’Université de Poitiers, (in french).

[6] Baize, D., Lamy, I., vanOort, F., Dère, C., Chaussod, R., Sappin-Didier, V., Bermond, A., Bourgeois, S., Schmitt, C., Schwartz, C. (2002). 100 years spreading of urban waste water on market-garden soils close to Paris (France) : subsequent impacts and hazards. 17 th World Congress of Soil Science, Bangkok. Symposium 29, paper n° 204.

[7] Baize, D. (2012). Les « terres d’Aubues » de Basse Bourgogne : nouvelle synthèse et bilan de matières à très long terme. Étude et Gestion des Sols , 19, 3-4, pp. 139-161, (in french).

[8] AFES. (2009). Référentiel pédologique 2008 . D. Baize et M.C. Girard (coord.). Quae éditions. Paris. 432 p., (in french).

[9] Derrien, D. et coll. (2016). Stocker du carbone dans les sols : quels mécanismes, quelles pratiques agricoles, quels indicateurs ? Étude et Gestion des Sols , Vol. 23, pp. 193 à 223, (in french).

[10] Hillel, D. (1988). L’eau et le sol. Principes et processus physiques . 2ᵉ édition. Pédasup 5, Academia, Louvain-la-Neuve, 294 p., (in french).

The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie ( www.a3e.fr ), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: Denis BAIZE (May 11, 2024), The importance of deep soil horizons, Encyclopedia of the Environment, Accessed May 14, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/soil/the-importance-of-deep-soil-horizons/ .

The articles in the Encyclopedia of the Environment are made available under the terms of the Creative Commons BY-NC-SA license, which authorizes reproduction subject to: citing the source, not making commercial use of them, sharing identical initial conditions, reproducing at each reuse or distribution the mention of this Creative Commons BY-NC-SA license.

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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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Soil horizon variation: A review

2019, Advances in Agronomy

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Agricultural and Food Science

Markku Yli-Halla

Eleven pedons in an agricultural landscape at elevations 80-130 m above sea level in Jokioinen, south-western Finland were investigated and classified according to Soil Taxonomy, the FAO-Unesco system (FAO), and the World Reference Base for Soil Resources system (WRB). The soils were related to geomorphology of the landscape which is characterized by clayey fields and forested bedrock high areas covered with glacial till. A Spodosol/Podzol was found in a coarse-sandy soil in an esker while the sandy loam in a bedrock high area soils did not have an E horizon. A man-made mollic epipedon was found in a cultivated soil which had a sandy plow layer while clayey plow layers were ochric epipedons. Cambic horizons, identified by structure and redox concentrations, were common in cultivated soils. In a heavy clay soil, small slickensides and wedge-shaped aggregates, i.e., vertic characteristics, were found. Histosols occurred in local topographic depressions irrespective of the absolute ele...

Oliver Chadwick

ABSTRACT Background and Aims Soil chronosequences on marine terraces along the Pacific Coast of California and Oregon show evidence of podzolization, though soils ultimately evolve to Ultisols. It is not clear if this pathway of soil evolution can be extended to the humid, inland Oregon Coast Range. Methods We analyzed soil properties for a fluvial terrace chronosequence sampled along the Siuslaw River (Oregon, USA) about 50 km from the Pacific coast. The seven terraces ranged in age from &amp;amp;lt;3.5 ky to nearly 1,000 ky. Results There was no evidence of early podsolization. Instead, evidence was found that andisolization starts early and occurs even in older soils when pedogenic iron accumulation and clay synthesis and illuviation dominate. Soils develop the morphology characteristic of Ultisols sometime between 20 and 70 ky, but high levels of oxalate extractable iron and aluminum satisfy criteria of an andic subgroup. Alfisols are not formed as an intermediary stage. Conclusions The lack of Spodosols inland is due to the inland shift from udic to ustic or xeric moisture regime, which favors summer drying and ripening of short-range order minerals rather than deep leaching or translocation. Other factors are higher pH, different organic chemistry and faster calcium cycling under the Douglas fir inland when compared to the Sitka spruce of the coastal terraces.

International Journal of Research Studies in Agricultural Sciences

Muluadam Birhan

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What Are Soils?

Soils are dynamic and diverse natural systems that lie at the interface between earth, air, water, and life. They are critical ecosystem service providers for the sustenance of humanity. The improved conservation and management of soils is among the great challenges and opportunities we face in the 21st century.

Soil is... a Recipe with Five Ingredients

Soil is a material composed of five ingredients — minerals, soil organic matter, living organisms, gas, and water. Soil minerals are divided into three size classes — clay , silt , and sand (Figure 1); the percentages of particles in these size classes is called soil texture . The mineralogy of soils is diverse. For example, a clay mineral called smectite can shrink and swell so much upon wetting and drying (Figure 2) that it can knock over buildings. The most common mineral in soils is quartz; it makes beautiful crystals but it is not very reactive. Soil organic matter is plant, animal, and microbial residues in various states of decomposition; it is a critical ingredient — in fact the percentage of soil organic matter in a soil is among the best indicators of agricultural soil quality (http://soils.usda.gov/sqi/) (Figure 3). Soil colors range from the common browns, yellows, reds, grays, whites, and blacks to rare soil colors such as greens and blues.

Soils are... Big

You may be surprised to hear " dirt " described as "big". However, in the late 1800's soil scientists began to recognize that soils are natural bodies with size, form, and history (Figure 4). Just like a water body has water, fish, plants, and other parts, a soil body is an integrated system containing soil, rocks, roots, animals, and other parts. And just like other bodies, soil systems provide integrated functions that are greater than the sum of their parts.

Soils are... Young to Very, Very Old

Soils are... diverse.

• Plinthite — which hardens irreversibly upon repeated wetting and drying (Figure 8a).
• Sulfidic — a horizon containing pyrite which, upon exposure to oxygen, can produce so much sulfuric acid that it kills plants and can cause fish kills (Figure 8b).
• Petrocalcic — in which so much calcium carbonate is accumulated that it literally forms a rock-like layer in the middle of a soil (Figure 8c).

Soils... Communicate

• O - Horizon containing a high percentage of soil organic matter.
• A - Horizon darkened by the accumulation of organic matter.
• E - Horizon formed through the removal ( eluviation ) of clays, organic matter, iron, or aluminum. Usually lightened in color due to these removals.
• B - Broad class used for subsurface horizons that have been transformed substantially by a soil formation process such as color and structure development; the deposition ( illuviation ) of materials such as clays, organic matter, iron, aluminum, carbonates, or gypsum; carbonate or gypsum loss; brittleness and high density; or intense weathering leading to the accumulation of weathering-resistant minerals.
• C - A horizon minimally affected or unaffected by the soil formation processes.
• R - Bedrock.

These master horizons may then be further annotated to give additional information about the horizon. Lower case letters can be placed as suffixes following the master horizon letter to give additional information about soil characteristics or soil formation processes. For example, the lower case "t" on the B horizon in Figure 9 indicates that the horizon is characterized by illuvial clay accumulation. Multiple letters can be used — Figure 8c depicts a Bkm horizon meaning that it is cemented (m) by illuvial carbonates (k). Numbers placed before the master horizon name (e.g., 2Bt) indicate a difference in parent material; numbers placed at the end of a horizon name are used to subdivide horizons that have the same designation but are different in some way (e.g., a red Bt1 over a yellow Bt2).

Soils are... Biological Bliss

Soils are... fertile.

Soils are the primary provider of nutrients and water for much of the plant life on earth. There are 18 elements considered essential for plant growth, most of which are made available to plants through root uptake from soils (Brady & Weil 2007). Soils retain nutrients by several mechanisms. Most nutrients are dissolved in soil water as either positively or negatively charged ions; soil particles are also charged and thereby are able to electrically hold these ions. Soils also hold nutrients by retaining the soil water itself.

Arguably the greatest of all the ecosystem services provided by soils is the retention of water — without soils our land would be little but rocky deserts. Plants use much more water than one might think because they are constantly releasing water into the atmosphere as a result of transpiration, which is a component of the process of photosynthesis. Clay and silt particles are the primary mineral components in soils that retain water — these small particles slow the drainage of water and, like a sponge, physically hold water through capillary forces. Clay provides such strong force that plants can't pull all the water away from it, which makes silt particles the ultimate ingredient for plant-available water storage — they hold large quantities of water but also release it to plant roots (Figure 3).

Soils are... Clay Factories

Soils are... service providers, soils are... degrading and polluted, soils are... home, soils are... a profession.

activity - A general term used to describe how chemically reactive a particle is with ions, water, and other particles.

clay - A mineral particle smaller than 0.002 mm.

clay synthesis - Clays are formed in soils through the transformation of existing clays or through the generation of entirely new clay particles from ions precipitating from solution.

desertification - The transformation of a non-arid landscape to an arid landscape, usually through a combination of climate changes and human-induced soil degradation.

dirt - 1. synonym for soil material; 2. soil out of place; 3. unclean material of any composition.

eluviation - The removal of materials such as clays, organic matter, iron, or aluminum from a horizon.

erosion - The surface removal of soil material from soils by the action of water or wind.

eutrophication - A process of excess algal growth that leads to oxygen depletion; often caused by excess nutrient inputs.

factors of soil formation - Factors from which soil scientists are able to predict the end result of soil formation processes: climate, organisms, topography, parent material, and time.

gas regulation - The absorption and release of gases that mediates the levels of these gases in the atmosphere.

illuviation - The deposition of materials such as clays, organic matter, iron, or aluminum into a horizon; generally the materials come from an upper horizon in the soil body.

leaching - The removal of dissolved ions from a soil.

natural bodies - Systems that form in nature with size, form, and history that act as in an integrated fashion to provide functions that differ from the sum of their parts.

remediate - To transform a chemical from a toxic form or state to a non-toxic form or state.

salinization - A build up of salts in soils to the point that they destroy the soil's physical and chemical properties and plants are not able to take up water due to the high salt concentration; often associated with improper irrigation.

sand - A mineral particle ranging in size from 0.02 to 2 mm.

silt - A mineral particle ranging in size from 0.002 to 0.02 mm.

soil - 1. A material composed of minerals, living organisms, soil organic matter, gas, and water. 2. A body composed of soil and other parts such as rocks, roots, and animals that has size, form, and history and provides integrated functions that are greater than the sum of its parts.

soil horizon - Layer present within soil bodies that are distinguishable from other layers; often generated through soil formation processes.

soil organic matter - Plant, animal, and microbial residues, in various states of decomposition.

soil texture - The percentages of sand, silt, and clay particles in a soil.

soil quality - The capacity of a soil to provide desirable ecosystem services.

transpiration - Evaporation of water from openings in plant tissues called stomata; associated with photosynthesis.

weathering - Physical, chemical, and biological processes that breakdown and transform rocks and minerals.

References and Recommended Reading

Ahrens, R. J. & Arnold, R. W. "Soil taxonomy," in Handbook of Soil Science , ed. M. Summer (CRC Press, 2000) E117-E135.

Brady, N. C. & Weil, R. R. T he Nature and Properties of Soils, 14th ed. Upper Saddle River, NJ: Prentice Hall, 2008.

Food and Agriculture Organization of the United Nations (FAO). Guidelines for Soil Description, 4th ed. FAO, Rome, 2006. ftp://ftp.fao.org/docrep/fao/009/a0541e/a0541e00.pdf

Haygarth P. M. & Ritz, K. The future of soils and land use in the UK: Soil systems for the provision of land-based ecosystem services. Land Use Policy 26S:S187-S197, 2009.

Jenny, H. The Factors of Soil Formation: A System of Quantitative Pedology . New York, NY: Dover Press, 1941.

Soil Survey Division Staff. Soil Survey Manual . Soil Conservation Service, United States Department of Agriculture, Handbook 18, 1993. http://soils.usda.gov/technical/manual/

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Soil Horizons

Soil profile.

There are different types of soil, each with its own set of characteristics. Dig down deep into any soil, and you’ll see that it is made of layers, or horizons (O, A, E, B, C, R). Put the horizons together, and they form a soil profile. Like a biography, each profile tells a story about the life of a soil. Most soils have three major horizons (A, B, C) and some have an organic horizon (O). The horizons are:

O  (humus or organic): Mostly organic matter such as decomposing leaves. The O horizon is thin in some soils, thick in others, and not present at all in others.

A  (topsoil): Mostly minerals from parent material with organic matter incorporated. A good material for plants and other organisms to live.

E  (eluviated): Leached of clay, minerals, and organic matter, leaving a concentration of sand and silt particles of quartz or other resistant materials – missing in some soils but often found in older soils and forest soils.

B  (subsoil): Rich in minerals that leached (moved down) from the A or E horizons and accumulated here.

C  (parent material): The deposit at Earth’s surface from which the soil developed.

R  (bedrock): A mass of rock such as granite, basalt, quartzite, limestone or sandstone that forms the parent material for some soils – if the bedrock is close enough to the surface to weather. This is not soil and is located under the C horizon.

Read about the 12 major soil orders in the United States!

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Essay on soil: meaning, origin and physical properties.

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Read this essay to learn about the Soil. After reading this essay you will learn about: 1. Meaning of Soil 2. Origin of Soil 3. Physical Properties.

Essay # Meaning of Soil:

“Soil is the fine earth covering land surface that has the important function of serving as a substratum of plant, animal and human life and acts as a reservoir of nutrients and water”. Soil is the material on the earth’s surface that results from the interactions of weather and biological activities with the underlying geologic formation. Soil is produced from broken down rocks, organic matter (decayed animals and plant life), water, and air”.

Dokuchalev, the father of Soil Science, gave the factors of Soil Formation.

Pedology is the study of origin, formation and geographic distribution of soils in nature, whereas Edaphology is the study of soils in relation to crop growth.

Essay # Origin of Soil:

(a) Rocks are the chief sources of soil materials over which soils are formed.

(b) Soil is formed from weathering of rocks.

(c) Disintegration breaks consolidate rocks into unconsolidated parent materials, which on further breaking and chemical decomposition form soil.

(d) Soil found at the site of formation is called sedentary soils, whereas soils found far away from the site of formation are called Cumulous or transported soils.

(e) Physical weathering involves agents such as temperature, water, wind, plant and animals and the process such as surface peeling off of rocks, alternate wetting and drying, freezing and thawing, burrowing of animals root penetration etc.

(f) Chemical weathering involves different reactions i.e. solution, hydration. Hydrolysis, Oxidation and reduction. Hydrolysis is the most important chemical weathering process.

Weathering minerals: Most resistant – Quartz

Moderate resistant – Feldspar

Least resistant – Calcite

(a) Igneous Rocks: Granite, Basalt and Syenite

(b) Sedimentary Rocks: Lime stone. Sand stone and Dolomite

(c) Metamorphic rocks: Gneiss, Marble, Quartzite and Slate

(d) Gneiss from Granite, Marble from Lime stone, Quartzite from Sand stone and Slate from Shale.

Chemical composition of earth’s crust:

Solum = A+B horizons

Regolith = A+B+C horizons

Essay # Physical Properties of Soil:

(a) Soil Structure :

(i) “Soil structure refers to the arrangement of primary particles of soil and their aggregates into certain defined patterns”. It is one of the important properties of soil, since it influences aeration, permeability and water capacity.

(ii) Soil structure refers to the shape of the soil clumps in any given soil. Soil structure is an important factor for water drainage through a soil and the suitability of a soil to hold structures etc.

(iii) Soil structure tells how the soil affects the movement of water, air and root penetration into the soil. The geometric shapes of the soil determine how it is put together.

(iv) Soil structure is denoted in order of Grade, Class and Type (GCT).

Types of Structure :

Four principal types of soil structure are recognized:

The pads (aggregates) are arranged in relatively thin horizontal layers, plates. This structure is found in recently deposited clay soils, surface layers of virgin soils but also in subsoil horizons i.e. ‘B’.

ii. Prismatic:

The vertically oriented aggregates or pillars. It occurs in ‘B’ horizon of clayey soils in arid and semi arid regions. When the tops are flat, these vertical aggregates are called Prismatic and when rounded, they are known as columnar. The size of prism like structure is up to 15 cm in diameter.

iii. Blocky:

Original aggregates have been reduced to block, irregularly, six- faced, cube like blocks of soil, 2-8 cm in size, common on heavy subsoil’s particularly of humid regions and in upper part of ‘B’ horizon.

iv. Spheroidal:

Original aggregates have been reduced to block, irregularly, six-faced, cube like blocks of soil, 2-8 cm in size, common on heavy subsoil’s particularly of humid regions and in upper part of ‘B’ horizon.

(b) Soil Texture :

(i) “Refers to the relative percentage of sand, silt and clay sized particles in the soil material”.

(ii) The varying proportions of particles of different size groups in a soil constitute is known as soil texture.

(iii) The principle textural classes are clay, clay loam, sandy clay, silt clay, sandy clay loam, silty clay loam, sandy loam, silt loam, sand, loamy sand and silt.

Types of Soil :

(i) Sandy soils are typically comprised of approximately 80 – 100 per cent sand, 0-10 per cent silt and 0 -10 per cent clay by volume. Sandy soils are light and typically very free draining, usually holding water very poorly due to very low organic content.

(ii) Loam soils are typically comprised of approximately 25 – 50 per cent sand, 30 – 50 per cent silt and 10 – 30 per cent clay by volume. Loam soils are somewhat heavier than sandy soils, but also tend to be fairly free draining, again, due to typically low organic content.

(iii) Clay soils are typically comprised of approximately 0 – 45 per cent sand, 0 – 45 per cent silt and 50 -100 per cent clay by volume. Clay soils are not typically free draining, and water tends to take a long time to infiltrate. When wet, such soils tend to allow virtually all water to run-off. Clay soils tend to be heavy and difficult to work when dry.

(c) Soil Color:

It is found out by using Mussel Color Chart. Three variable are used to denote soil color i.e. Hue – dominant wavelength. Value – relative lightness of the color and Chrome – purity of the color.

(d) Soil Plasticity and Cohesion :

(i) Plasticity is the capacity of the soil to change its shape under moist conditions.

(ii) Cohesion is the capacity to stick together.

(iii) Plastic soils are cohesive.

(e) Soil Colloids :

(i) Soil colloid is made up of inorganic colloid (clay) and organic colloid (humus).

(ii) Particles smaller than 1 micron are said to exhibit colloidal activity.

(iii) Soil colloids have high exchange capacity, which increases with silica sesuioxides ratio.

(f) Soil water :

(i) Water has maximum density at 4°C. One molecule of water is attached to four molecules in the neighborhood.

(ii) The surface tension of water is 72.7 dyne/cm 2 at 25°C.

(iii) Structure of water molecules is hexagonal lattice and the angle is 104.5°.

(iv) Types of Soil water: Hygroscopic water, capillary water & gravitational water.

(v) Hygroscopic water: Water held at tension of more than 31-atm and not available to the plants.

(vi) Gravitational water: water held below 1 /3 rd atm and drained from the soil due to gravity.

(vii) Wilting point: Water held at tensions beyond 15-atm and is not available to the plants.

(viii) Field capacity: if water is allowed to drain by gravity after supplying water, some water remains even after drainage due to gravity is called field capacity. Water at field capacity is held at 1 atm.

(ix) Available water: water held between and 15 atm.

(x) Water in soil moves in response to difference to tension or pressure.

(xi) Darcy’s law in soil deals to hydraulic gradient.

(g) Soil Air :

(i) Soil air contains 10 times Co 2 concentration (0.3%) as that of air.

(ii) Ideally 1/3 rd of soil pores are filled with water and 1 with air.

(iii) Flick’s law deals about the diffusion of gases in soils.

(iv) Soil air is characterized by ODR – Oxygen Diffusion Rate.

(h) Soil Temperature :

(i) In soil, heat is mainly transferred through conduction.

(ii) Fourier’s law deals with heat conduction in soils.

(iii) Sandy soils absorb more heat than clays soils.

(i) Soil Chemical Properties :

(i) pH is the negative logarithm of H + ion concentration.

(ii) Sorenson gave pH scale.

(iii) Soil pH is also called soil reaction.

(iv) There are two types of acidity in soil – active acidity and potential acidity.

(v) pH measures only active acidity.

(vi) Soil with pH less than 6.5 are acidic, 6.5 to 7.5 are neutral and above 7.5 are alkaline.

(vii) One unit change in pH changes H + ion concentration by 10 times, 2 units by 100 times and so on.

(j) Soil Electric Conductivity :

(i) Measures of soluble salts in m.mhos/cm or dS/m in conduct meter.

(ii) CEC is measured at pH 7 and expressed as meq/100g of soil. CEC varies greatly with nature and amount of clay and OM.

(iii) Kaolinite has 3-10, lilt – 10-30, Montmorillonite – 80-150 and Organic matter->200.

(k) Soil Organic matter :

(i) OM on decomposition by humidification process gives humus. Humus is amorphous in nature.

(ii) In hilly and high altitudes, OM is above 1%.

(iii) CN ratio of OM is 10:1, whereas an average of 14:1 of Indian soil.

(iv) Histosols are called Organic soils.

(l) Soil microbial properties :

(i) The smell of soil after fresh shower is due to Actinomycetes.

(ii) Bacteria occur in neutral soil, fungi in acidic soil and Algae in shade areas.

(m) Soil Microbial Properties :

(n) Minerogical Properties :

(i) There are primary, secondary, accessory and amorphous minerals.

(ii) Primary minerals:

Feldspar > Quartz > Mica > Limestone > Hornblende and augite > Olivine and serpentine.

(iii) Secondary minerals:

1:1 – One silica and one alumina layer: Kaolinite, Halloysite and Dickite.

2:1 – Two silica and one alumina layer: Monmorillionite, Vermiculite and Illite.

2:1:1 or 2:2 – the crystal unit is composed of one 2:1 unit: Chlorite.

Accessory minerals: Tourmaline, Topaz, Apatile, Rutile and Anatase.

Amorphous minerals: Allophane.

Related Articles:

• Importance and Quality of Soil Suitable for Orchard Cultivation
• Short Notes on Soil (Profile, Texture, Formation and Classification of Soil)

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• 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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What factors decide the moisture content in soil.

Many factors such as precipitation, soil characteristics, temperature and more affect soil moisture.

where is the substratum or the weathered rock in your soil profile?

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Soil Profile and Horizons – UPSC World Geography Notes

Table of Contents

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, known as soil horizons .

Soil Profile

A vertical section through various layers of the soil is termed the soil profile . The soil comprises three horizontal layers: true soil at the top, subsoil, and bedrock.

Each layer varies in feel (texture), color, depth, and chemical composition . These layers, termed horizons, are shaped by internal processes such as leaching or capillary movements/upward movements of materials and water.

The study of the soil profile involves examining a hexagonal column of soil taken as a sample .

A soil horizon is a layer typically parallel to the soil surface, exhibiting physical characteristics distinct from the layers above and below.

Horizons are primarily defined by noticeable physical features, especially color and texture .

The uppermost horizon, rich in humus and minerals, is generally dark in color, fostering soil fertility and providing nutrients to plants. This layer, known as the topsoil or the A-horizon, is soft, porous, and has a higher water retention capacity.

The next layer, the B-horizon or the middle layer, contains less humus but more minerals. It is generally harder and more compact.

The third layer, the C-horizon, is composed of small lumps of rocks with cracks.

Components of the Soil Profile

A soil horizon constitutes a distinct layer within the soil. Running approximately parallel to the soil surface, this horizon possesses unique properties and characteristics different from the adjacent layers above and below. The soil profile , a vertical section depicting all horizons, extends from the soil surface to the parent rock material.

Within the regolith , encompassing all weathered material in the profile, two components stand out: the solum and the saprolite . The solum includes the upper horizons, representing the most weathered portion of the profile. The saprolite , situated directly above the solid, consolidated bedrock but beneath the regolith, is the least weathered portion.

Soil Horizons

They evolve through interactions involving climate, living organisms, and the land surface, unfolding over time. Horizons typically form through either the selective removal or accumulation of specific ions, colloids, and chemical compounds. This process is often driven by water seeping through the soil profile from the surface to deeper layers, resulting in diverse soil textures and colors within the horizons.

There are two primary types of soil horizons : organic and mineral .

Organic horizons , denoted with the capital letter O, rest above mineral horizons and originate from plant and animal matter. The upper Oi horizon consists of decomposing organic matter easily recognizable by sight, such as leaves or twigs. The lower Oa horizon contains humus, which has broken down beyond recognition.

Mineral horizons : There are four main mineral horizons: A, E, B, C.

O Horizon: Layers characterized by organic material . Some O layers consist of undecomposed or partially decomposed litter (such as leaves, needles, twigs, moss, and lichens). They may be on top of either mineral or organic soils .

A Horizon or Surface soil: Part of the topsoil where organic matter is mixed with mineral matter. This layer has the most accumulation of organic matter and soil life. Nutrients like iron, aluminum, clay, and organic matter are sometimes dissolved and carried out in this layer. Depleted of (eluviated of) iron, clay, aluminum, organic compounds, and other soluble constituents. When depletion is pronounced, a lighter-colored “E” subsurface soil horizon is apparent at the base of the “A” horizon.

E Horizon: “E” stands for eluviated layer. Light-colored eluviated layer, eroded of nutrients. Significantly leached of clay, iron, and aluminum oxides, leaving a concentration of resistant minerals like quartz in the sand and silt sizes. Present only in older, well-developed soils, generally occurring between the A and B horizons.

B Horizon or Subsoil: Subsurface layer reflecting chemical or physical alteration of the parent material . Accumulates all leached minerals from the A and E horizons. Iron, clay, aluminum, and organic compounds accumulate in this horizon (illuviation, opposite of eluviation).

C Horizon or Parent rock: Partially weathered parent material accumulates in this layer, especially in sedimentary deposits. Least weathered horizon, also known as saprolite; unconsolidated, loose parent material. May accumulate more soluble compounds (inorganic material).

R Horizon or Bedrock: Denotes the layer of unweathered bedrock at the base of the soil profile . Unlike the above layers, R horizons largely comprise continuous masses of hard rock. Soils formed in situ will exhibit strong similarities to this bedrock layer. These areas of bedrock are under 50 feet of the other profiles.

Significance

The examination of the soil profile holds importance in agricultural sciences as it allows for the determination of land use patterns . Land capability classification relies on the study of the soil profile and horizon .

FAQs on Soil Profile and Horizons

Q: what is a soil profile.

A: The soil profile is the vertical section of soil that comprises three main layers – A-horizon, B-horizon, and C-horizon.

Q: How does the soil profile impact plant growth?

A: The composition of each soil horizon directly influences nutrient availability and structural support for plant growth.

Q: What defines a soil horizon?

A: A soil horizon is a distinct layer in the soil profile with unique physical and chemical properties.

Q: How are soil horizons named?

A: Soil horizons are named using letters (A, B, C) based on their position in the profile, indicating specific characteristics.

Q: What factors influence the thickness of the soil profile?

A: Climate, parent material, vegetation, topography, and time collectively shape the thickness of the soil profile.

Q: What causes the shimmering effect above farmland during hot days?

A: The heat haze or mirage is a result of rapid temperature variations between the warm ground and the cooler air, causing light to refract.

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Home » World Geography » Physical Geography of the World » Biogeography » Soils | Biogeography » Soil Profile

EU Mission: A Soil Deal for Europe

What this EU mission is, how EU Missions will be implemented, mission boards, meetings, news, events.

What this EU Mission deals with

The main goal of the Mission 'A Soil Deal for Europe' is to establish 100 living labs and lighthouses to lead the transition towards healthy soils by 2030.

Life on Earth depends on healthy soils. Soil is the foundation of our food systems. It provides clean water and habitats for biodiversity while contributing to climate resilience. It supports our cultural heritage and landscapes and is the basis of our economy and prosperity.

However, it is estimated that between 60 and 70% of EU soils are unhealthy. Soil is a fragile resource that needs to be carefully managed and safeguarded for future generations. One centimetre of soil can take hundreds of years to form, but can be lost in just a single rainstorm or industrial incident.

The Mission leads the transition towards healthy soils by  

• funding an ambitious research and innovation programme with a strong social science component 
• putting in place an effective network of 100 living labs and lighthouses to co-create knowledge, test solutions and demonstrate their value in real-life conditions
• developing a harmonised framework for soil monitoring in Europe
• raising people’s awareness on the vital importance of soils

The 8 Mission objectives

• reduce desertification
• conserve soil organic carbon stocks
• stop soil sealing and increase re-use of urban soils
• reduce soil pollution and enhance restoration
• prevent erosion
• improve soil structure to enhance soil biodiversity
• reduce the EU global footprint on soils
• improve soil literacy in society

The Mission will support the EU’s ambition to lead on global commitments, notably the Sustainable Development Goals (SDGs) , and contribute to the European Green Deal targets on sustainable farming, climate resilience ,biodiversity and zero-pollution. It is also a flagship initiative of the long-term vision for rural areas .

To know more about how the Mission will achieve its goal, read the Mission implementation plan .

Mission living labs and lighthouses

Living labs are places where to experiment on the ground . Soil health living labs will  be partnerships between multiple partners and different actors, like researchers, farmers, foresters, spatial planners, land managers, and citizens who come together to co-create innovations for a jointly agreed objective. Living Labs will be established at territorial, landscape or regional scale, with several experimental sites covered underneath. 

This is an innovative way to do research and innovation: in a Living Lab, experimentations happen in real-life conditions, operating with end-users i.e. commercial farms or forest exploitations, real urban green parks or industrial sites, and other actors such as NGOs or local authorities. This is key to make sure that research and innovation find solutions to societal challenges and challenges that land managers face on the ground. 

Lighthouses are single sites, like a farm or a park, where to showcase good practices . These are places for demonstration and peer-to-peer learning. Here good practices are tested or in place and can be showed to inspire other practitioners to move towards sustainable land management. In addition, in lighthouse sites, researchers work together with land managers to ensure that research responds to concrete needs encountered in the field.

Mission Soil platform website

In the Mission Soil platform website you find:

• information about Mission Soil and the opportunities to engage in its activities
• the Mission projects, funding opportunities, news, and events
• a contact point and helpdesk to reply to requests for information on the Mission and its activities

Visit the Mission Soil Platform

Mission Soil Manifesto

We are calling on regions, municipalities, private or public companies, organisations, associations, schools, educational institutes, universities, research institutions and a wide range of stakeholders as well as individuals to sign the Mission Soil Manifesto and become part of a community that cares for soil.

Please click below to sign the Mission Soil Manifesto

Sign the Manifesto

Funding opportunities

New funding opportunities are open to contribute to the Mission Soil.

There are 9 different topics open for the submission of proposals. The projects are called to work on soil erosion, nitrogen fluxes, soil health and pollinators, soil biodiversity, healthy crops, carbon farming, and forest peatsoils. Moreover, new funding opportunities are available to establish Living Labs, and to cooperate for soil health in Africa.

The Horizon Europe Work Programme 2024 - EU Missions is published.

Calls for applications are open from 8 May until 8 October 2024.

What are EU Missions?

EU Missions are a new way to bring concrete solutions to some of our greatest challenges. They have ambitious goals and will deliver tangible results by 2030. 

They will deliver impact by putting research and innovation into a new role, combined with new forms of governance and collaboration, as well as by engaging citizens.

EU Missions are a novelty of the Horizon Europe research and innovation programme for the years 2021-2027.

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Related links

Support at least 150 European regions and communities towards climate resilience by 2030

improving the lives of more than 3 million people by 2030 through prevention, cure and for those affected by cancer including their families, to live longer and better

The Mission will help achieve the marine and freshwater targets of the European Green Deal

Deliver 100 climate-neutral and smart cities by 2030 and ensure that these cities act as experimentation and innovation hubs to enable all European cities to follow suit by 2050

Healthy soil The common agricultural policy ensures compliance with rules to protect soil and encourages farmers to take extra steps to improve soil management

Long-term vision for the EU's rural areas Building the future of rural areas together

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