SOIL: CHARACTERISTICS, FORMATION, COMPOSITION, LAYERS, TYPES - ENVIRONMENT - 2025

The soil is the upper layer of the lithosphere caused by the weathering of the bedrock due to the action of climate and biological entities. Understanding by weathering the fragmentation of the rock forming an unconsolidated material with a defined structure and texture.

The aggregation of the solid particles that make up the soil determines its structure and the relative proportion of particles smaller than 2 mm define the texture. These particles are grouped into three general classes, ranging from larger to smaller diameter: sand, silt, and clay.

Soil. Source: Road to Gaia

The action of climatic factors such as precipitation and temperature as well as living organisms are responsible for the formation of the soil. These factors exert an action on the parent material or bedrock, fragmenting it over long periods of time.

This process gives rise to a complex porous structure composed of various minerals, water, air, and organic matter. This structure occurs in more or less defined horizons or layers with characteristic color, composition, texture and structure.

There is a wide diversity of soil types, which are described and classified according to different classification systems. The soil is the base of support of the vegetal cover, both natural and agricultural, being a fundamental element of the ecosystem.

However, the soil is being degraded and lost due to erosion, a consequence of climatic factors and human action. While pollution degrades the soil by introducing toxic substances into it or that affect its physical, chemical and biological properties.

Soil characteristics

The soil is a matrix made up of abiotic elements such as minerals, water and air, with biotic factors, under certain conditions of climate and relief. This matrix has a defined texture, structure, density and porosity, and constitutes an ecosystem with its characteristic biota.

- texture

The texture of a soil is determined by the relative proportion of sand, silt and clay present in it. This constitutes the fine fraction of the soil (fine earth), where the sand has coarser particles, with a diameter of 2 to 0.08 mm. The second component in diameter is silt with 0.08 to 0.02 mm and finally clay with less than 0.02 mm, This composition depends on the parent material or bedrock that gave rise to the soil, as well as the factors that took part in its formation. Any fragment with a diameter greater than 2 mm is already considered a coarse fraction of the soil or gravel.

The sand

The composition of sand is silica for the most part, since this is the most abundant mineral in rocks on Earth. However, there are also calcareous sands from the erosion of corals or volcanic sand from volcanic rocks.

The limo

It is a heterogeneous sediment of intermediate fractions, composed of both inorganic and organic elements.

Clay

Clays are hydrated alumina silicates and are considered chemically active in soil. They have a colloidal behavior, electrically charged and are important in the retention of moisture and mineral elements.

- Structure

The structure of the soil is given by the union of the solid particles of the soil forming clods or structural units called peds. The formation of these structures is the product of the flocculation or aggregation process caused by physical-chemical events.

Soil structure. Source: No machine-readable author provided. Pastranec assumed (based on copyright claims).

This occurs due to the attraction of opposite electrical charges between particles, involving water, humus and aluminum and iron oxides.

Humic complexes

Humus is a colloidal substance caused by the decomposition of organic matter due to the action of decomposing bacteria and fungi. The aggregates of humus form complexes that agglutinate soil particles, forming peds.

Living organisms that add soil

The roots of plants and the substances they exude also contribute to agglutinate particles forming structure in the soil. In the same way, organisms such as earthworms are fundamental in the processing of the soil and the definition of its structure.

- Density and porosity

The texture and structure of the soil determine the existence of pores in it, which are of variable diameter. The composition and porosity of the soil also determine a variable density, being that the lower the porosity, the higher the density of the soil.

Soil pores are important because they constitute the system of spaces through which water and air circulate in the soil. Both the water and the air in the soil are vital for the development of life on and within it.

- Interface and ecosystem

In the soil the mineral elements of the lithosphere, the water of the hydrosphere, the air of the atmosphere and the living beings of the biosphere interact. The soil maintains an exchange of chemical elements with water, as well as gases with the atmosphere, such as O2 and CO2.

On the other hand, living beings from the soil obtain nutrients and water, providing organic matter and minerals. In this context, the soil is an ecosystem where abiotic and abiotic factors are interrelated.

Rhizosphere

It is the environment that surrounds the roots of plants in the soil and forms a particular condition in the soil. In this environment the roots obtain water and mineral nutrients from the soil and provide various exudates, in addition to establishing symbiotic relationships.

The rhizosphere is where most of the life of the soil takes place, since that is where there is more carbon availability.

- Fertility

A fundamental property of the soil is its fertility, as it contains essential mineral elements for the development of terrestrial plants. Among these minerals are macronutrients such as nitrogen, phosphorus and potassium as well as micronutrients (iron, boron, zinc, manganese, nickel, molybdenum, among others).

- Water

Water is found circulating in the porous part of its structure, adhered to colloidal particles (clays) and plays a fundamental role in the formation of the soil structure. The main source of water for vegetation is the soil and essential minerals for plants are dissolved in it.

Soil formation

The process of soil formation or pedogenesis, is the product of the action of several factors. These range from the rock that gives rise to it to the factors that weather it.

- Parental material

The bedrock that forms the lithosphere is a continuous layer of varied mineralogical composition depending on its nature. They can be sedimentary, metamorphic or igneous rocks formed by different processes.

View of the bedrock. Source: Road to Gaia

Regolith

Under the action of climatic and biological factors, the rock gradually disaggregates or fragments, forming a variable layer of thick material called regolith. The climate and living beings continue to act on this material until they form the soil.

- Weather

The earth's surface is subjected to different climatic conditions, generating a temperature and humidity gradient. Each region has a regime of rainfall, winds and temperatures that varies during the day and year.

These conditions act on the parent material, degrading it and giving it a particular structure, creating different types of soils.

Precipitation

Water affects soil formation both by the physical erosive effect on the rock and by the water supply itself. Water, as a universal solvent, is a fundamental element in the chemical reactions that occur in the formation of the soil.

In addition, excess moisture and the alternation between wet and dry periods influence the type of soil that is formed.

Temperature

High temperatures favor various chemical processes that contribute to the formation of soil. While the extreme variations in temperatures promote structural stresses in the rock, generating fractures.

- Biotic factors

The activity of living beings that inhabit the soil and on it is decisive in the formation of the soil.

Vegetation

The presence of a vegetation cover plays a role in the stability of the substrate, providing an environment conducive to soil formation. Without vegetation cover, erosion increases and consequent loss of the soil in formation.

On the other hand, the roots of the plants and their exudates contribute to the fragmentation of the parental material and are soil binders.

Other organisms

The microorganisms and macroorganisms that inhabit the soil contribute substantially to its formation. Decomposers such as bacteria, archaea, fungi, and protozoa process organic matter and form humus.

Earthworms drill tunnels and ingest soil, processing organic matter in such a way that they contribute to generating structure in the soil. This increases the porosity of the soil and therefore the flow of water and air.

There are also a large number of larger digging animals that also contribute to soil formation, such as moles, shrews and others.

- relief

It is very important in the formation of the soil, because a steep slope prevents the permanence of the soil in formation. On the other hand, a plain or depression near a mountainous area will receive the washed soil material.

- Weather

Soil formation requires a long process of bedrock weathering and regolith processing. Therefore the time factor is fundamental for the evolution of the soil until it reaches its climax.

- climax floor

Once a balance has been reached in the formation process in relation to environmental conditions, a climax ground has been formed. At this point, the soil in question is considered to have reached its highest evolutionary level.

Soil composition

The composition of the soil varies according to the source rock that gave rise to it and the soil-forming processes involved.

Minerals

Almost all known minerals can be found in the soil, the most abundant groups being silicates, oxides, hydroxides, carbonates, sulfates, sulfides and phosphates.

Organic material

Depending on the biome where it develops, a soil will have a higher or lower content of organic matter. Thus, in the tropical rain forest most of the organic matter is in the superficial litter (horizon 0) and the underlying soil is poor in humus.

In the temperate deciduous forest there is a higher rate of accumulation of decomposed organic matter and in desert areas this accumulation of organic matter is very low.

Water

In the porous matrix of the soil, water circulates both in liquid form and as water vapor. Some of the water is strongly attached to the colloidal soil particles.

Air

The porous matrix has air, and therefore oxygen, carbon dioxide and atmospheric nitrogen. The air in the soil is important for maintaining life in the soil, including radical respiration.

Layers (horizons)

In soil formation, gravity, water infiltration, particle size, and other factors create a layered structure. These horizontal layers are arranged in a vertical gradient and are called soil horizons, together forming the so-called soil profile.

Soil horizons. Source: Mariiana QM

Traditionally 3 fundamental horizons are identified in a soil identified from top to bottom with the letters A, B and C. While the Soil Survey Division Staff of the United States defines 5 fundamental horizons and 2 possible layers.

Horizon 0

It is the presence of a layer of superficial organic matter with a mineral composition of less than 50% by volume. In this case, the level of decomposition of the organic matter present does not matter.

Horizon A

It is the surface horizon or below the horizon 0, characterized by the content of humus mixed with the mineral component. It is dark in color and there are roots, as well as alteration due to biological activity.

Horizon E

There is a predominance of sand and silt due to the loss of clays, showing a light color.

Horizon B

It is a horizon rich in minerals with accumulation of clays and other substances, which can form impervious clay blocks or layers.

Horizon C

It is the horizon closest to the bedrock and therefore less subject to the processes of pedogenesis. It is made up of fragments of rocks, accumulations of plaster or soluble salts, among other substances.

Layer R

Identify layers of hard rock, which requires the use of heavy equipment for drilling.

Layer W

This layer was recently added to refer to the presence of a layer of water or ice at any level. That is, this layer can be located between any of the aforementioned horizons.

Types of soil

There are various criteria for classifying soils, from very simple schemes based on texture or climate, to complex systems. Among the latter are the USDA (Unites States Department of Agriculture) and the FAO-UNESCO.

- According to the texture

It is based on the texture of the soil, according to its proportion of sand, silt and clay. To define it, the soil textural triangle (FAO or United States Department of Agriculture) is used.

Thus the textural classes are established, presenting sandy, clayey or silty soils, and the various combinations, such as a clay-sandy soil.

- According to the weather

This classification applies to soils in whose formation the fundamental element is the climate and give rise to the so-called zonal soils.

Humid climate soils

High humidity accelerates soil formation processes, while dissolving calcium carbonate and disrupting silicates and feldspars. Iron and aluminum predominate, being soils of low fertility and high content of organic matter such as the laterite types of the tropical rainforest.

Dry climate soils

The low humidity retards the soil-forming process, so they are thin and with the presence of scarcely weathered parent material. They present little organic matter given the scarce vegetation that they support and abundant calcium carbonate such as aridisols.

Temperate climate soils

The humidity and temperature conditions are moderate and deep and fertile soils are formed over time. They present significant amounts of organic matter and insoluble minerals such as iron and aluminum as in alfisols.

- USDA

It is the soil taxonomy system of the United States Department of Agriculture, which recognizes 12 orders as a superior category. It follows the suborder category with 64 classes, the groups with more than 300 classes and the subgroups with more than 2,400 classes.

Diagnostic characteristics

This system uses as diagnostic elements to assign a soil to a class, the type of soil moisture as well as the temperature regime. Likewise, the presence of certain horizons both on the surface (epipedons) and inside the soil (endopedons).

FAO-UNESCO

The top category in this system, equivalent to the order in the USDA system, is the Major Soil Group and includes 28 classes. The next level in the hierarchy is the Soil Unit and covers 152 classes.

Roles and importance

Soil is a fundamental component of terrestrial ecosystems and the basis of most human activities.

Support and nutrition of terrestrial vegetation

The soil provides the support on which land plants are established through their root system. Additionally, it provides mineral nutrients and water that plants require for their development.

Basis of agriculture and breeding

It is an essential production factor in agriculture, although there are modern techniques that dispense with it, such as hydroponics. However, mass production of most crops is only possible on large areas of soil.

Carbon cycle and sequestration

In its gas exchange with the atmosphere, the soil supplies and absorbs CO2. In this sense, the soil contributes to reducing the greenhouse effect and therefore global warming.

Permafrost

It is a layer of organic soil frozen in circumpolar latitudes, which constitutes an important reserve of CO2 in the soil.

Construction foundation

The soil is the base of support for human constructions, such as roads, canals, buildings, among many others.

Soil erosion

Erosion is the loss of the soil due to the action of climatic factors or human activity. Extreme soil erosion produces desertification and is one of the greatest threats to agricultural soils.

Soil erosion. Source: US Fish and Wildlife Service

Water erosion

Precipitation causes soil loss due to the impact of water droplets on aggregates and subsequent surface runoff. The more exposed the ground and the steeper the slope, the greater the drag caused by runoff.

Eolic erosion

The wind carries soil particles, especially in arid conditions, where it is dry and with little adhesion. Vegetation acts as a wind barrier, so its absence contributes to increasing the effects of wind erosion.

Anthropic erosion

Among the most erosive activities are deforestation and intensive crops, especially due to agricultural mechanization. As well as mining, especially open-pit mines, and infrastructure construction.

Soil contamination

Soils can be contaminated by both natural and man-made causes, but the most serious cases are due to human activities.

Agrochemicals

The application of chemical pesticides and fertilizers is one of the main causes of soil contamination. Many of these products are residual, taking long periods of time to biodegrade.

Effluent and runoff water

Poorly channeled and untreated sewage, as well as runoff water from urban and industrial areas, are the cause of pollution. Runoff waters carry waste such as lubricants, motor oils and paint residues that contaminate the ground.

Mining

This activity not only physically degrades the soil, but is also a source of polluting chemicals. Such is the case of mercury and arsenic used in the extraction of metals such as gold.

In the same way, the use of high-power hydropneumatic pumps to erode the soil in search of metal, releases polluting heavy metals.

Oil industry

Oil spills at drilling facilities and leaks from mud retention dikes pollute the ground.

Acid rain

Acid rain map. Source: Alfredsito94

Acid rain produced by industrial gases when reacting in the atmosphere with water vapor, causes acidification of soils.

Garbage

Accumulations of solid waste, especially plastics and electronic waste, are a source of soil contamination. Among other things, plastics release dioxins and electronic waste contributes heavy metals to the soil.

References

1. FAO (2009). Guide for the description of soils. Food and Agriculture Organization of the United Nations.
2. INIA (2015). Science and Technology Week Open Day. National Institute of Agricultural Research, Tacuarembó.
3. Jaramillo, DF (2002). Introduction to soil science. Faculty of Sciences, National University of Colombia.
4. Lal, R. (2001). Soil degradation by erosion. Land Degradation and Development.
5. Morgan, RPC (2005). Soil erosion and conservation. Blackwell Publishing.