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what is the soil

The soil is the biologically active surface layer of the earth’s crust, it functions as a seat for plant and animal life, providing nutrients, oxygen and water.

It is an essential natural resource for the production of other resources and plays a fundamental role in natural cycles and in food chains.

In the composition four essential elements participate in the soil:

  1. The mineral materialis the inorganic component, made up of rocky and mineral fragments.
  2. The organic materialare remains of plants and animals in different states of decomposition that derive in humusresulting substance that serves to provide consistency, nutrients and other benefits to the soil.
  3. The Wateris retained in the pores and is responsible for transporting nutrients, determines soil moisture and the concentration of mineral salts.
  4. The airwhich is stored in larger pores and is characterized by containing less oxygen than atmospheric air, because living organisms consume it.

Types of soil

According to its structure and texture

Types of soil

1. Image of a clay soil. 2. Silty soil. 3. Sandy soil.

The particles that make up the soil, give it biological and productive capacities, and can be of three types. We can find coarse particles, which are called sandsmedium-sized particles called slimes and fine particles known as clays.

Depending on the proportion of particles and their arrangement, a soil can be:

Clayey: it can be suitable for cultivation if it is mixed with humus to enhance its fertility.

limestone: contains a large amount of salts, is typical of dry climates and not suitable for agriculture.

Sandy: does not retain water, lacks organic matter, so it is not very fertile.

Stony: it is composed mainly of stones, does not retain moisture and is not suitable for agriculture.

humiferous: Named for the amount of humus it contains, its color is dark and it is ideal for growing.

See also Hummus.

According to soil taxonomy

Soil taxonomy (Soil Taxonomy) is a universal classification made by the Department of Agriculture of the United States.

Based on parameters such as evolution, physical characteristics or the area where they develop, he establishes the following classification:

  1. Entisolsare young, poorly developed soils that do not present differentiated horizons and are mainly composed of rocky material.
  2. Inceptisolssoils that are beginning to evolve, may present horizons that have been generated by rapid processes, such as the translocation of materials.
  3. mollisols, are soils whose surface layer is dark in color and rich in organic matter. They can develop in different climates and at high latitudes.
  4. Alfisolsare soils that develop in temperate, well-drained zones with a high density of vegetation.
  5. Ultisolsare typical of temperate and rainy climates, present clay accumulation in the lower levels due to water filtration.
  6. spodosolsare characterized because the organic matter is mixed with aluminum and iron, the lack of clay gives it a sandy texture, it develops in humid environments and with a lot of vegetation.
  7. Oxisolsare mature soils, with a high oxide content, typical of warm and humid areas where humus develops easily.
  8. Aridisols, are soils that develop in arid zones where vegetation is scarce due to the high salt content. They usually develop in mountainous areas, where sudden changes in temperature cause alterations in the rocks.
  9. Andisols, develop in volcanic soil and the weathering of their materials depends on the level of rainfall and temperature. In humid climates, the superficial layer presents a dark color due to the accumulation of humus.
  10. Vertisols, are soils with a high clay content, with a cracked appearance during the dry seasons. This lack of constancy in the level of humidity limits the growth of vegetation.
  11. Histosolsdevelop in cold and humid areas, so the presence of undecomposed organic matter is high.
  12. Gelisolsare characterized by having a permanently frozen layer called permafrost, that limits their development. The climatic conditions and the lack of nutrients condition the growth of vegetation.
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Soil properties and characteristics

physical properties

The texture refers to the solid component of the soil, indicates the ease of working it, the accumulation of water it possesses, its filtering capacity and the amount of air it retains.

The porosity It is the amount of space free of solid elements, intended for the circulation of water, air, gases and nutrients.

The color of the soil reveals the type of minerals it contains, the amount of organic matter, its degree of humidity or its fertility. To determine the color of the soil, tables are used, such as the Munsell scale, which shows colors according to tone, brightness and intensity.

The structure indicates the way the particles are arranged. When mixed with water and organic matter, the particles form compact groups called aggregates. The spaces between these aggregates are called pores and serve to facilitate the circulation of other components.

The depth of the soil indicates to what level the roots can reach without difficulty for the absorption of nutrients.

Chemical properties

nutrient load, indicates the capacity of the soil to feed the living beings that inhabit it. Elements such as carbon (C) and hydrogen (H) are essential for the development of vegetation.

cation exchange capacity, indicates the amount of ions that the soil can store and emit. This trait is linked to the presence of nutrients and is essential for plant growth.

pH levelis the potential level of hydrogen, is related to soil fertility and determines whether we are dealing with an acid, alkaline, or neutral soil.

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biological properties

The organisms that inhabit the soil (worms, slugs, insects, etc.) contribute to the circulation of nutrients, transport and mix mineral and organic material, aerate the soil with their movements and facilitate the passage of roots.

The microorganisms (fungi, bacteria, etc.) play an important role in the formation and transformation of the soil. They degrade organic and inorganic matter, contribute to nitrogen fixation in the soil, mineralize organic compounds and favor the assimilation of minerals.

soil formation

ground view

The rocky base of the soil, after various natural processes, becomes fertile and allows the growth of plants, trees and vegetation in general.

Soil formation is the process by which rocks on the earth’s surface are transformed into a certain type of soil.

Rocks of different origins are involved in the first phase of the process. The igneous rocks They are those that are formed by the solidification of the molten material inside the earth. The sedimentary are the rocks formed by accumulation of sediments, and the metamorphic They are rocks that are formed by changes in temperature or pressure of the two previous types.

This rocky material undergoes a process called weathering, wear due to precipitation and other atmospheric phenomena. Its function is to transform the mineral into original material through various reactions, both physical and chemical.

The source materialin turn, can be sedentary, because it remains in the place where it was formed, or transported, by wind, water or glacial currents.

Weathering facilitates the existence of organic matter that degrades the rock. The organic substrate has to reach a certain thickness for plant life to develop and take root.

With the passage of time, and thanks to edaphogenesis, the original material is transformed into soil.

The edaphogenesis It is the process of soil formation, and it can occur in different ways:

  • By means of the revenue either lost of materials.
  • through the translocationthat is, the movement of certain compounds.
  • For transformation, physical or chemical changes in composition.
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Soil formation, in summary, is the result of the combination of five factors that interact with each other.

  1. original materialis the geological material that forms or supports it, depending on the type of rock the soil will have certain properties.
  2. Topographywhich analyzes the soil taking into account the shape and unevenness of the terrain.
  3. Weatherwhich can alter the properties of the soil due to the action of rain, solar radiation, temperature, etc.
  4. Living organisms that interact with the soil, such as vegetation, microorganisms and animals.
  5. Timewhich determines the age of the soil based on its level of development.

soil layers

soil horizons

Division of the soil into layers or horizons.

The soil layers are each of the horizontal and parallel strata into which it is divided, they are also known as horizons. A set of horizons constitutes the profile of the soil, where each layer differs from the rest by its physical or chemical properties. The order of the horizons is the result of the forming processes.

The main layers that make up the profile of a soil are:

OR horizonknown as the organic layer, is the most superficial level and is formed, above all, by decomposing organic matter, such as leaves and animal remains.

A-horizon, this layer presents the mineral element and a greater load of humus, in relation to the lower levels. On agricultural soil, this layer can suffer alterations due to the transit of animals and machinery.

B-horizon, is a level where different materials such as clays and metal oxides from higher levels accumulate. The presence of humus is scarce and the minerals have undergone many alterations.

horizon Calso known as subsoil, is a mineral layer that preserves the properties of the original material because the transformations it has undergone are minimal.

horizon R, is the undisturbed layer of rock material. It may present cracks in which certain materials such as clay accumulate, but their size is usually incompatible with root growth.


  • Burbano-Orjuela, H. (2016). The soil and its relationship with ecosystem services and food security. Journal of Agricultural Sciences, 33(2), 117-124.
  • Ciancaglini, N. (2009). Guide for the determination of soil texture by organoleptic method. San Juan: Inta EEA.
  • Lopez, AJ (2005). Edaphology manual. Department of Crystallography, Mineralogy and Chemistry. Agriculture of the University of Seville.