Properties of Soil: Soil is defined as the topmost layer of the earth’s surface that supports life and is rich in minerals and microorganisms that cover the entire land visible to us. All living creatures, including plants, animals, and microorganisms, grow and reside on the soil. 

We notice different types of soil based on colour, texture, and composition in different places. The varieties of vegetation in different places are evidence of different types of soil. Soil has various physical, chemical, and biological properties. Based on these categories, the soil is classified into sandy, clayey, and loamy based on some physical properties like texture, colour, and water-holding capacity. Some chemical properties like \({\rm{pH}}\), salinity, organic matter content, etc., also decide the soil type. Many microorganisms like an earthworm and some other bacteria live in the soil and play a vital role in increasing soil fertility. In this article, let’s discuss everything about the different properties of soil in detail.

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Nature and Properties of Soils

The soil is a whole part of nature, embracing the topsoil and the subsoil and other inner layers above its parent rocks. It is formed by weathering of the rocks, which is its parent material. Based on the climate under which it weathers, on the kind of topography upon which it is present, on the vegetation and other organisms with which the entire development of the soil takes place, and on the time duration that takes the soil to develop, the soil has different nature and properties. Soil consists of splintered rocks and humus. Some of the essential properties of soil are listed below:

Physical Properties of Soil

The physical properties of soil include texture, particle size, porosity, water holding capacity, soil horizon, etc. These properties are visible to us.

Soil Texture

Soil texture deals with the relative content of different sizes of soil particles. The different proportions of the soil make up the mineral component of soil, such as sand, silt, and clay. These soil types have the following range of particle size:

Soil Type	Particle Size
Sand	\(0.05\) to \(2\,{\rm{mm}}\)
Silt	\(0.002\) to \(0.05\,{\rm{mm}}\)
clay	\(< 0.002\,{\rm{mm}}\)

Sand and silt are commonly composed of quartz or some other inactive minerals; due to their large size, they do not contribute to a soil’s ability to retain soil water or nutrients.

On the other hand, clay has tiny size and sheet-like structure, clay has a large amount of surface area per unit mass, and this surface charge helps to attract water. Because of this, clay has the highest water holding capacity.

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Soil Porosity

Soil porosity refers to the voids or pores within the soil that facilitate air and water movement. More porous soil is considered to be healthier. Poor quality soils have few visible pores, cracks or voids.

Soil Colour

Soil has distinct colours due to the minerals present and by the organic matter content. Different areas have different colours of soil. Soil colours range mostly from black to red to white due to organic matter and iron.  Red soil indicates the presence of oxidised iron. Dark brown or black coloured topsoil indicates that the soil has a high content of organic matter.

Properties of Red Soil

1. It is porous.
2. Red soil has a high percentage of iron oxide. 
3. Its \({\rm{pH}}\) value ranges from \(4.5\) to \(6.5.\)
4. It is not fertile.

Properties of Black Soil

1. Black soil is also known as ‘black cotton soil’ because cotton is the most important crop grown on this soil.
2. It retains a good amount of water, thus has a clayey texture and is highly fertile.
3. It is rich in Calcium carbonate, Potash, Magnesium, etc.
4. It is deficient in Nitrogen, Phosphorus and Humus. 

Soil Profile

The vertical section of the soil from the top surface to the bottom-most layer where the soil meets the underlying rocks is defined as a soil profile. Based on the physical, chemical, and biological properties, the soil is classified into various horizons, which are as follows:

The O-horizon – This is the topmost layer of the soil surface composed of organic materials such as dried leaves, dead animals, etc., that are decomposed. The soil of this layer has dark brown or black colour.

The A-horizon – This surface horizon, also known as the topsoil, consists of considerable organic matter such as hummus. It is predominantly the fertile surface layer of many soils in agricultural lands. This layer is rich in several microorganisms such as bacteria, fungi, earthworms, etc.

The B-horizon – This layer is mainly composed of minerals (sand, silt, and clay). It contains less humus, minerals, and organic matter. Due to the presence of clayey soil, this layer holds more water than the topsoil.

The C-horizon – This horizon is known as parent rock and mainly consists of weathered as well as partially decomposed rock. The R-horizon – The lowest layer consists of a mass of rock such as granite, basalt, limestone or sandstone, quartz and is known as bedrock.

Chemical Properties of Soil

Some chemical properties of soil include the following:

Soil pH

1. \({\rm{pH}}\) is defined as a measure of the active hydrogen ion \(\left( {{{\rm{H}}^{\rm{ + }}}} \right)\) concentration. Soil \({\rm{pH}}\) is an indication of the acidity or alkalinity of the soil.
2. The \({\rm{pH}}\) scale ranges from \(0\) to \(14,\) with values below \(7.0\) in the acidic medium and values above \(7.0\) in the alkaline medium. A \({\rm{pH}}\) value of \(7\) is considered neutral, where \({{{\rm{H}}^{\rm{ + }}}}\) and \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) are equal.
3. Microbial and plant growth in the soil is mainly affected by the pH of the soil, as ion solubility in soil is directly affected by \({\rm{pH}}\). A \({\rm{pH}}\) range of \(6.0\) to \(6.8\) is ideal for most crops because it is accompanied by optimum solubility of the essential plant nutrients. 
4. In acidic soils, hydrogen and aluminium are the dominant exchangeable cations as hydrolysis produces hydrogen ions. Whereas, calcium and magnesium are basic cations.
5. Factors that affect soil \({\rm{pH}}\) include parent material, vegetation, and climate. The addition of certain fertilisers to soil can also produce hydrogen ions and make the soil acidic. Liming the soil adds calcium, which replaces hydrogen ions with hydroxide ions, and raises the \({\rm{pH}}\). In this manner \({\rm{pH}}\) of the soil is balanced in a particular area.

Cation Exchange Capacity (CEC)

Most heavy metals exist as cations in the soil environment. The most common cations found in soils are hydrogen \(\left( {{{\rm{H}}^{\rm{ + }}}} \right),\) aluminium \(\left( {{\rm{A}}{{\rm{l}}^{{\rm{3 + }}}}} \right){\rm{,}}\) calcium \(\left( {{\rm{C}}{{\rm{a}}^{{\rm{2 + }}}}} \right){\rm{,}}\) magnesium \(\left( {{\rm{M}}{{\rm{g}}^{{\rm{2 + }}}}} \right){\rm{,}}\) and potassium \(\left( {{{\rm{K}}^{\rm{ + }}}} \right).\) Clay and organic matter can hold cations from being “leached” or washed away as clay and organic matter are predominantly negatively charged (anions). The cations that are adsorbed, are replaced by other cations in a rapid, reversible process called “cation exchange”.

The “cation exchange capacity,” or “CEC,” of a soil is a measurement of the amount of cations retained by the negative charge (anions) per unit weight of soil or the number of cations in an exchangeable form that a given sample of soil has. The higher the clay and organic matter content, the more negatively charged the CEC should be.

Cations leaving the exchange sites enter the soil solution, where plants can absorb them. Cation exchange is an essential mechanism in soils for retaining and supplying plant nutrients and for adsorbing contaminants. Due to this reason, it plays a vital role in wastewater treatment in soils.

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Biological Properties of Soil

The influence of microorganisms living in the soil regulates the biological properties of soil. Organic matter in soil contains waste, metabolites, and residue from plants and animals that act as a good fertiliser on decomposition. Microbes in the soil convert organic matter into usable forms such as ammonia, sulphate, phosphate, etc. Some examples of soil microorganisms and other living organisms are bacteria, viruses, fungi, nematodes, protozoa, rodents, etc. All these living creatures help in maintaining the soil environment.

Engineering Properties of Soil

Engineering properties of soil include:

1. Cohesion – As we know, cohesion is a property of internal molecular attraction that resists the rupture of a substance. In small soil particles, water films help in binding the soil particles into an individual mass. Cohesion is greater in clays that are well compacted.
2. The angle of internal friction – The resistance in sliding soil particles in a mass of soil depends upon the angle of internal friction.
3. Capillarity – Water rises through the pores of the soil due to capillary action. By this action, water flows through all the directions in the soil irrespective of the gravitational force.
4. Permeability – Soil moves across the soil pores due to permeability or porosity.
5. Elasticity – When stress is applied to the soil, it does not get deformed permanently. Instead, it gets back to its original form once the stress is removed.
6. Compressibility – Sand, gravels, and silt are not compressible. At the same time, clays are compressible.

Index Properties of Soil

The properties of soil that help identify and classify soil for general engineering purposes are known as Index properties of soil. These properties help in determining the soil quality and the agricultural practices that can be done in a particular soil type. Some of the index properties of soil are as follows:

1. Particle-size distribution – It measures the size of the soil particles and the distribution of the particles that make up a soil mass. This property stands out to be the most fundamental of all soil properties as it decides how densely packed soil is.
2. Consistency – Soil consistency is the property by which soil resists any deformation or rupture. The consistency of non-cohesive soil depends primarily on the factors like particle shape and size distribution. In contrast to cohesive soils, this property depends on water content to a great extent.
3. Clay-Minerals Content – Clayey soil mainly contains minerals such as silica, alumina, magnesia, and water. Depending on these factors, particle size and elasticity of clayey mineral content are decided.
4. Water content – It is one of the most crucial index properties of finely grained soil as it decides the soil’s fertility and mineral absorbing capacity.

Summary

In brief, the soil is the foundation of all life on the earth. The composition of soil varies from one place to another. We can see many variations in the soil as we travel to different places. These variations occur due to various properties of soil. Soil has different physical properties like texture, colour, structure, porosity, water holding capacity, etc. Based on the texture, the soil is of different types, such as sand, silt, and clay. 

Due to mineral content, soil gains colours such as red, blue, or even whitish. Along with these physical properties, chemical, and biological properties such as soil \({\rm{pH}}\), presence of ions, organic matter content, etc., also deal with soil quality in a particular area. Along with all soil properties, soil moisture is also essential to help plant growth.

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FAQs

Q.1. What are the properties of desert soil?
Ans: Desert soil is sandy. They are very dry and soak in water quickly. In deserts, the demand for water from the atmosphere, and plants, is much greater than precipitation. Water-soaked in the upper layer of desert soil evaporates very fast.

Q.2. What are the physical properties of soil?
Ans: The physical properties of soil include texture, particle size, porosity, water holding capacity, soil profile, etc. These properties are visible to us. Based on the texture, the soil is of different types, such as sand, silt, and clay. Soil colour is another physical property of soil, such as black soil, red soil, etc.

Q.3. What are the chemical properties of soil?
Ans: Chemical properties of soil include soil pH and cation exchange capacity (CEC). Soil with a low pH value is considered acidic soil, while soil with a high pH value is considered basic. Soil contains many minerals that plant uptake in the form of cations and anions. This cation-anion exchange occurs between soil particles and plant roots.

Q.4. What are the shear strength parameters of soil?
Ans: The stability of soil particles is gained by the maximum internal resistance against shearing force. Some of the shearing parameters include cohesion, which is a property of internal molecular attraction that resists the rupture of a substance, and angle of internal friction that deals with the resistance in sliding of soil particles in a mass.

Q.5. How does deforestation change the physical properties of soil?
Ans: Deforestation is the act of cutting trees and removing forests. Due to the reckless cutting of trees, the temperature of a particular place increases, and the region’s humidity relatively decreases. Due to these changes in the climate, a small amount of organic matter is decomposed, resulting in a decline in soil structural properties and thus increasing bulk density. Also, soil fertility decreases.

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