Soils constitute a major element in the natural environment, linking climate and vegetation, and they have a profound effect on man’s activities through their relative fertility. The scientific study of soils is known as pedology; the process of soil formation is referred to as pedogenesis (soil genesis).
Soil is the uppermost layer of Earth's crust, formed by the continuous weathering of mountain rocks over thousands of years. It consists of four basic components:
Minerals
Organic materials (including humus)
Air
Water
Process of Soil Development
The texture of the soil depends on the proportions of sand, silt, and clay, which contribute to its mineral texture. Organic constituents, including leaves, decompose to form the upper organic layer, humus, which plays a crucial role in soil fertility.
Soil Genesis (Pedogenesis)
Pedogenesis refers to the process of soil formation. Soils are a key component of the natural environment, linking climate, vegetation, and human activity through their fertility. The study of soils is called pedology.
A vertical section through soil is called a soil profile, and within this profile, there are several distinguishable layers or horizons that help identify different types of soil.
Key Features of Soil
Mineral Matter: Soil is primarily composed of inorganic material, part of the lithosphere.
Organic Material: Contains a significant amount of organic material capable of supporting plant life.
Plant Nutrients: Soil can produce and store essential nutrients, supporting plant growth and productivity.
Interaction of Earth Spheres: Soil serves as an interface between the atmosphere, lithosphere, hydrosphere, and biosphere.
Soil is typically thinly distributed over the land surface, but it plays an essential role in supporting life by providing a habitat for plants and microorganisms.
Soil Development: The Process of Weathering
Images of soil formation
Weathering: The process where rocks are physically and chemically broken down by atmospheric conditions and the action of water. Weathering results in the breakdown of solid rock, transforming large rock masses into smaller fragments.
Regolith: The principal product of weathering is a loose layer of inorganic material called regolith, which forms like a blanket over unfragmented rock.
Material Composition: Regolith may consist of materials transported by wind, water, or ice, leading to variations in its composition across different locations.
Soil Formation: The upper portion of regolith, approximately half a meter, becomes the soil. This layer is biologically and chemically active, containing plant roots, decaying organic matter, and microorganisms.
Soil is not the final product of weathering, but rather a stage in an ongoing cycle of physical, chemical, and biological processes.
The Role of Soil in the Ecosystem
Process of Soil formation
Soil functions as a fundamental part of the ecosystem, interacting with various elements such as water, air, and living organisms. Its primary role is to support plant life by storing and producing nutrients, which is crucial for sustaining terrestrial ecosystems.
In soil enrichment, matter—organic or inorganic—is added to the soil. Surface mineral enrichment of silt by river floods or as wind-blown dust is an example.
Soil Enrichment Leaching
Organic enrichment occurs as water carries humus from the O horizon into the A horizon below.
Removal Process
In removal processes, material is removed from the soil body. This occurs when erosion carries soil particles into streams and rivers.
Leaching, the loss of soil compounds and minerals by solution in water flowing to lower levels, is another important removal process.
Cheluviation is the downward movement of materials in the soil, similar to leaching, but occurring through the influence of organic agents (chelating agents).
Translocation Process
Downward Translocation:
Fine particles, particularly clays and colloids, are translocated downward, a process called eluviation. This leaves behind grains of sand or coarse silt, forming the E horizon.
Material brought downward from the E horizon—clay particles, humus, or sesquioxides of iron and aluminum—accumulates in the B horizon, a process called illuviation.
The topmost layer of the soil is a thin deposit of wind-blown silt and dune sand, which has augmented the soil profile.
Humus, moving downward from decaying organic matter in the O horizon, has enriched the A horizon, giving it a brownish color.
Translocation and Transformation of Soil
Eluviation has removed colloids and sesquioxides from the whitened E horizon, and illuviation has added them to the B horizon, which displays the orange-red colors of iron sesquioxide.
The translocation of calcium carbonate is another important process. In moist climates, a large amount of surplus soil water moves downward to the groundwater zone, leaching calcium carbonate from the soil in a process called decalcification.
In dry climates, calcium carbonate is carried down to the B horizon, where it is deposited as white grains, plates, or nodules, in a process called calcification.
Calcification can produce a cemented layer, known as a hard pan, that interferes with both eluviation and illuviation, rendering the soil less fertile.
Upward Translocation:
In desert climates, upward translocation can occur. Groundwater is drawn upward to replace evaporating surface water by capillary tension.
This water is often rich in dissolved salts. When the salt-rich water evaporates, the salts are deposited and build up in a process called salinization.
Large amounts of these salts are toxic to many plants, and in irrigated desert lands, salinization can ruin the soil.
Transformation Process
The last class of soil-forming processes involves the transformation of material within the soil body, such as the conversion of minerals from primary to secondary types.
Humification is the decomposition of organic matter by microorganisms to produce humus.
In warm, moist climates, the transformation of organic matter to carbon dioxide and water can be nearly complete, leaving virtually no organic matter in the soil.
Soil is an ever-evolving material. Metaphorically, soil acts like a sponge – taking in inputs and being acted upon by the local environment – changing over time and when the inputs or local environment change.
Factors that are responsible for soil development are:
Climate
Organisms
Relief
Parent Material
Time
Human Activity
Climate
Climate, measured by precipitation and temperature, is an important determinant of soil properties. Precipitation controls the downward movement of nutrients and other chemical compounds in soils by translocation.
If precipitation is high, water will wash nutrients deeper into the soil and out of reach of plant roots. If precipitation is low, salts will build up in the soil and restrict fertility.
Soil temperature affects the chemical development of soils and the formation of horizons. Below 10°C, biological activities are slowed, and at or below the freezing point (0°C; 32°F), biological activity stops, and chemical processes affecting minerals are inactive.
Decomposition is slow in cold climates, and organic matter accumulates to form a thick O horizon. In contrast, bacteria rapidly decompose plant material in warm, moist climates of low latitudes, where O horizons are lacking.
Role of Precipitation:
In areas with a lot of rainfall, water percolating down through soil tends to leach nutrients and organic matter out of the upper layers, unless modified by other soil components like plant roots.
For example, soils underlying tropical rain forests tend to be nutrient-poor because of intensive leaching due to heavy rains.
In arid regions with little annual precipitation, high rates of evaporation encourage the accumulation of salts in the soil.
Role of Temperature:
Solar energy controls the form of water falling onto the soil surface, increases reaction rates, evapotranspiration, and biological processes. Temperature fluctuations cause shrinking, swelling, frost action, and general weathering in soils.
Laterite soils are found in climates with alternate wet and dry seasons, while in Rajasthan, high temperatures and wind erosion form sandy soils regardless of the parent rock.
Organisms
Living plants and animals, as well as their nonliving organic products, have an important effect on soil. Plant roots mix and disturb the soil, providing organic material directly to upper soil horizons.
Role of Organisms in Soil
Earthworms continually rework the soil through burrowing and bypassing soil through their intestinal tracts, improving soil structure and fertility.
Burrowing animals like moles, gophers, rabbits, badgers, and prairie dogs also create tube-like openings in the soil.
The cultivating activities of earthworms improve soil structure, increase fertility, lessen the danger of accelerated erosion, and deepen the soil profile. Their presence is an indicator of productive soil.
Relief
The configuration or shape of the ground surface (relief) influences soil formation. Soil horizons are thicker on gentle slopes and thinner on steep slopes due to erosion on steep slopes.
Slopes facing away from the Sun tend to have cooler, moister soils, while slopes facing toward the Sun have higher soil temperatures and increased evapotranspiration.
Topography Affects Soil Process
Topography redistributes water, causing wetter conditions in lowlands and creating saline or organic soils. It also affects soil processes, distribution, and vegetation types.
Parent Material
Soil chemistry is influenced by the parent material. For example, iron-rich bedrock produces soils rich in iron oxides, while limestone forms calcium-rich soils. Soil texture is largely determined by the size of mineral grains in the parent material.
Soil inherits many properties from its parent material, including mineral composition, color, particle size, and chemical elements.
Examples:
The peninsular soils reflect the parent rock composition.
Crystalline and metamorphic rocks like granite, gneiss, and schist form red soils due to iron oxide content.
Soils derived from lava rocks are black-colored, while sandy soils are derived from sandstone.
Soils of the northern plains are deposited from Himalayan and peninsular blocks and have little relation to the in-situ rock material.
Time
Soil and Relief Impacts
Soil-forming processes are slow, and many centuries may be required for a thin layer of soil to form on newly exposed surfaces. Warm, moist environments are conducive to soil development.
Soil develops more quickly from sediments and slower from bedrock. A soil scientist’s rule of thumb is that it takes about 500 years to form 2.5 cm (1 in.) of topsoil.
Human Activity
Human activities, such as clearing native vegetation for crops, can induce erosion and remove upper soil layers rich in organic matter.
Agricultural soils that have been plowed and planted for centuries have undergone significant changes in structure and composition. These altered soils are recognized as distinct soil classes, just as important as natural soils.
Soil is the upper weathered layer of the Earth’s crust, affected by plants and animals.
A vertical section through this zone is called a soil profile, which typically contains several distinguishable layers or horizons, helping to identify different types of soil.
Physical and Chemical Properties of Soil
States of Matter in Soil
Soil contains matter in all three states: solid, liquid, and gaseous.
The solid portion is partly organic and partly inorganic.
Inorganic and Organic Components
The inorganic, or mineral, part of the soil consists of particles derived from the parent material (rocks) that weather to form the soil.
The organic portion consists of living and decayed plant and animal materials such as roots and worms.
The end-product of decay is humus, a black amorphous organic matter.
Soil Water and Atmosphere
Soil water is a dilute but complex chemical solution derived from direct precipitation, run-off, seepage, and groundwater.
The soil atmosphere fills the pore spaces of the soil when they are not occupied by water.
Texture of Soil
The texture of soil refers to the sizes of the solid particles composing the soil, ranging from gravel to clay.
The proportions of different sizes vary from soil to soil and from layer to layer.
Soil Texture Diagram
Texture largely determines the water-retention properties of the soil.
Sandy soil has large pore spaces and drains water rapidly.
Clay soil has small individual pore spaces, leading to poor drainage.
Loam textures are considered best for plant growth.
Soil Acidity
Soil acidity is related to the proportion of exchangeable hydrogen ions in the soil compared to other elements.
The degree of acidity is measured on the pH scale, ranging from 0 (extreme acidity) to 14 (extreme alkalinity).
A pH value of about 6.5 is regarded as ideal for the growth of cereal crops.
Colour of Soil
Soil colour varies considerably and can reveal much about how the soil is formed and its composition.
In recently formed soils, the colour reflects that of the parent material, while in others, the colour can be different from the underlying rock.
Soils range from white to black, generally influenced by the amount of humus present.
In cool, humid areas, soils tend to be black or dark brown due to high humus content.
In desert or semi-desert areas, soils are light brown or grey with little humus.
Reddish colours in soils indicate the presence of ferric compounds (oxides and hydroxides), suggesting well-drained soil, though it could also come from red-coloured parent material.
Properties of Soil
All soils contain mineral particles, organic matter, water, and air. The combinations of these factors determine the soil’s properties.
The key properties of soil include:
Texture
Structure
Porosity
Chemistry
Colour
Understanding the proportions of water, minerals, and organic components in soil can help determine its productivity and best usage.
Several soil properties can be readily tested or examined to describe and differentiate soil types.
Soil texture defines the proportion in which the soil separates to form the mineral component of the soil.
These separates can be classified as sand, clay, and silt.
Sand and silt are of no major importance to the soil as they don’t contribute significantly to the soil’s ability to restore water or nutrients.
Clay, however, is an active part of soil texture because it has small particle sizes, a large amount of surface area per unit mass, and helps in storing ions and water.
Soil Coarseness/Fineness
Soil texture refers to the coarseness or fineness of the mineral matter in the soil.
The texture is determined by the proportion of sand, silt, and clay particles.
The equal proportion of all three is known as loam.
Soil texture affects the water holding capacity, nutrient retention, nutrient fixation, drainage, compressibility, and aeration of the soil.
Particle Size Classification
Clay: Particle Size – diameters less than 0.002 millimeters.
Silt: Particle Size – diameters between 0.002 millimeters to 0.05 millimeters.
Sand: Particle Size – diameters between 0.05 and 2 millimeters.
Rocks larger than 2 millimeters are regarded as pebbles, gravel, or rock fragments and technically are not considered soil particles.
Loamy Soil
Loamy soil is a soil in which none of the three components (sand/silt/clay) dominates the other two.
In particular, loamy soil contains about 40% sand, 40% silt, and 20% clay.
Soil Structure
Example of Soil Structure
It is the arrangement of soil particles into certain patterns like plate-like structure, block-like structure, prism-like structure, etc.
Soil structure describes how the sand, silt, and clay particles are clumped together.
Organic matter (decaying plants and animals) and soil organisms like earthworms and bacteria influence soil structure.
Clays, organic matter, and materials excreted by soil organisms bind the soil particles together to form aggregates.
Soil structure is important for plant growth, regulating the movement of air and water, influencing root development, and affecting nutrient availability.
Good quality soils are friable (crumbly) and have fine aggregates so the soil breaks up easily when squeezed.
Poor soil structure has coarse, very firm clods or no structure at all.
Soil Structural Characteristics
Permeability – The ease with which liquids/gases can pass through rocks or a layer of soil is called permeability. It depends on the size, shape, and packing of particles. It is usually greatest in sandy soils and poor in clayey soils.
Porosity – The volume of water that can be held within the soil is called its porosity. It is expressed as a ratio of the volume of voids (pores) to the total volume of the material.
Basic Types of Structural Units
Platy: Plate-like aggregates that form parallel to the horizons like pages in a book. This type of structure may reduce air, water, and root movement. It is a common structure in an E horizon.
Blocky: Two types – angular blocky and subangular blocky. These types of structures are commonly seen in the B horizon. Angular blocky is cube-like with sharp corners, while subangular blocky has rounded corners.
Five Types of Structural Units of Soil
Prismatic: Vertical axis is longer than the horizontal axis. If the top is flat, it is referred to as prismatic. If the top is rounded, it is called columnar.
Granular: Peds are round and porous, spheroidal. This is usually the structure of A horizons.
Structureless: No observable aggregation or structural units. Examples include single grain-sand and massive-solid mass without aggregates.
Soil Colour
Soil color (brown, yellow, red) depends on oxidized or ferric iron compounds.
Darker the color of the soil, the more organic content it contains. The higher the organic content, the higher the soil temperature as it absorbs more heat due to the darker color.
Soils rich in humus tend to be dark because decomposed organic matter is black or brown. Soils with high humus content are usually very fertile, so dark brown or black soils are often referred to as ‘rich’.
Red or yellow soils typically indicate the presence of iron.
Soil Colour Parameters
Soil colour is described by the parameters called hue, value, and chroma.
Hue represents the dominant wavelength or colour of the light; value refers to the lightness of the colour; chroma refers to the relative purity or strength of the colour.
The colour of the soil can be determined by comparison with a standard set of colour chips in a notebook called Munsell Soil Colour Charts.
Soil Permeability
Soil permeability is a broad term used to define the ability of the soil to transmit water.
Soil Permeability
It is important for understanding water dynamics and the water balance of the soil, and it is essential for accurate management of irrigation.
Permeability depends partly on soil texture, with sandy soils having high permeability as compared to clayey soils.
A soil with high organic content tends to have high porosity and permeability.
Soil Horizons
A-Horizon: This is the uppermost layer of soil, also called topsoil. It is rich in humus and minerals and holds most of the water compared to other layers. This layer consists of sand, silt, and clay.
Soil Profiles and Horizon
B-Horizon: This is the second layer from the top. It is rich in minerals and supports moisture. This layer consists of silt, clay, weathered rocks, and some nutrients.
C-Horizon: This layer consists of small pieces of rocks broken down due to weathering.
BedRock: This is the last layer, consisting of solid, unweathered rock.
The chemical properties of soil are influenced by factors such as:
Inorganic Matter: The mineral content of the soil, which varies and plays a significant role in differentiating soil types.
Organic Matter: Though present in small quantities, it is essential for soil fertility.
Colloidal Properties: Colloids, crucial for water retention and nutrient storage, include:
Clay Colloids: Important for adsorbing large quantities of water.
Organic Colloids: Help retain moisture and nutrients in the soil.
pH of Soil: The measure of the chemical reaction in the soil, determining its acidic or basic nature.
Range in pH for Most of Inorganic Soils
Soil pH impacts plant growth and microbial activity. Most complex plants grow in soils with a pH between 4 and 10, with an ideal pH of 6.0 to 6.8 for most crops.
Low pH values indicate acidic soil, while high pH values indicate alkalinity.
In arid regions, soils tend to be alkaline, while in humid regions, they are typically acidic.
Soil acidity or alkalinity can be adjusted by adding lime (for acidity) or flushing with irrigation water (for alkalinity).
Effects of pH: pH affects ion solubility, microbial growth, and nutrient availability. Some elements, like iron, are more soluble at lower pH, which may lead to leaching or groundwater contamination.
Soil pH Management: Lime is used to raise pH, while sulfur can lower it. The amount of lime needed depends on the soil's cation-exchange capacity (CEC).
Soil Colloids
Soil Colloids: These are the most active constituents of the soil, with surfaces that attract positively charged mineral ions (cations) like calcium (Ca++), magnesium (Mg++), potassium (K+), and sodium (Na+).
Soil colloids hold and exchange cations, which are vital for plant growth.
Soil Colloids: Cation Exchange Capacity
Cation Exchange Capacity (CEC): The ability of the soil to hold and exchange these cations, ensuring nutrient availability for plants and preventing leaching.
Without soil colloids, essential nutrients would be washed away by percolating water.
Improvement of Soil Structure: Organic matter enhances soil structure by increasing its nutrient and water-holding capacity. It also provides food for soil biology.
Humus: Biological Properties of Soil
Effects of Low Organic Matter: Soils with low organic matter tend to have poor structure, low water retention, and are prone to erosion and nutrient leaching.
Effects of High Organic Matter: Soils with high organic matter levels have good structure, high water-holding capacity, and reduced erosion and nutrient leaching.
Exception: In cracking clay soils, the clay minerals have a more significant effect on soil structure than organic matter.
Biological Properties of Soil
The biological properties of soil include:
Organic Matter: Contributes to soil fertility and structure.
Soil Organisms: Play a crucial role in the decomposition and nutrient cycling processes.
Presence of Disease-Causing Organisms: Affect soil health and plant growth.
Biological Processes in Soil Formation: The presence and activities of living plants and animals, along with their non-living organic products, contribute to soil formation.
Biomass: The production of organic matter, both above ground (stems and leaves) and below ground (roots), which is decomposed into humus by decomposers.
Nutrient Recycling: The cycling of nutrients from dead plant tissues helps prevent nutrient loss through leaching by surplus water in the soil.
Nutrients Recycling
Role of Soil Animals: Animals like earthworms contribute to soil formation by burrowing and processing soil through their intestinal tracts.
Factors Affecting Biological Behavior of Soil
Respiration Rate: CO2 evolution measured under laboratory or field conditions.
Potential N/C Mineralization: Increase in mineral nitrogen or carbon content under controlled conditions.
Earthworms: Density of earthworms present in the soil.
Bacterial Biomass: Total bacterial biomass for a given soil mass.
Bacterial Diversity: Can be determined by functional groups or genetic diversity.
Presence of Pathogens: Determined through pathology techniques such as cultures or DNA profiling.
Organic amendments, like compost and manure, help maintain long-term soil health.
Soil Testing
Testing provides insight into soil nutrient levels, pH, and organic content, guiding appropriate management practices.
Regular testing helps identify deficiencies and optimize fertilizer use for sustainable crop production.
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