Soil Structure

Structure and Composition of Soil – UPSC World Geography Notes

Soil, the upper weathered layer of the Earth’s crust influenced by plants and animals, forms a soil profile when viewed in a vertical section. Each soil profile typically consists of distinguishable layers or horizons, allowing the identification of different soil types.

Comprising matter in solid, liquid, and gaseous states, soil’s solid portion is a combination of organic and inorganic components. The inorganic or mineral part originates from particles derived from the parent material—rocks undergoing weathering to form soil. Meanwhile, the organic portion includes living and decomposed plant and animal materials, such as roots and worms, leading to the formation of humus, a black amorphous organic substance. Soil water, a complex chemical solution, arises from precipitation, run-off, seepage, and groundwater, filling the soil atmosphere in pore spaces when not occupied by water.

Soil texture, defined by the sizes of solid particles, ranges from gravel to clay, with varying proportions of these sizes in different soils and layers. Texture significantly influences the soil’s water-retention properties, with loam textures generally considered optimal for plant growth.

Soil acidity, related to the proportion of exchangeable hydrogen ions compared to other elements, is measured on the pH scale, ranging from 0 (extreme acidity) to 14 (extreme alkalinity). A pH value around 6.5 is typically considered most favorable for cereal crop growth.

Soil color provides insights into its formation and composition. In newly formed soils, the color often mirrors that of the parent material, while in other cases, it differs. Colors can range from white to black, indicating varying humus content. Cool humid areas usually have dark soils due to high humus content, while desert or semi-desert soils are light brown or grey with minimal humus. Reddish colors suggest the presence of ferric compounds, particularly oxides and hydroxides, often indicating well-drained soil, though local variations may be due to a red-colored parent material.

Properties of Soil

Every soil comprises mineral particles, organic matter, water, and air, and the interplay of these elements defines the soil’s characteristics, encompassing its texture, structure, porosity, chemistry, and color.

Understanding the composition and proportions of a soil’s water, mineral, and organic components enables the assessment of its productivity and guides decisions about its optimal use. Various easily testable or observable soil properties are employed to describe and distinguish between different soil types.

Physical Properties of Soil

It relies on factors such as the quantity, size, shape, arrangement, and mineral composition of soil particles. Additionally, the organic matter content and the presence of pore spaces play a significant role.

Soil Texture

Soil texture determines how the mineral component of the soil separates, with classifications such as sand, clay, and silt. Sand and silt, being less influential, do not significantly contribute to the soil’s capacity to retain water or nutrients. In contrast, clay plays an active role in soil texture due to its small size and substantial surface area per unit mass, facilitating the storage of ions and water.

The coarseness or fineness of the mineral matter in the soil constitutes soil texture, influenced by the proportions of sand, silt, and clay particles. When these three components are present in equal proportions, it is termed as loam. Soil texture has profound effects on various soil properties, including water holding capacity, nutrient retention, nutrient fixation, drainage, compressibility, and aeration.

To provide clarity on particle sizes:

  • Clay particles have diameters less than 0.002 millimeters.
  • Silt particles range in size from 0.002 to 0.05 millimeters.
  • Sand particles have diameters between 0.05 and 2 millimeters. Particles larger than 2 millimeters, such as pebbles, gravel, or rock fragments, are technically not considered soil particles.

Loamy soil is characterized by a balanced composition where none of the three components (sand, silt, clay) dominates the others. Specifically, loamy soil typically consists of approximately 40% sand, 40% silt, and 20% clay.

Soil Structure

The arrangement of soil particles forms specific patterns, such as plate-like structures, block-like structures, prism-like structures, and more.

Soil structure refers to how sand, silt, and clay particles are grouped together. Influenced by organic matter (from decaying plants and animals) and soil organisms like earthworms and bacteria, these factors contribute to the formation of aggregates. Clays, organic matter, and substances excreted by soil organisms bind the soil particles into aggregates, and soil structure plays a crucial role in plant growth. It regulates the movement of air and water, influences root development, and impacts nutrient availability.

High-quality soils are characterized by being friable (crumbly) with fine aggregates, allowing the soil to break up easily when squeezed. In contrast, poor soil structure exhibits coarse, very firm clods or lacks structure altogether.

Key structural characteristics of soil include:

  1. Permeability: This denotes the ease with which liquids/gases can pass through rocks or a soil layer and depends on particle size, shape, and packing. Sandy soils typically have high permeability, while clayey soils exhibit lower permeability.
  2. Porosity: It represents the volume of water that can be held within the soil and is expressed as a ratio of the volume of voids (pores) to the total material volume.

There are five fundamental types of structural units in soil:

  1. Platy: These are plate-like aggregates that form parallel to the horizons, resembling pages in a book. This structure may impede air, water, and root movement, and it is commonly found in an E horizon, rarely in other horizons.
  2. Blocky: There are two variations—angular blocky and subangular blocky. These structures are typically observed in the B horizon. Angular blocky structures are cube-like with sharp corners, while subangular blocky structures have rounded corners.
  3. Prismatic: With a longer vertical axis than the horizontal axis, prismatic structures are categorized as prismatic if the top is flat and columnar if the top is rounded.
  4. Granular: Peds in this structure are round, porous, and spheroidal. This is commonly the structure found in A horizons.
  5. Structureless: This category entails no observable aggregation or structural units. Examples include single grain (sand) and massive structures, which present as a solid mass without visible aggregates.

Soil Colour

Essentially, the color of soil, be it brown, yellow, or red, is influenced by oxidized or ferric iron compounds. A darker soil color signifies increased organic content, with higher levels of organic matter absorbing more heat due to their darker shade.

Soils abundant in humus appear dark, given that decomposed organic matter tends to be black or brown. Consequently, soils with significant humus content are often exceptionally fertile, earning the label of ‘rich’ when they exhibit a dark brown or black hue.

Red or yellow soil colors commonly signal the presence of iron.

Soil color is characterized by three parameters: hue, value, and chroma. Hue signifies the dominant wavelength or color of the light, value relates to the lightness of the color, and chroma indicates the relative purity or strength of the color.

To swiftly determine the soil color based on these parameters, one can compare a sample with a standardized set of color chips found in a notebook known as MUNSELL SOIL COLOUR CHARTS. In these charts, the top right corner represents the hue, the vertical axis represents the value, and the horizontal axis represents the chroma.

Soil Permeability

Soil permeability is a comprehensive term that describes the soil’s capacity to transmit water. Understanding the dynamics and balance of water in the soil is crucial for effective irrigation management. Permeability is influenced in part by the soil’s texture, with sandy soils exhibiting higher permeability compared to clay soils. Additionally, soil management practices can modify permeability.

  • While most porous rocks are permeable, clay presents an exception due to its extremely small pore spaces, often sealed with groundwater held by surface tension. Another exception is granite, a non-porous but permeable crystalline rock.
  • Although individual crystals absorb minimal water, the presence of numerous joints and cracks allows water to pass through, rendering it permeable.
  • High organic content in soil also contributes to increased porosity.

Soil Horizon

The soil is vertically stratified into distinct horizons, progressing from top to bottom:

  1. A-Horizon (Topsoil): Positioned at the surface, this layer is commonly known as topsoil. Abundant in humus and minerals, it retains the majority of water compared to the underlying layers. Comprising sand, silt, and clay, it serves as a habitat for various organisms such as snakes and earthworms.
  2. B-Horizon: The layer beneath the topsoil, the B-horizon is moderately rich in humus and supports moisture. It includes silt, clay, weathered rocks, and additional nutrients. The mineral content is higher in this layer than in the topsoil.
  3. C-Horizon: This layer consists of small rock fragments resulting from weathering processes.
  4. Bedrock: The lowest layer, bedrock, comprises solid, unweathered rock strata.

Chemical Properties of Soil

The chemical characteristics of soils are influenced by the following factors:

  1. Inorganic Matter in the Soil: The mineral composition of the soil is a crucial factor that distinguishes various soil types, given its abundance in the soil.
  2. Organic Matter in the Soil: Despite their low concentrations, organic components play a significant role in determining soil fertility.
  3. Colloidal Properties of Soil: Soil colloids, primarily classified into clay and organic colloids, contribute distinct functions. Clay colloids aid in water adsorption, while organic colloids enhance moisture and nutrient retention capacity.
  4. Soil pH: The soil’s chemical reactivity is gauged by its pH value, which determines whether the soil is acidic or basic.

Acidity & Alkalinity

  • Soil chemistry involves assessing acidity, alkalinity, or neutrality.
  • Low pH values indicate acidity, while high pH values suggest alkaline conditions.
  • Most complex plants thrive in soils with pH levels between 4 and 10.
  • Optimal pH varies by plant species.
  • Arid and semi-arid regions tend to have alkaline soils, while humid regions tend to have acidic soils.
  • To correct soil alkalinity and enhance productivity, irrigation water can be used to flush the soil.
  • Strongly acidic soils can be detrimental to plant growth.
  • Soil acidity can generally be corrected by adding lime to the soil.
  • pH in the soil affects ion solubility, influencing microbial and plant growth.
  • An optimal pH range of 6.0 to 6.8 is ideal for most crops, facilitating the solubility of essential plant nutrients.
  • Lower pH increases the solubility of minor elements (e.g., iron) and most heavy metals, impacting heavy metal movement and groundwater contamination.
  • pH management is crucial for controlling heavy metal migration in soil.
  • Lime requirement, the amount of liming material needed to raise soil pH, increases with cation exchange capacity (CEC).
  • To lower soil pH, sulfur can be added, leading to the production of sulfuric acid.

Soil Colloids

The primary and highly active component of soil, soil colloids play a crucial role by attracting soil nutrients that are dissolved in soil water, presenting as positively charged mineral ions or cations.

Certain cations, essential for plant growth such as calcium (Ca++), magnesium (Mg++), potassium (K+), and sodium (Na+), must be dissolved in a soil-water solution to be accessible to plants when near root membranes.

The fertility of the soil-water solution crucially relies on the soil’s ability to retain and exchange cations, a characteristic known as cation-exchange capacity. Without soil colloids, vital nutrients would be prone to leaching, carried away by percolating water and streams.

Biological Properties of Soil

The presence of organic matter in the soil contributes to the enhancement of soil structure, augmenting both nutrient retention and water-holding capacity. Additionally, organic matter serves as a source of nourishment for soil biology. Soils characterized by low organic matter often exhibit poor structure, minimal water retention, and susceptibility to erosion or nutrient leaching, except in the case of cracking clay soils where the primary influence on structure comes from clay minerals. Conversely, soils with elevated organic matter levels display favorable characteristics such as good structure, enhanced water-holding capacity, and reduced risks of erosion and nutrient leaching.

Biological properties encompass:

  • Organic matter
  • Soil organisms
  • Presence of disease-causing organisms.

Biological processes play a comprehensive role in soil formation, encompassing the presence and actions of living plants and animals, as well as their non-living organic byproducts. Living plants contribute to soil formation through two fundamental mechanisms:

(i) Biomass Production: This involves the generation of organic matter, with biomass occurring both above the soil as stems and leaves, and below the soil as roots. Biomass serves as the raw material for organic matter in the O horizon and lower horizons. Decomposer organisms break down this raw material, transforming it into humus and eventually into its original components, carbon dioxide, and water.

(ii) Nutrient Recycling: This process revolves around the cycling of nutrients contained in dead plant tissues back into the soil. Nutrient recycling acts as a mechanism to prevent the escape of nutrients through the leaching action of excess soil water moving downward through the soil.

Organisms residing in the soil play a crucial role in soil biological processes. For instance, earthworms contribute to soil modification not only through burrowing but also by passing soil through their intestinal tracts.

Several significant factors influencing the biological dynamics of soil include:

  1. Respiration Rate: The evolution of CO2 under standardized laboratory conditions or in the field.
  2. Potential N/C Mineralization: The increase in mineral Nitrogen or Carbon content under standardized laboratory conditions.
  3. Earthworms: The density of earthworms in the soil.
  4. Bacterial Biomass: The total bacterial biomass per given soil mass.
  5. Bacterial Diversity: Determined by functional groups or by describing genetic diversity.
  6. Presence of Pathogens: Evaluated through various pathology techniques, ranging from cultures to DNA profiling.

FAQs on Structure and Composition of Soil

Q1: What do you mean by Soil structure?

Answer: Soil structure refers to the arrangement and organization of soil particles into aggregates or clumps. It influences the soil’s ability to hold water, allow root penetration, and support plant growth.

Q2: What is the Granular structure of soil?

Answer: Granular structure is a type of soil structure where soil particles form small, rounded aggregates resembling granules. This structure enhances soil fertility, drainage, and aeration, making it favorable for plant growth.

Q3: What are the various forms of soil structure?

Answer: Various forms of soil structure include:

  1. Granular Structure: Small, rounded aggregates resembling granules.
  2. Platy Structure: Flat, horizontal layers of soil particles.
  3. Prismatic Structure: Vertical columns or pillars of soil particles.
  4. Blocky Structure: Irregular, angular blocks or chunks of soil particles.
  5. Spheroidal Structure: Round or oval-shaped aggregates of soil particles.

Each structure has unique properties affecting the soil’s characteristics and suitability for different plants and land uses.

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