Introduction to Soil


SoilSoil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Earth's body of soil is called the pedosphere.

Soil is a product of several factors: the influence of climate, relief (elevation, orientation, and slope of terrain), organisms, and the soil's parent materials (original minerals) interacting over time. It continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion. Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem.

Soil science has two basic branches of study: edaphology and pedology. Edaphology studies the influence of soils on living things. Pedology focuses on the formation, description (morphology), and classification of soils in their natural environment.

Functions of the Soil

Soil has four important functions:

  1. as a medium for plant growth
  2. as a means of water storage, supply and purification
  3. as a modifier of Earth's atmosphere
  4. as a habitat for organisms

Composition of the Soil

Basic Composition

Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a porous phase that holds gases (the soil atmosphere) and water (the soil solution). Accordingly, soil scientists can envisage soils as a three-state system of solids, liquids, and gases.

Soil compositionA typical soil is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas. The pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil. Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.

Components of the soil are mineral particles, humus, water and air. The actual amount of each of these depend upon the type of soil. Some soils are deficient in one or more of these, while there are some others that have varied combinations.

Soil Profile

The soil profile is defined as a vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock. In other words, it is the description of the various layers that soil forms as they get buried under the surface of the earth.

A soil horizon is horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath layers. The naming of a horizon is based on the type of material of which it is composed. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions. No soil profile has all the major horizons. Some may have only one horizon.

Example of a Soil Profile

Soil HorizonsO) Organic surface layer: Litter layer of plant residues, the upper part often relatively undecomposed, but the lower part may be strongly humified.

A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. Additionally, due to weathering, oxides (mainly iron oxides) and clay minerals are formed and accumulated.

E) Zone of Eluviation: In some soils, the constituents are soluble and move downwards. When this eluviation is pronounced, a lighter coloured E subsurface soil horizon is apparent at the base of the A horizon.

B) Subsoil: This layer has normally less organic matter than the A horizon, so its colour is mainly derived from iron oxides. The B horizon has generally a soil structure.

E) Zone of IlluviationIn a soil, where substances move down from the topsoil, this is the layer where they accumulate. The process of accumulation of clay minerals, iron, aluminium and organic compounds, is referred to as illuviation.

C) Substratum: Layer of non-indurated poorly weathered or unweathered rocks. This layer may accumulate the more soluble compounds like CaCO3. Soils formed in situ from non-indurated material exhibit similarities to this C layer.

R) Bedrock: R horizons denote the layer of partially weathered or unweathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. Soils formed in situ from bedrock will exhibit strong similarities to this bedrock layer.

Soil Formation


Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological and anthropogenic processes working on soil parent material. Soil is the result of evolution from more ancient geological materials, under the action of biotic and abiotic (not associated with life) processes.

Soil is said to be formed when organic matter has accumulated and colloids are washed downward. These constituents are moved from one level to another by water and animal activity. As a result, layers (horizons) formin the soil profile.


How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil. They are: parent material, climate, topography (relief), organisms, and time. When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.

Physical Properties

There are three kinds of soil mineral particles, called soil separates: sand, silt, and clay. Most of their properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.

Properties of Sands, Silt and Clay
Property/behavior Sand Silt Clay
Water-holding capacity Low Medium to high High
Aeration Good Medium Poor
Drainage rate High Slow to medium Very slow
Soil organic matter level Low Medium to high High to medium
Decomposition of organic matter Rapid Medium Slow
Warm-up in spring Rapid Moderate Slow
Compactability Low Medium High
Susceptibility to wind erosion Moderate (High if fine sand) High Low
Susceptibility to water erosion Low (unless fine sand) High Low if aggregated, otherwise high
Shrink/Swell Potential Very Low Low Moderate to very high
Sealing of ponds, dams, and landfills Poor Poor Good
Suitability for tillage after rain Good Medium Poor
Pollutant leaching potential High Medium Low (unless cracked)
Ability to store plant nutrients Poor Medium to High High
Resistance to pH change Low Medium High

Last modified: Friday, 18 October 2019, 10:33 PM