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Introduction to Soil /
Soil Formation
LA7014 - Ecology / Technology I

What is the difference between dirt and soil?

Textbook Definition of Soil

Preferred Definition - “The unconsolidated material at or near the earth’s surface that has properties due to its proximity to the surface”

Long Version - “the unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows the effects of genetic and environmental factors of: climate (including water and temperature effects) and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time”

Importance of Soil

"Essentially, all life depends upon the soil .... There can be no life without soil and no soil without life; they have evolved together." Charles E. Kellogg

"The nation that destroys its soil, destroys itself."
Franklin D. Roosevelt

"While the farmer holds the title to the land, actually it belongs to all the people because civilization itself rests upon the soil." Thomas Jefferson

Importance of Soil

Medium in which plants are grown for food and fiber.

Mechanical support for plant roots, such as trees.

Physical support for structures, roads, sidewalks, etc.

Home for millions of organisms.

Air-storage facility.

Mineral supplement for people (in some regions of the world)

Earth’s compost pile.

Soil Sphere Theory

Pedo = Soil

Bio = Plant

Litho = Rock

Hydro = Water

Atmo = Air

Can you see how all of these are interrelated? A Few Definitions

Unconsolidated material: refers to loose materials

Typically soil profiles consist of layers (horizons) of unconsolidated material over a parent material such as bedrock Soil Profile: A vertical section of soil through multiple horizons that extends into the parent material

Ex: Ruston series

State soil of Louisiana

Soil Horizons

Defined: A layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and number of organisms present, degree of acidity or alkalinity, etc.

6 Master horizons (O, A, E, B, C, R)

Numerous subordinates (lower case letters)

Further describe master horizon attributes

Parent Material

“The unconsolidated and more or less chemically weathered mineral or organic matter from which soils form” (Brady)

“The kind of mineral or organic materials in which the soil formed, including kinds of rock from which the regolith was derived” (USDA Soil Survey Manual)

Types of Parent Material

Residuum: Original unweathered material

Lacustrine: Lake deposited

Marine: Ocean deposited

Colluvium: Gravity deposited

Eolian: Wind deposited

Moraine/till: Ice deposited

Alluvium: Stream/river deposited

Parent Material Diagram
From Brady, Nature and Property of Soils 13th ed.

Soil Formation Factors

Parent Material


Living organisms, microbes, etc.


Precipitation, temperature


Inorganic vs. organic

Relief, landscape position


Soil Formation

Pedogenesis - The formation and development of the soil profile.

The “Pedon” is the three-dimensional body of soil used as the soil base of reference.

“Genesis” is often referred to as “beginning.”

The Rock Cycle

Rocks are merely combinations of minerals.

Rock formation is determined by different environmental factors. The Earth’s Crust

Chemists recognize over 100 elements that make up everything tangible on earth.

The earth’s crust is made up of many of the elements but the most abundant are the following:









Rock Types

Igneous Rocks - Include granite and their metamorphic associates.

Make up the bedrock foundation of some continents.
Crystalline structure forms as magma cools. The slower the cooling, the larger the crystal structure.

Sedimentary Rocks - Sandstone, Shale, and Limestone.

Make up the bedrock foundation of 3/4 of the land area on earth.
Deposited as loose layers of sediment.

Metamorphic Rocks - Quartzite, Marble, Slate, and Gneiss.

Igneous and Sedimentary Rocks are altered by heat and pressure.

Sandstone forms Quartzite, Limestone forms Marble, etc.

Rock Weathering

Just as plants and animals decay, rocks are weathered through naturally occurring physical and chemical forces causing rocks to be weathered into

Collectively saprolite is call the Regolith of the earth, which is composed of the loose mineral materials above solid bedrock.

Rock Weathering

Lichens are composed of an alga and a fungus and are often one of the first organisms to colonize exposed rocks.

These organisms, along with cyanobacteria, produce weak acids that dissolve nutrients within the rock and begin to build a humid mat capable of supporting mosses, and eventually higher plants.

Rock Weathering

Physical Weathering - The breaking down of rocks into smaller pieces with no change in the molecular arrangement within the minerals.



Freeze - Thaw

Tree Roots

Chemical Weathering - Chemical reactions that alter the molecular composition of rocks.






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