Geology - Soil Formation
Geology -- Soil Formation
Soil--Introduction
“... the Latin name for man, homo, derived from humus, the stuff of life in the soil.”
~ Dr. Daniel Hillel
Image: USGS
Weathering is a crucial part of the rock cycle. As rocks are exposed at the Earth’s surface, weathering is inevitable. When solid rock is subjected to air, moisture, and organic matter, it breaks down, and regolith is formed. The part of the regolith that can support rooted plants is the soil. More definitively, soils are regolith weathered from solid rock combined with water, air, and organic material called humus. Thus, soils are a result of the interactions among the lithosphere, hydrosphere, atmosphere, and biosphere. The lithosphere provides the parent material of solid rock. In the hydrosphere, rain, snow, ice, and the flow of streams and rivers breaks down and transports this rock. Gases from the atmosphere chemically react with minerals and water to further break down the rock. The actions of organisms within the soil and the decaying of plant matter in the biosphere complete the formation of soils.
Soil is one of our most important renewable resources. However, in many areas within the United States and around the world, human activities are contributing to the its loss. In the United States, more than three billion tons of soil are lost from croplands each year via water and wind erosion. In addition, the minerals (nitrogen in particular) in soils are often depleted at a more rapid rate than they are restored. To reverse those conditions, sustainable practices should be adopted. In addition to agricultural practices that reduce soil loss, such as no-till or low-till of croplands, sustainable practices of development should also be implemented. Sustainability involves using natural resources to meet the needs of today without degrading the environment or compromising the ability of future generations to meet their own needs.
Guiding Questions and Objectives
- What are the general properties of soil and how do they vary from one region to another?
- What are the processes that affect the formation of soil?
- What role does soil classification play in evaluating and preserving a region's soil?
- What is the overall importance of soil conservation?
Soil Composition
Soil is composed of about 50% minerals. The remaining 50% is composed of air, water, and organic matter. The majority of the organic matter of soil is called humus, the insoluble residue from the partial decomposition of plants and animals. Humus is an important source of minerals. Equally important, humus binds soil particles together resulting in a spongy texture that retains moisture, which is critical to the survival and growth of most plants.
Soil Profiles
Soil profiles are a series of horizontal layers in the ground that are usually visible because of differences between chemical, physical and organic composition. Each recognizable layer is known as a horizon. The upper-most horizon (layer) generally has a higher concentration of organic matter and nutrients than the lower horizons.
If we were to cut a section of soil like the one pictured below, we can see how the difference in appearance of each distinct horizon is a result of the varying chemical, physical, and organic composition.
- The O horizon is entirely made up of organic matter. O for Organic! This layer consists of newly added matter, such a leaf litter, and decayed matter, or humus.
- The A horizon, also called topsoil, is the region mixed with organic matter and mineral particles. Roots tend to grow best in this horizon, and seeds also germinate here.
- The E horizon, once considered part of the A horizon, is the leaching layer. It loses most of the organic matter, clay, and nutrients through leaching by rain and snow. It is the light colored sandy layer.
- The B horizon, or subsoil, contains clay, minerals, and some organic matter that drip down from the above layers.
- The C horizon, where most plant roots cannot reach, consists of materials like gravel and sand.
- The R horizon (not pictured) lies between 20 to 60 inches below the surface. This is the region of underlying bedrock.
The depths of the horizons differ from one area to the next. For example, on steep hillsides the upper horizon is generally thinner than those in a valley because rain and snow-melt wash off organic matter before it can bind with the inorganic components. In a river valley, the ground adjacent to the running water can periodically receive large quantities of decaying plants and animals during a flood. As a result, such “bottom land” is generally rich in nutrients and more capable of supporting crops than the land on the slopes. The fertility of the soil also depends on its porosity, which is directly related to the ratio of various soil components.
Now let’s take a look at the type of soil based on soil series and horizon that predominates the state of North Carolina. Please follow this LINK for a explanation of “What is a state soil?” Then proceed to the bottom of the page and read the soil type for the state of North Carolina.
Soil Property -- Particle Size
There are more than 20,000 different types of soil in the United States based primarily on differences in physical and chemical composition. One of the factors employed in the classification of soils as well as the assessment of the value of a soil for different uses is the size of the inorganic particles in a sample.
Soil particles are divided into three classifications based on size.
Soils that have the highest potential for supporting plant life (fertile soil) as well as the animals that live in that environment are classified as loam . Loam is a mixture of sand, silt and clay, which provide a varied mixture of particle sizes, so the “best” quality loam for growing many crops generally consists of 40% sand, 40% silt and 20% clay. Soil that is capable of supporting plant and animal life is also referred to as fertile soil.
On the other end of soil composition, soils that permit the rapid downward movement of water generally are nutrient poor because the chemicals that are important for the growth of organisms are leached out of the soil. Leaching is a process in which dissolved and/or suspended chemicals are removed from materials by a solvent. With soils, the primary solvent is water. When water contains high concentrations of acids, such as the nitric and sulfuric acids in acid rain, the amount of chemicals leached from the soil is accelerated.
Sand, which is composed primarily of silicon dioxide, is important in helping to create spaces for air and water. Commercially, sand is employed in the production of glass, and to produce concrete, a high sand composition is required. Clay particles, which have a large surface area in contrast to their overall size, help to keep minerals from leaching out of soils as well as help to bind soils and retain water. Soils that are 80% or more clay, which is called hardpan, generally do not drain well. For that reason, clay is used to line artificial ponds and landfills. Soils used in the manufacture of pottery also have very high clay content. The properties of silt fall between those of clay and sand.
To view information concerning the fundamental concepts behind soil review the LINK.
Biological Life of Soil
The following web-sources cover how soil composition affects the interaction of various biological organism that reside in the soil. The first site is a Discovery.com site, so this site will work in a classroom. The site has three sections: “Down and Dirty”, “Field Guide”, and “Soil Safari”.
Importance of Soil and a Soil Survey
The LINK entitled “10 Key Messages” was prepared by the NRCS. It discusses the overall importance of soil and how a soil survey helps people to conserve this valuable resource.
Soil Taxonomy
There are twelve soil orders, identified by soil scientists, which provide the basis for a soil classification system.
Soil classification provides a foundation for scientists, local, state and federal agencies and concerned citizens to understand and utilize their soil to the best potential. The following section discusses the problems, causes and potential solutions for soil erosion.
Soil Erosion
“The nation that destroys its soil destroys itself.”
~Franklin D. Roosevelt
What Causes Soil Erosion?
Soil erosion is the movement of soil components, especially surface litter and topsoil, from one place to another. The two main agents of erosion are flowing water and wind. Some soil erosion is natural, and some is the result of human activities. In undisturbed vegetated ecosystems, the roots of plants help anchor the soil, and usually soil is not lost faster than it forms.
Farming, logging, construction, overgrazing by livestock, off-road vehicles, deliberate burning of vegetation, and other activities that destroy plant cover leave soil vulnerable to erosion. Such human activities can speed up erosion and destroy in a few decades what nature took hundreds to thousands of years to produce.
The two major harmful effects of soil erosion are (1) loss of soil fertility and its ability to hold water and (2) runoff of sediment that pollutes water, kills fish and shellfish, and clogs irrigation ditches, boat channels, reservoirs, lakes. Because natural processes regenerate it, soil (especially topsoil) is classified as a renewable resource. However, in tropical and temperate areas it takes 200-1,000 years (depending on climate and soil type) for 2.54 centimeters (1 inch) of new topsoil to form. If topsoil erodes faster than it forms on a piece of land, the soil becomes a nonrenewable resource. In the USA, soil erosion rates are about 16 times faster than soil formation rates, and we are among the best at conserving our soils!
Several studies document the seriousness of soil erosion:
How Serious Is Soil Erosion in the United States?
According to the National Resources Conservation Service, about one-third of the nation’s original prime topsoil has been washed or blown into streams, lakes,and oceans, mostly as a result of overcultivation, overgrazing, and deforestation.
According to the U.S. Department of Agriculture (USDA), soil on cultivated land in the United States is eroding about 16 times faster than it can form. Erosion rates are even higher in heavily farmed regions, including the Great Plains, which has lost on-third or more of its topsoil in the 150 years since it was first plowed. Some of the country’s most productive agricultural lands, such as those in Iowa, have lost about half their topsoil. Because of soil conservation efforts, the USDA estimates that soil erosion in the United States decreased by about 40% between 1985 and 1997. Using these data, USDA researchers estimate that soil erosion cost the United States about $30 billion in 1997, and average loss of $3.4 million per hour.
Critics such as Pierre Crosson say that these estimates of soil erosion and damages from such erosion are exaggerated and are based on inexact models instead of field measurements of soil loss and sedimentation rates in nearby bodies of water. According to Crosson, if current rates of cropland erosion in the United States continue for 100 years, crop yields will be only 3-10% less than they would be without such erosion.
However, David Pimentel and others point out that current estimates by models and a few on-site measurements do not include all the ecological effects of soil erosion. Such effects include reduction in (1) soil depth, (2) availability of soil water for crops, and (3) soil organic matter and nutrients. When such effects are included, some soil scientists and ecologists estimate that soil erosion causes a 15-30% reduction in crop productivity.
Erosion of cropland soil necessitates costly use of inorganic fertilizers to help replace lost soil nutrients. However, because fertilizers are not a substitute for fertile soil, there is a limit to the amount of fertilizer that can be applied before crop yields level off and then begin to decline.
How Can Conservation Tillage Reduce Soil Erosion?
Soil conservation involves reducing soil erosion and restoring soil fertility. For hundreds of years, farmers have used various methods to reduce soil erosion, most of which involve keeping the soil covered with vegetation.
In conventional-tillage farming, the land is plowed and then the soil is broken up and smoothed to make a planting surface. In areas such as the Midwestern United States, harsh winters prevent plowing just before the spring growing season. Thus, cropfields often are plowed in the fall. This leaves the soil bare during the winter and early spring and makes it vulnerable to erosion.
To reduce erosion, many U.S. farmers are using conservation-tillage farming (either minimum-tillage or no-till farming). The idea is to disturb the soil as little as possible while planting crops. With minimum-tillage farming, special tillers break up and loosen the subsurface soil without turning over the topsoil, previous crop residues, and any cover vegetation. In no-till farming, special planting machines inject seeds, fertilizers, and (usually) herbicides into slits made in the unplowed soil.
Besides reducing soil erosion, conservation tillage (1) saves fuel, (2) cuts costs, (3) holds more water in the soil, (4) keeps the soil from getting packed down, (5) allows more crops to be grown during a season (multiple cropping), (6) gives yields at least as high as those from conventional tillage, and (7) reduces the release of carbon dioxide from the soil to the air, which helps ease the threat of global warming.
Reference for Above : Soil Erosion & Conservation Excerpt from Sustaining the Earth, 5th Edition, G. Tyler Miller, Jr. Copyright 2002 by Wadsworth Group and Thomson Learning, Inc.
Additional Information and Resources: We have provided links to five NRCS publications that offer even more detail about soil, soil taxonomy and soil surveying.
From The Surface Down
Rationale for Soil Classification
Key to Soil Taxonomy
Twelve Orders of Soil Taxonomy (poster)
Twelve Ordes of Soil Taxonomy (web-site)
At This Point - Proceed to the next subtopic: AP Notes - Soils
Now let’s take a look at the type of soil based on soil series and horizon that predominates the state of North Carolina. Please follow this LINK for a explanation of “What is a state soil?” Then proceed to the bottom of the page and read the soil type for the state of North Carolina.
Soil Property -- Particle Size
There are more than 20,000 different types of soil in the United States based primarily on differences in physical and chemical composition. One of the factors employed in the classification of soils as well as the assessment of the value of a soil for different uses is the size of the inorganic particles in a sample.
Soil particles are divided into three classifications based on size.
- Sand particles range from 0.05 to 2 mm in size.
- Silt includes those particles from 0.002 to 0.05 mm.
- Clay consists of particles that are less than 0.002 mm.
Soils that have the highest potential for supporting plant life (fertile soil) as well as the animals that live in that environment are classified as loam . Loam is a mixture of sand, silt and clay, which provide a varied mixture of particle sizes, so the “best” quality loam for growing many crops generally consists of 40% sand, 40% silt and 20% clay. Soil that is capable of supporting plant and animal life is also referred to as fertile soil.
On the other end of soil composition, soils that permit the rapid downward movement of water generally are nutrient poor because the chemicals that are important for the growth of organisms are leached out of the soil. Leaching is a process in which dissolved and/or suspended chemicals are removed from materials by a solvent. With soils, the primary solvent is water. When water contains high concentrations of acids, such as the nitric and sulfuric acids in acid rain, the amount of chemicals leached from the soil is accelerated.
Sand, which is composed primarily of silicon dioxide, is important in helping to create spaces for air and water. Commercially, sand is employed in the production of glass, and to produce concrete, a high sand composition is required. Clay particles, which have a large surface area in contrast to their overall size, help to keep minerals from leaching out of soils as well as help to bind soils and retain water. Soils that are 80% or more clay, which is called hardpan, generally do not drain well. For that reason, clay is used to line artificial ponds and landfills. Soils used in the manufacture of pottery also have very high clay content. The properties of silt fall between those of clay and sand.
To view information concerning the fundamental concepts behind soil review the LINK.
Biological Life of Soil
The following web-sources cover how soil composition affects the interaction of various biological organism that reside in the soil. The first site is a Discovery.com site, so this site will work in a classroom. The site has three sections: “Down and Dirty”, “Field Guide”, and “Soil Safari”.
- Start looking through the Discovery.com site by using this LINK, and read through the “Down and Dirty” section. Please do not forget to look through the pages that are listed on the left.
- Use this LINK to go to the “Field Guide” section and read this overview of common soil organisms that are pictured at the bottom of the page.
- Use this LINK to go the “Soil Safari” section and take the flash enabled "Earth Ship” on a soil safari.
- Use this LINK to take a virtual museum tour of soil organisms. The link for the virtual tour is at the bottom of the page.
- This LINK will take you to the Smithsonian National Museum of Natural History's "Dig It! The Secrets of Soil" website.
Importance of Soil and a Soil Survey
The LINK entitled “10 Key Messages” was prepared by the NRCS. It discusses the overall importance of soil and how a soil survey helps people to conserve this valuable resource.
Soil Taxonomy
There are twelve soil orders, identified by soil scientists, which provide the basis for a soil classification system.
- Please follow this LINK to a web-site by the soil and land resources division at the University of Idaho.
- Read the home page and then select the link at the left of the page entitled “Information about each of the twelve orders." Go through all twelve orders by reading the description, viewing where the soil order is located, and looking at some of the number examples that run down the right side of each soil order page.
- Use this LINK to view the distribution of soil orders across the United States. Use the zoom tool to zoom in on North Carolina and determine the soil orders located in North Carolina.
- Use this LINK to view the distribution of soil orders on a global scale. Use the zoom tool to locate a region in the world that shares a similar group of soil orders as North Carolina.
Soil classification provides a foundation for scientists, local, state and federal agencies and concerned citizens to understand and utilize their soil to the best potential. The following section discusses the problems, causes and potential solutions for soil erosion.
Soil Erosion
“The nation that destroys its soil destroys itself.”
~Franklin D. Roosevelt
What Causes Soil Erosion?
Soil erosion is the movement of soil components, especially surface litter and topsoil, from one place to another. The two main agents of erosion are flowing water and wind. Some soil erosion is natural, and some is the result of human activities. In undisturbed vegetated ecosystems, the roots of plants help anchor the soil, and usually soil is not lost faster than it forms.
Farming, logging, construction, overgrazing by livestock, off-road vehicles, deliberate burning of vegetation, and other activities that destroy plant cover leave soil vulnerable to erosion. Such human activities can speed up erosion and destroy in a few decades what nature took hundreds to thousands of years to produce.
The two major harmful effects of soil erosion are (1) loss of soil fertility and its ability to hold water and (2) runoff of sediment that pollutes water, kills fish and shellfish, and clogs irrigation ditches, boat channels, reservoirs, lakes. Because natural processes regenerate it, soil (especially topsoil) is classified as a renewable resource. However, in tropical and temperate areas it takes 200-1,000 years (depending on climate and soil type) for 2.54 centimeters (1 inch) of new topsoil to form. If topsoil erodes faster than it forms on a piece of land, the soil becomes a nonrenewable resource. In the USA, soil erosion rates are about 16 times faster than soil formation rates, and we are among the best at conserving our soils!
Several studies document the seriousness of soil erosion:
- A United Nations (UN) Environment Programme survey found that topsoil is eroding faster than it forms on about one-third of the world’s cropland, causing an estimated 85% of the world’s land degradation from human activities.
- A 1992 study by the World Resources Institute and the UN Environment Programme found that soil on an area equal to the size of China and India combined had been seriously eroded since 1945. The study also found that about 15% of land scattered across the globe was too eroded to grow crops anymore because of a combination of (1) overgrazing (35%) (2) deforestation (30%), and (3) unsustainable farming (28%). Two-thirds of these seriously degraded lands are in Asia and Africa.
- According to a 2000 study by the consultative Group on International Agricultural Research, (1) nearly 40% of the world’s land (75% in Central America) used for agriculture is seriously degraded by erosion, salt buildup (salinization), and waterlogging, and (2) soil degradation has reduced food production on about 16% of the world’s cropland.
- According to agricultural expert Lester R. Brown, the topsoil that washes and blows into the world’s streams, lakes, and oceans each year would fill a train of freight cars long enough to encircle the planet 150 times. At that rate, the world is losing about 7% of its topsoil from actual or potential cropland each decade.
- Soil expert David Pimentel estimates that world-wide soil erosion causes at least $375 billion per year (an average of $42 million per hour) in (1) direct damage to agricultural lands and (2) indirect damage to waterways, infrastructure, and human health.
How Serious Is Soil Erosion in the United States?
According to the National Resources Conservation Service, about one-third of the nation’s original prime topsoil has been washed or blown into streams, lakes,and oceans, mostly as a result of overcultivation, overgrazing, and deforestation.
According to the U.S. Department of Agriculture (USDA), soil on cultivated land in the United States is eroding about 16 times faster than it can form. Erosion rates are even higher in heavily farmed regions, including the Great Plains, which has lost on-third or more of its topsoil in the 150 years since it was first plowed. Some of the country’s most productive agricultural lands, such as those in Iowa, have lost about half their topsoil. Because of soil conservation efforts, the USDA estimates that soil erosion in the United States decreased by about 40% between 1985 and 1997. Using these data, USDA researchers estimate that soil erosion cost the United States about $30 billion in 1997, and average loss of $3.4 million per hour.
Critics such as Pierre Crosson say that these estimates of soil erosion and damages from such erosion are exaggerated and are based on inexact models instead of field measurements of soil loss and sedimentation rates in nearby bodies of water. According to Crosson, if current rates of cropland erosion in the United States continue for 100 years, crop yields will be only 3-10% less than they would be without such erosion.
However, David Pimentel and others point out that current estimates by models and a few on-site measurements do not include all the ecological effects of soil erosion. Such effects include reduction in (1) soil depth, (2) availability of soil water for crops, and (3) soil organic matter and nutrients. When such effects are included, some soil scientists and ecologists estimate that soil erosion causes a 15-30% reduction in crop productivity.
Erosion of cropland soil necessitates costly use of inorganic fertilizers to help replace lost soil nutrients. However, because fertilizers are not a substitute for fertile soil, there is a limit to the amount of fertilizer that can be applied before crop yields level off and then begin to decline.
How Can Conservation Tillage Reduce Soil Erosion?
Soil conservation involves reducing soil erosion and restoring soil fertility. For hundreds of years, farmers have used various methods to reduce soil erosion, most of which involve keeping the soil covered with vegetation.
In conventional-tillage farming, the land is plowed and then the soil is broken up and smoothed to make a planting surface. In areas such as the Midwestern United States, harsh winters prevent plowing just before the spring growing season. Thus, cropfields often are plowed in the fall. This leaves the soil bare during the winter and early spring and makes it vulnerable to erosion.
To reduce erosion, many U.S. farmers are using conservation-tillage farming (either minimum-tillage or no-till farming). The idea is to disturb the soil as little as possible while planting crops. With minimum-tillage farming, special tillers break up and loosen the subsurface soil without turning over the topsoil, previous crop residues, and any cover vegetation. In no-till farming, special planting machines inject seeds, fertilizers, and (usually) herbicides into slits made in the unplowed soil.
Besides reducing soil erosion, conservation tillage (1) saves fuel, (2) cuts costs, (3) holds more water in the soil, (4) keeps the soil from getting packed down, (5) allows more crops to be grown during a season (multiple cropping), (6) gives yields at least as high as those from conventional tillage, and (7) reduces the release of carbon dioxide from the soil to the air, which helps ease the threat of global warming.
Reference for Above : Soil Erosion & Conservation Excerpt from Sustaining the Earth, 5th Edition, G. Tyler Miller, Jr. Copyright 2002 by Wadsworth Group and Thomson Learning, Inc.
Additional Information and Resources: We have provided links to five NRCS publications that offer even more detail about soil, soil taxonomy and soil surveying.
From The Surface Down
Rationale for Soil Classification
Key to Soil Taxonomy
Twelve Orders of Soil Taxonomy (poster)
Twelve Ordes of Soil Taxonomy (web-site)
At This Point - Proceed to the next subtopic: AP Notes - Soils