GSLM - Soil (Lesson)

Soil

How much of the Earth is covered with soil? Click here to download the handout and read to find out! Links to an external site.

Introduction

Without mechanical and chemical weathering working to break down rock, there would not be any soil on Earth. It is unlikely that humans or most other creatures would be able to live on Earth without soil. Wood, paper, cotton, medicines, and even pure water need soil. So the soil is a precious resource that must be carefully managed and cared for. Although soil is a renewable resource, its renewal takes a lot of time.

Characteristics of Soil

Even though the soil is only a very thin layer on Earth's surface over the solid rocks below, it is where the atmosphere, hydrosphere, biosphere, and lithosphere meet. Within the soil layer, important reactions between solid rock, liquid water, air, and living things take place. Soil is a complex mixture of different materials.

• About half of most soils are inorganic materials, such as the products of weathered rock, including pebbles, sand, silt, and clay particles.
• About half of all soils are organic materials, formed from the partial breakdown and decomposition of plants and animals. Organic materials are necessary for soil to be fertile. The organic portion provides the nutrients, such as nitrogen, needed for strong plant growth.
• In between the solid pieces, there are tiny spaces filled with air and water.

In some soils, the organic portion could be missing, as in desert sand. Or a soil could be completely organic, such as the materials that make up peat in a bog or swamp. Soil is an ecosystem unto itself. In the spaces of soil, there are thousands or even millions of living organisms. Those organisms could be anything from earthworms, ants, bacteria, or fungi.

 

 

Climate

Scientists know that climate is the most important factor determining soil type because, given enough time, different rock types in a given climate will produce a similar soil. Even the same rock type in different climates will not produce the same type of soil. This is true because most rocks on Earth are made of the same eight elements and when the rock breaks down to become soil, those elements dominate.

The same factors that lead to increased weathering also lead to greater soil formation.

• More rain equals more chemical reactions to weather minerals and rocks. Those reactions are most efficient in the top layers of the soil where the water is fresh and has not yet reacted with other materials.
• Increased rainfall increases the amount of rock that is dissolved as well as the amount of material that is carried away by moving water. As materials are carried away, new surfaces are exposed, which also increases the rate of weathering.
• Increased temperature increases the rate of chemical reactions, which also increases soil formation.
• In warmer regions, plants and bacteria grow faster, which helps to weather material and produce soils. In tropical regions, where temperature and precipitation are consistently high, thick soils form. Arid regions have thin soils.  
• Soil type also influences the type of vegetation that can grow in the region.

Please watch this USDA Water Movement in Soil video to learn more.  Links to an external site.

Rock Type

The original rock is the source of the inorganic portion of the soil. The minerals that are present in the rock determine the composition of the material that is available to make soil. Soils may form in place or from material that has been moved.

• Residual soils form in place. The underlying rock breaks down to form the layers of soil that reside above it. Only about one-third of the soils in the United States are residual.
• Transported soils have been transported in from somewhere else. Sediments can be transported into an area by glaciers, wind, water, or gravity. Soils form from the loose particles that have been transported to a new location and deposited.

Slope

The steeper the slope, the less likely material will be able to stay in place to form soil. Material on a steep slope is likely to go downhill. Materials will accumulate and soil will form where land areas are flat or gently undulating.

Mass Wasting

The steeper the slope, the more likely mass wasting events can occur taking soil, rock, and other materials downhill. These events are often triggered by seismic activity, deforestation, and/or heavy precipitation. Examples of mass wasting include rockslides, landslides, mudslides, and avalanches.

 

Time

Soils thicken as the amount of time available for weathering increases. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration.

Biological Activity

The partial decay of plant material and animal remains produces organic material and nutrients in the soil. In soil, decomposing organisms break down the complex organic molecules of plant matter and animal remains to form simpler inorganic molecules that are soluble in water. Decomposing organisms also create organic acids that increase the rate of weathering and soil formation. Bacteria in the soil change atmospheric nitrogen into nitrates.

The decayed remains of plant and animal life are called humus, which is an extremely important part of the soil. Humus coats the mineral grains. It binds them together into clumps that then hold the soil together, creating its structure. Humus increases the soil's porosity and water holding capacity and helps to buffer rapid changes in soil acidity. Humus also helps the soil to hold its nutrients, increasing its fertility. Fertile soils are rich in nitrogen, contain a high percentage of organic materials, and are usually black or dark brown in color. Soils that are nitrogen-poor and low in organic material might be gray, yellow, or even red in color. Fertile soils are more easily cultivated.

Soil Texture and Composition

The inorganic portion of soil is made of many different size particles, and these different size particles are present in different proportions. The combination of these two factors determines some of the properties of the soil.

• A permeable soil allows water to flow through it easily because the spaces between the inorganic particles are large and well connected. Sandy or silty soils are considered 'light' soils because they are permeable, water-draining types of soils.
• Soils that have lots of very small spaces are water-holding soils. For example, when the clay is present in the soil, the soil is heavier, holds together more tightly, and holds water.
• When soil contains a mixture of grain sizes, the soil is called a loam.

 

Please watch this video below on soil texture.

Click here to watch the Soil Texture Video Links to an external site.

Soil Texture Triangle

When soil scientists want to precisely determine soil type, they measure the percentage of sand, silt, and clay. They plot this information on a triangular diagram, with each size particle at one side ( Figure below). The soil type can then be determined from the location on the diagram. At the top, the soil would be clay; at the left corner, it would be sand, and at the right corner it would be silt. Soils in the lower middle with less than 50% clay are loams.

 

 

Determine the soil types or percentage of particle type by using the Soil Texture Triangle above. Work through each problem, and then double-check your answer by clicking on the answer. If you have any trouble using this chart, watch the video, Texture Triangle, in the sidebar to help you.

1. What percentage of each of the three particles is silt?
2. What type of soil is 18% clay, 17% silt, and 65% sand? 
3. What type of soil is 35% clay, 35% silt, and 30% sand? 

Correct Answers:

1. Clay 0-12%, Silt 88-100% , and Sand 0-20% 

2. Sandy loam

3. Clay loam

Soil Horizons and Profiles

A residual soil forms over many years, as mechanical and chemical weathering, slowly change a solid rock into soil. The development of residual soil may go something like this.

1. The bedrock fractures because of weathering from ice wedging or another physical process.
2. Water, oxygen, and carbon dioxide seep into the cracks to cause chemical weathering.
3. Plants, such as lichens or grasses, become established and produce biological weathering.
4. Weathered material collects until there is soil.
5. The soil develops soil horizons, as each layer becomes progressively altered. The greatest degree of weathering is in the top layer. Each successive, lower layer is altered just a little bit less. This is because the first place where water and air come in contact with the soil is at the top.

A cut in the side of a hillside shows each of the different layers of soil. Altogether, these are called soil profiles (see images and diagram below).

Humus

The O horizon, if present, is the top layer and is composed of decaying plant and animal material or humus. Decomposers such as fungi and bacteria and detritivores like earthworms and certain insects reside in this layer to speed up the breakdown of organic matter. This provides nutrients and adds water-holding properties to soil increasing the fertility and health of the soil. Remember the O horizon is the source of "Organic" material in the soil.

Topsoil

Called the A horizon, the topsoil is usually the darkest layer of the soil because it has the highest proportion of organic material. The topsoil is the region of most intense biological activity: insects, worms, and other animals burrow through it and plants stretch their roots down into it. Plant roots help to hold this layer of soil in place. In the topsoil, minerals may dissolve in the freshwater that moves through it to be carried to lower layers of the soil. Very small particles, such as clay, may also get carried to lower layers as water seeps down into the ground.

Subsoil

The B horizon or subsoil is where soluble minerals and clays accumulate. This layer is lighter brown and holds more water than the topsoil because of the presence of iron and clay minerals. There is less organic material.

C horizon

The C horizon is a layer of partially altered bedrock. There is some evidence of weathering in this layer, but pieces of the original rock are seen and can be identified.

Not all climate regions develop soils, and not all regions develop the same horizons. Some areas develop as many as five or six distinct layers, while others develop only very thin soils or perhaps no soils at all.

Types of Soils

Although soil scientists recognize thousands of types of soil - each with its own specific characteristics and name - let's consider just three soil types. This will help you to understand some of the basic ideas about how climate produces a certain type of soil, but there are many exceptions to what we will learn right now

How Does Your Garden Grow?

Download the file below to explore the key nutrients present in the soil to support plant growth.

Click here to download "How Does Your Garden Grow?" Links to an external site.

Self-Assessment: Soil Texture and Horizon Summary

 Download the Soil Texture and Horizon Summary sheet by clicking here.  Links to an external site. Complete it and use it as part of your notes for this section.

 

RESOURCES IN THIS MODULE ARE OPEN EDUCATIONAL RESOURCES (OER) OR CREATED BY GAVS UNLESS OTHERWISE NOTED. SOME IMAGES USED UNDER SUBSCRIPTION.