Particle Size, Soil Composition and Water Retention

Content Benchmark E.8.C.8

Students know soils have properties, such as color, texture, and water retention, and provide nutrients for life according to how they form. E/S

Soil, NOT dirt! Often people, especially students, use the term dirt to describe the material in which plants grow. Dirt can be thought of as the material under an unclean fingernail. Soil can be thought of as one of our most important natural resources. Without soil most plants would not be able to grow. Plants are the primary producers, and therefore, the primary food source within nearly every ecosystem. In fact the United States is home to more than 50,000 different soils.To understand how vitally important soil is we can think about the time during the Great Depression when the United States’ soil quality was so greatly diminished, due to over farming, that basic food needs of our population could not be met. To better understand the importance of soil we must first outline some characteristics of rocks. Then examine properties of soils and how soil forms. And finally, describe how abiotic (nonliving parts of an ecosystem) and biotic (living parts of an ecosystem) factors influence each other.

Particle Size, Soil Composition and Water Retention

Water is clearly needed for all living organisms, and plants are no exception. One of the main components of a soil is its water concentration. A soil’s ability to retain water is directly related to the size of the particles of rock fragments that compose the soil. Through weathering processes rocks are broken down into progressively smaller fragments. These fragments form the building blocks for a soil. The smaller the fragments become the more compacted they can get.

If you would like to learn more about the weathering processes that form soils, go to

The figure below shows the basic shapes of particles (rock fragments) as they breakdown.

Figure 1.Basic Particle Shapes

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The data table below shows the name and definition of each particle size. It is important to note that particle size determines that name, not the type of material. In other words sand is a size, not a material.

Table 1. Particle Size and Definition

Name / Definition
Blocks / Angular particles greater than 256 mm in size.
boulders / Rounded particles greater than 256 mm in size.
Cobbles / Rounded particles between 64 and 256 mm in size.
Pebbles / Rounded particles between 2 and 64 mm in size.
sand / Particles between .0625 and 2 mm in size.
Silt / Particles between 2µm and .0625 mm in size.
Clay / Particles less than 2µm in size.

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The rock fragments in most soils typically consist of three sizes: sand, silt, and clay. Sand is much coarser than silt, and clay is very fine. Infiltration is the ability of water at the surface of the soil to percolate down through the soil due to the force of gravity. In general, the smaller the particles are, the more compact they become. The size of the soil particles, and their spacing, determine how much water can flow through the soil. The larger the spacing, or pore size, the greater the infiltration rate. Thus, sandy soils will have high infiltration rates because pore sizes are large and there are no finer materials to block the pores. The soil’s ability to retain water, a vital ingredient in a healthy soil, is mainly controlled by the particles within the soil. As the particle size increase so does the spaces between the particles. This space allows for greater infiltration, but it also allows for greater evaporation.

For more information about water in the soil go to

For a video that explains how water moves through the soil go to

ftp://ftp-fc.sc.egov.usda.gov/NSSC/Educational_Resources/Water_Movement_V2.wmv.

Being that there are over 50,000 different types of soils in the U.S. alone one would think that each type is vastly different. Most of that difference is in the concentrations of various components shared by all soils. The basic components of every soil are: rock fragments and mineral, water, gases, remains of dead organisms, and living organisms (soil biota: ranging from bacteria, fungi, and earthworms). These basic components are found, to varying degrees, in each soil.

NevadaState Soil (Orovada)

Figure 2. Nevada state soil (Orovada)

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Properties of Soils

The properties of the soil are controlled by the particles found within the soil. The main constituent rock fragments control the water infiltration, retention and ultimately the life that can survive in the soil. The data table below (Table 2) shows the relationship between the particle size and various characteristics of the soil.

Table 2. Soil Properties and Particle Size

Sand / Silt / Clay
Porosity / mostly large pores / small pores predominate / small pores predominate
Permeability / rapid / low to moderate / slow
Water holding capacity / limited / medium / very large
Soil particle surface / small / medium / very large

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Color is another characteristic of a soil that denotes the fertility of the soil. The dark material found throughout the upper profile of the soil (horizon O and A) is formed from decomposing organic matter. This material is called humus. Humus is part of any fully developed soil, and is vital to plant health. Other colors found within the layers of the soil profile denote the presence of specific mineral and rock fragments.

To learn more about the organic matter of the soil go to

To learn more about the different colors found within the soil go to

General Soil Profile

Figure 3. Soil profile

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To learn more about soil properties go to

Soil Formation

The factors that affect soil formation are climate,types of rock (referred to as the parent material or bedrock), the slope of theland, amount of moisture and organisms present (plants further the weathering process), and length of time the bedrock has been weathering. The time frame for soil formation varies greatly from location to location due to these factors, however the process takes so long that soil is considered a nonrenewable resource. In the pictures below (figure #3), measured with the same type of shovel, notice the difference in the time it takes to form relative small amounts of soil.

Figure 4.Soil profiles

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A soil typically starts forming when exposed bedrock weathers to form what is called the regolith. This material may be removed through erosion, or may stay and provide the basic building blocks for soil formation. As the upper layers of the regolith continue to weather into finer and finer particles the lower layers of regolith become protected. The well weathered fragments of rock form the basic component of the soil. Within the broken fragments of rock small plants can take root. The plants further break down the regolith, and as they die they decompose to form the humus. As time passes the soil layers (soil profile) continually grow. Each layer of the soil profile is called a horizon. The characteristics of each horizon are controlled by the particles contained within that layer. The diagram below (figure #4) is a representation of a well developed soil profile (the measurements are, unfortunately, in inches); notice (in figure #4) that each horizon of the profile varies in color and composition and therefore characteristics.

Figure 5. Soil profile

(From

Soils generally consist of three major horizons: A, B, and C. Horizon A contains a large amount of organic material, humus, which is referred to as topsoil. Nearly all living organisms live of the soil live in the A horizon, thus the large amount of humus. The A horizon is also the layer responsible for allowing water to infiltrate the soil. As water enters the soil it dissolves minerals from horizon A and transports them to horizon B. This process is called leaching. Horizon B, below horizon A, is commonly called the subsoil. It contains leached minerals from horizon A, as well as rock fragments and sometimes humus. Horizon C, below horizon B, is the bottom layer. It contains rock fragments (regolith) and the original parent rock (bedrock).

To learn more about the factors that affect soil formation go to

To view state soils go to

To learn more about the different characteristics of each general soil type go to

Content Benchmark E.8.C.8

Students know soils have properties, such as color, texture, and water retention, and provide nutrients for life according to how they form. E/S

Common misconceptions associated with this benchmark

1. Students incorrectly think that weathering and erosion are synonymous.

Students often have difficulty separating the two concepts. This may be a result of teaching the concepts together or may be due to a difficulty with the vocabulary used to define each concept. Weathering is the breaking down of rock through mechanical or chemical process. This process is vital for soil formation. Erosion is the removal of weathered rock through a physical process (driven with the force due to Earth’s gravitational attraction on a mass).

For more information, a student work page, andsoil formation notes go to

1. Students often think soil is alive.

While it is true that living things exist in soil, andcomponents of the soil were once-living,students need to understand that soil itself is not alive. Soil biota (earthworms, bacteria, and the such) are an essential component in a healthy soil, however the soil overall is not alive.

For more information about the composition of the soil go to

1. Students often think that potting soil from a home improvement store is the only type of soil.

Soil is the top layer of earth and is composed of various particle sizes both organic and inorganic. In actuality, soils are only not found in a few locations around the Earth (solid rock outcrops, icecaps, & shifting sands). A sandy shore for example doesn’t contain the components of a soil while the Mojave Desert does. Students must be taught that soils look vastly different due to the constituent materials that make the soil.

For a world map that shows locations of soils types go to

For a description about each soil type listed on the map go to

1. Students often think soil type is determined by color.

Soil type is actually determined based on particle size. Color is dependent upon the rock type from which the soil is formed over time. As water infiltrates the soil and leaches minerals through the subsoil color can change when the minerals of the soil oxidize. These changes in color do not change the soil type. Again, soil is classified by texture (particle size).

For more information about characteristics of soils, and the soils constituent materials go to

1. Students often think that soil layers go deeply into the crust (that soil layers are very thick).

Students often think that soils are the crust, or that soils extend deeply into the crust. Actually soils only form relatively thin layers above the bedrock. It takes a substantial amount time to form even a small layer of soil and erosion is constantly removing what weathering produces. Needless to say, soils are very thin when compared to even the thickness of solid rock features.

For more information soil depths go to

Content Benchmark E.8.C.8

Students know soils have properties, such as color, texture, and water retention, and provide nutrients for life according to how they form. E/S

Sample Test Question

Questions and Answers to follow on a separate document

Content Benchmark E.8.C.8

Students know soils have properties, such as color, texture, and water retention, and provide nutrients for life according to how they form. E/S

Answers to Sample Test Questions

Questions and Answers to follow on a separate document

Content Benchmark E.8.C.8

Students know soils have properties, such as color, texture, and water retention, and provide nutrients for life according to how they form. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. The Dirt on Soil is a web site from Discovery.com that allows students to explore the many layers of the soil, examine the living organisms in the soil, and even provides a virtual safari through the soil to collect samples and make observations. This web site is a great resource that will allow students to explore the soil.

To access the site and go exploring visit

1. Porosity and Particle Size are commonly compared to each other when examining the soil. The Science Buddies web site has a great experiment that will allow your students to test if the porosity of a rock matrix is affected by particle size. This is relatively a print and go resource that could easily be implemented within a middle school classroom.

To access the resource go to

1. Thinkquest’s “Get the Latest Dirt on Soils,” website is a great resource written at a middle school level. This would work well as a review for students, introduction to the unit, or as an independent learning task. The site offers easy to understand vocabulary and visual representations as an online tutorial about soils.

Visit this resource at

1. Natural Resources Conservation Services has a project in which students make models of soil profiles on index cards. The site has directions with pictures that fully describe the activity. This activity will allow students to model soil profiles and gain a better understanding of horizons. “You can demonstrate erosion severity by altering the depth of the A horizon or display various types for soils found in different locations. Displays can be made of several soils on a drawing of a hill slope or other landscape by using the tape on a larger card or poster.”

To access this resource go to

1. Visual Geology, a San DiegoStateUniversity site, is an online tutorial full of notes, diagrams, and animated models about soils. This extremely comprehensive website is a wealth of information that could be used as a reference for teachers that want to know more or need an in-depth answer for that special student.

To access this informative website go to

1. Soil Science Resource website is a tremendously valuable site that offers subject specific resources organized by grade level. For example, there are 17 resource sites with lesson plans and activities for general soil teaching at the 5-8 grade level. This site is a great resource for planning and implementing inquiry instruction. Students could carry out various activities and peer share what they learned about various aspects of soils.

To access this useful directory of lessons and activities go to

1. The Great Plant Escape is a web resource that allows students to play detective while learning about the soil. The students are given information about soils, and asked to complete activities. These activitieswould bea great way to introduce to a soil unit.

To access thisresource go to