Unit 1 Lesson 3 Soil Formation

OBJECTIVES

Define soil and describe its composition. Describe how soil is classified. Identify the four main layers in a soil profile. Explain the four main factors that determine soil formation.VOCABULARY

horizon layer of distinct soil

soil top layer of mineral and organic material on Earth's surface

soil profile arrangement of horizons in a soil sample

topography elevation and slope of land

Lesson Outline

Soil is made of weathered rock, organic material, air, and water. Soil is classified by chemical and physical properties, such as texture and color. A soil profile shows the arrangement of horizons. Most soils have four main

horizons—the organic horizon (O), the surface horizon (A), the subsoil (B), and the substratum (C).

Temperature and moisture are the most important climactic factors in soil formation.

Fertile soil usually occurs in areas with moderate to low incline. The higher the number of living organisms in an area, the greater will be the rate

of weathering and soil formation. Fertile soil develops from parent material high in essential elements (Ca, Mg, K,

and Na).

Soil Definition

Did you know there are over 20,000 different types of soil in the United States? Soil is the top layer of mineral and organic material on the earth's surface. It forms from weathered rocks and organic material. The weathered rock can be anything from sand to clay and the organic matter is from decomposed animals and plants. There are many different types of soil, made from thousands of combinations of

minerals and organic material.

[Pie chart of makeup of soil: Organic matter, air, minerals, water]

Geologists named most soils after the location where they first discovered the soils. For example, Sacramento soil occurs in the Sacramento and northern San Joaquin Valleys in California.

Geologists classify soil based on physical and chemical properties.

[Chart of physical properties of soil] Color, texture, and structure are some of the properties used to classify soil. Over time, soil develops layers called horizons. Physical and chemical characteristics can be different from one horizon to the

next, and this arrangement of horizons is a soil profile.

[Diagram showing the three different soil horizons]

There are three main soil horizons: surface horizon (labeled here as A), subsoil (labeled as B), and substratum (labeled as C. Many soils also have an O horizon above the A horizon. The O horizon is mostly decomposed organic material and rich with nutrients, like nitrogen and potassium.

The A horizon contains a mixture of organic material and inorganic minerals. It is usually dark in color.

People often call the O and A horizons topsoil.

The B horizon is a dense layer of fine soil located below the A horizon. The C horizon is located below the B horizon. It is made of the rock from which

the soil developed. Below the C horizon lays the hard bedrock of the Earth's crust.

But how does soil form? When rock is weathered, it breaks down into smaller pieces, and erosion transports these sediments in different locations. Soil forms when these sediments combine with organic material from decomposed organisms. There are four main factors that determine soil formation: climate, topography, living organisms, and parent material.

Climate

Have you gone on vacation to a tropical climate? The soil in tropical areas is very different from the soil in other climates.

Temperature and moisture greatly influence the formation of soil.

The high temperatures and abundant moisture in tropical climates cause rocks to weather quickly.

Warm, humid conditions are also comfortable for many living organisms.

The greater the number of plants and animals in an area, the faster will be the rate of biological weathering and the more organic material the soil will contain.

Few soils form directly from the rocks below them. Most form from sediments transported from other locations by wind, water, and ice.

The length of transport can be as little as a few feet or as long as several miles.

The type of climate in an area determines the type of erosional transport that will occur.

In dry climates, wind erosion greatly influences soil formation.

In cold climates, glaciers often carry sediment.

Topography

Imagine you are building a farm. You can buy land in a wide, flat field, or on a narrow, steep mountain. Which location would you pick?

The topography of land has a significant effect on soil formation.

Fertile soil tends to form in areas with moderate or gentle slopes.

Soil formed on steep slopes, like a mountain, is usually poor in quality.

Remember that erosion transports sediment from one location to another.

Water flowing down a hill or mountain drops off larger sediments first. Smaller particles carry over a longer distance and deposit further away. As a result, coarse, infertile soil usually occurs near the base of a mountain. We usually find fine, fertile soil farther away from the base.

Living Organisms in Soil

Have you ever added fertilizer to a garden, or planted trees in your backyard?

Humans can have a large effect on the formation of soil. By adding fertilizers in the soil, the nutrients in the fertilizer can change the

chemical composition of original soil. Plant roots create channels in soil, which can open pathways into the denser "B"

horizon. When living organisms die, they decompose and return organic nutrients to the

ground. Then other organisms that are found in soil break down these organic matters

and use the nutrients to live and grow. A soil food web is created because soil organisms rely on each other for survival. The cycling of nutrients between living organisms and the ground helps

determine the fertility of soil. There are several types of living organisms that can be found in soil, such as

bacteria, fungus, arthropods, and nematodes. Bacteria and fungus are essential in decomposing organic matter and complex

organic compound in the soil. Arthropods and nematodes help by causing bacteria and fungus to be distributed

throughout the soil as they move. Each of these organisms plays a vital role in creating a fertile soil.

Living organisms also affect the rate of biological weathering.

The higher the number of living organisms in an area, the greater will be the rate of weathering.

Living organisms need water to survive. For example, dry areas like deserts have very little topsoil because of the limited number of living organisms available to weather rock and contribute organic matter.

Parent Material

The type of rock weathered is the parent material. Remember that rocks are made of minerals. Some minerals weather more quickly than others do.

Minerals formed under high temperature and pressure are the most easily weathered.

The most difficult minerals to weather are those formed under low temperatures and pressures.

The chemical composition of parent material has a direct impact on the fertility of soil.

Materials rich in calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) weather into fertile soil. These elements dissolve easily in water, which makes them readily available to growing plants. Other materials release hydrogen ions (H+), which makes soil acidic and infertile.

You have learned that soil is made of weathered rock, water, air, and organic material. The size of particles is an important soil characteristic. According to the United States Department of Agriculture (USDA), there are three main particle sizes—sand, silt, and clay.

Clay is particles smaller than 0.002 mm. Sand is particles larger than 0.05 mm. Particles in between are silt.

The size of particles in soil determines how much water the soil will hold. Small particles hold more water than large particles.

Soil usually settles into different layers called horizons. The arrangement of horizons is the soil profile.