Name: _____________________________________Date: __________________Period: ______ Unit 9: Earth’s Destructive Forces

All About Soil A. Soil Formation

Objective: Describe the different components of soil and explain how weathering results in the formation of soil.

Soil is a mixture of weathered rock and organic matter that usually covers bedrock. Bedrock is the solid rock that underlies all soil. Organic matter includes the remains of plants and animals that have been broken down by bacteria. This decayed organic material is called humus. Humus [pronounced hyoo-mus] should not be confused with hummus! The ingredients of soil include minerals, air, water, and organic material. About 50% of a soil’s volume is minerals and organic matter. The other 50% is just space that can be filled with oxygen and other gases or with water. The development of a soil is the result of both chemical and mechanical weathering.

• Chemical weathering turns hard minerals into soft ones. • Mechanical weathering breaks solid rock into smaller pieces. • Plant and animals add organic materials in the form of waste products and dead organisms. • The decay of organic matter produces acids, which accelerate chemical weathering. • Burrowing animals, such as earthworms, insects, and rodents, help circulate air and water

through the soil and mix mineral and organic remains.

The material from which soil forms is called its parent material. Soil that has weathered directly from the bedrock beneath it and therefore matches its parent material is called residual soil. Soil that does not match the bedrock it is over is called transported soil. It did not weather from bedrock beneath it but was brought there by agents of erosion. Much of New England and the Midwest are covered by soil that was deposited by the movement of glaciers after the last Ice Age. Test Yourself: Circle the right choice.

1. Soil is formed from… a. Weathered rock and decayed remains of plants and animals. b. Oxidation and rock particles c. Horizons and silt

2. Humus is… a. Mineral Fragments b. Gravel c. Decayed organic matter

3. A soil that matches its parent material is called a (residual, transported) soil.

B. Soil Texture Objective: Describe the different particle sizes that make up soil and how to interpret a soil triangle diagram. The size of the rock particles in soils can vary. Coarse particles are called gravel. Smaller particles are called sand. Silt and clay are made out of rock particles that are very small, mostly too small to see without a magnifying glass. Soils different in the amount of gravel, sand, silt, and clay they contain. The soil texture is determined by the amount of each that is present in the soil. To determine which type of soil is present at a location, scientists use the soil texture triangle chart. Each side of the triangle represents one of the main soil components; clay, silt, and sand. Each side of the triangle has a scale from 0% to 100%. Let’s use an example to learn how to read the soil texture diagram. We are going to classify a soil sample that is 30% clay, 15% silt, and 55% sand.

• First locate 30% on the clay axis, and draw a line horizontally from left to right. • Then locate 15% on the silt axis, and draw a line going down diagonally to the left. • Finally locate 55% on the sand axis, and draw a line going up diagonally to the left.

The intersection point will fall in a region of the diagram. The region label is the name of our soil. What is our example soil texture called? ____________________________

Test Yourself: Use the soil triangle diagram on the next page to determine the type of soils for the following combinations. 1. 60% clay, 20% silt, and 20% sand ___________________________

2. 15% clay, 40% silt, and 45% sand ___________________________

3. 30% clay, 60% silt, and 10% sand ___________________________

C. Soil Profile

Objective: Describe the different layers that develop in a soil and the factors that influence the formation of these layers. A cross-section of soil exposed by digging is called the soil profile. The weathering of soil produces layers known as soil horizons. The O Horizon is decomposing organic material at the top of the soil. The A-horizon, which is also called topsoil, is usually rich in dark-colored humus. The B-horizon is often referred to as the subsoil. This layer does not contain any humus. Instead, it is usually composed of tightly packed clay. The B-horizon contains minerals that have been transported by groundwater from the topsoil through a process known as leaching. The C-horizon contains partially weathered bedrock. This layer sits on top of the solid bedrock, sometimes referred to as the R-horizon, which acts as the parent material. It takes thousands of years to form soil with well-developed horizons.

In a mature soil, all of the horizons will be present, while in an immature soil only two layers will be present. In addition to time, parent material, and living organism, climate, which refers to the general weather patterns of an area, has a large impact on soil development. For example:

• Polar regions have thin or indistinct horizons. • Desert regions have little to no humus and a small A-horizon. Because of evaporation, there

can be lots of salts and gypsum present. • Tropical regions have very little humus or nutrients, and the soil tends to have a red color

because of oxidation. Topography affects how soil moves across the surface of the land. Steep slopes tend to have less soil, while valleys and meadows have thicker, more fertile soils. Summarize: Give at least one characteristic of each soil horizon. O-Horizon: _________________________________________________________________________

A-Horizon: _________________________________________________________________________

B-Horizon: _________________________________________________________________________

C-Horizon: _________________________________________________________________________

R-Horizon: _________________________________________________________________________

Complete the blanks below with each of the following words.

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Factors that Build Soil

Word Choices: • Climate • Living Organisms • Topography • Time • Parent Material