weathering and soil -...

180

Weathering and Soil

Andrew Brown, Ecoscene/CORBIS

Soil is a natural resourcethat must be monitored,managed, and protected.

SECTION 1WeatheringMain Idea Weatheringprocesses weaken andbreak apart rock materialinto smaller pieces.

SECTION 2The Nature of SoilMain Idea Soil is a mixtureof weathered rock, decayedorganic matter, mineralfragments, water, and air.

SECTION 3Soil ErosionMain Idea Soil erosion isharmful because plants donot grow as well when top-soil has been removed.

What’s a tor?A tor, shown in the photo, is a pile of boulders left on the land.Tors form because of weathering, which is a natural process thatbreaks down rock. Weathering weakened the rock that used to bearound the boulders. This weakened rock then was eroded away,and the boulders are all that remain.

Write a poem about a tor. Use words in your poem that rhymewith the word tor.Science Journal

181

Start-Up Activities

Stalactites and Stalagmites During weathering, minerals can be dissolvedby acidic water. If this water seeps into acave, minerals might precipitate. In this lab,you will model the formation of stalactitesand stalagmites.

1. Pour 700 mL of water into two 1,000-mLbeakers and place the beakers on a largepiece of cardboard. Stir Epsom salt intoeach beaker until no more will dissolve.

2. Add two drops of yellow food coloring toeach beaker and stir.

3. Measure and cut three 75-cm lengths ofcotton string. Hold the three pieces ofstring in one hand and twist the ends of allthree pieces to form a loose braid of string.

4. Tie each end of the braid to a large steel nut.

5. Soak the braid of string in one of thebeakers until it is wet with the solution.Drop one nut into one beaker and theother nut into the second beaker. Allowthe string to sag between the beakers.Observe for several days.

6. Think Critically Record your observa-tions in your Science Journal. How doesthis activity model the formation of stalactites and stalagmites?

Weathering and Soil Makethe following Foldable to helpyou understand the vocabularyterms in this chapter.

Fold a vertical sheet of notebookpaper from side to side.

Cut along every third line of only thetop layer to form tabs.

Label each tab.

Build Vocabulary As you read the chapter, listthe vocabulary words about weathering and soilon the tabs. As you learn the definitions, writethem under the tab for each vocabulary word.

STEP 3

STEP 2

STEP 1

Preview this chapter’s contentand activities at earth.msscience.com

Andrew Brown, Ecoscene/CORBIS

http://www.glencoe.com

182 A CHAPTER 7 Weathering and Soil

Apply It! Before you read, skimthe questions in the Chapter Review. Choosethree questions and predict the answers.

Learn It! A prediction is an educated guess based on whatyou already know. One way to predict while reading is to guess what youbelieve the author will tell you next. As you are reading, each new topicshould make sense because it is related to the previous paragraph or passage.

Practice It! Read the excerpt below from section. Basedon what you have read, make predictions about what you will read in therest of the lesson. After you read section, go back to your predictions to seeif they were correct.

Different regions on Earth haveDeserts are dry, prairies are semidry,

and temperate forests are mild and moist.These places also have

affectthe quality of soils. Soils in deserts contain lit-tle organic material and are thinner than soilsin wetter climates. Prairie soils have thick, darkA horizons because the grasses that grow therecontribute lots of organic matter.

—from page 193

Soil temperature and moisture contentdifferent types of soils.

climates.different

Think about howyou would describedifferent climatesin different regionson Earth.

Predict some dif-ferent types of soilsfor different places.What factors mightproduce differenttypes of soils?

Determine how soiltemperature andmoisture contentcould affect thequality of soils.

182 B

As you read, check the predic-

tions you made to see if they

were correct.Use this to focus on the main ideas as you read the chapter.

Before you read the chapter, respond to the statements

below on your worksheet or on a numbered sheet of paper.

• Write an A if you agree with the statement.

• Write a D if you disagree with the statement.

After you read the chapter, look back to this page to see if you’ve

changed your mind about any of the statements.

• If any of your answers changed, explain why.

• Change any false statements into true statements.

• Use your revised statements as a study guide.

Before You Read Statement After You ReadA or D A or D

1 Weathering breaks rocks into smaller and smallerpieces, such as sand, silt, or clay.

2 Exposure to atmospheric water and gases causesrocks to change chemically.

3 Soil is a mixture of weathered rock, decayedorganic matter, mineral fragments, water, and air.

4 Because of weathering, new soil is usually pro-duced rapidly in all regions on Earth.

5 The different layers of soil are called horizons.

6 Climate does not affect the type of soil producedin Earth’s different regions.

7 Most plants grow well when topsoil erodes.

8 In tropical, deforested areas, soil is useful to farm-ers for only a few years before the topsoil is gone.

9 Contour farming is a practice of planting crops inlarge, circular mounds.

Print out a worksheetof this page at earth.msscience.com

Print out a worksheetof this page at earth.msscience.com


182 CHAPTER 7 Weathering and Soil

Weathering and Its EffectsCan you believe that tiny moss plants, a burrowing vole

shrew, and even oxygen in the air can affect solid rock? Thesethings weaken and break apart rock at Earth’s surface. Surfaceprocesses that work to break down rock are called weathering.

Weathering breaks rock into smaller and smaller pieces,such as sand, silt, and clay. These particles are called sediment.The terms sand, silt, and clay are used to describe specific parti-cle sizes, which contribute to soil texture. Sand grains are largerthan silt, and silt is larger than clay.

Soil texture influences virtually all mechanical and chemicalprocesses in the soil, including the ability to hold moisture andnutrients.

Over millions of years, weathering has changed Earth’s surface. The process continues today. Weathering wearsmountains down to hills, as shown in Figure 1. Rocks at thetop of mountains are broken down by weathering, and thesediment is moved downhill by gravity, water, and ice.Weathering also produces strange rock formations like thoseshown at the beginning of this chapter. Two different types ofweathering—mechanical weathering and chemical weather-ing—work together to shape Earth’s surface.

■ Explain how mechanical weath-ering and chemical weatheringdiffer.

■ Describe how weathering affectsEarth’s surface.

■ Explain how climate affectsweathering.

Through time, weathering turnsmountains into sediment.

Review Vocabularysurface area: the area of a rock orother object that is exposed tothe surroundings

New Vocabulary ● weathering● mechanical weathering● ice wedging● chemical weathering● oxidation● climate

Weathering

Figure 1 Over long periods oftime, weathering wears mountainsdown to rolling hills.Explain how this occurs.

Mechanical WeatheringMechanical weathering occurs when rocks are broken apart

by physical processes. This means that the overall chemicalmakeup of the rock stays the same. Each fragment has charac-teristics similar to the original rock. Growing plants, burrowing animals, and expanding ice are some of the things that canmechanically weather rock. These physical processes produceenough force to break rocks into smaller pieces.

What can cause mechanical weathering?

Plants and Animals Water and nutrientsthat collect in the cracks of rocks result inconditions in which plants can grow. As theroots grow, they enlarge the cracks. You’veseen this kind of mechanical weathering ifyou’ve ever tripped on a crack in a sidewalknear a tree, as shown in Figure 2. Sometimestree roots wedge rock apart, also shown inFigure 2.

Burrowing animals also cause mechanicalweathering, as shown in Figure 3. As theseanimals burrow, they loosen sediment andpush it to the surface. Once the sediment isbrought to the surface, other weatheringprocesses act on it.

183

Figure 3 Small animalsmechanically weather rock whenthey burrow by breaking apartsediment.

Figure 2 Growing tree roots can be agents of mechanical weathering.

Tree roots also can grow into cracks and break rock apart.Tree roots can crack a sidewalk.

183

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Ice Wedging A mechanical weathering processcalled ice wedging is shown in Figure 4. Ice wedgingoccurs in temperate and cold climates where waterenters cracks in rocks and freezes. Because waterexpands when it turns to ice, pressure builds up in thecracks. This pressure can extend the cracks and breakapart rock. The ice then melts, allowing more water toenter the crack, where it freezes and breaks the rockeven more. Ice wedging is most noticeable in themountains, where warm days and cold nights arecommon. It is one process that wears down mountainpeaks. This cycle of freezing and thawing not onlybreaks up rocks, but also can break up roads and high-ways. When water enters cracks in road pavement andfreezes, it forces the pavement apart. This causes pot-holes to form in roads.

Surface Area Mechanical weathering by plants,animals, and ice wedging reduces rocks to smallerpieces. These small pieces have more surface area thanthe original rock body, as shown in Figure 5. As theamount of surface area increases, more rock is exposedto water and oxygen. This speeds up a different type ofweathering called chemical weathering, which contin-ues to reduce the particle size of sediments from acoarse to a finer texture.

Figure 4 When waterenters cracks in rock andfreezes, it expands, causingthe cracks to enlarge and the rock to break apart.

Figure 5 As rock is brokenapart by mechanical weather-ing, the amount of rock surfaceexposed to air and waterincreases. The backgroundsquares show the total number of surfaces exposed.

W. Perry Conway/CORBIS

SECTION 1 Weathering 185

Chemical WeatheringThe second type of weathering, chemical weathering, occurswhen chemical reactions dissolve or alter the minerals in rocks orchange them into different minerals. This type of weatheringoccurs at or near Earth’s surface and changes the chemical com-position of the rock, which can weaken the rock.

Natural Acids Naturally formed acids can weather rocks.When water reacts with carbon dioxide in the air or soil, a weakacid, called carbonic acid, forms. Carbonic acid reacts with min-erals such as calcite, which is the main mineral that makes uplimestone. This reaction causes the calcite to dissolve. Over manythousands of years, carbonic acid has weathered so much lime-stone that caves have formed, as shown in Figure 6.

Chemical weathering also occurs when naturally formedacids come in contact with other rocks. Over a long period oftime, the mineral feldspar, which is found in granite, some typesof sandstone, and other rocks, is broken down into a clay mineralcalled kaolinite (KAY oh luh nite). Kaolinite clay is common insome soils. Clay is an end product of weathering.

How does kaolinite clay form?

Plant Acids Some roots and decaying plants give off acids thatalso dissolve minerals in rock. When the minerals dissolve, therock is weakened. Eventually, the rock breaks into smaller pieces.As the rock weathers, nutrients become available to plants.

Figure 6 Caves form whenslightly acidic groundwater dissolveslimestone.Explain why the groundwater isacidic.

Topic: ChemicalWeatheringVisit for Weblinks to information aboutchemical weathering.

Activity List different types ofchemical weathering. Next to eachtype, write an effect that you haveobserved.

earth.msscience.com

Carbon dioxide � Water � Carbonic acidCarbonic acid dissolves limestone.Carbon dioxide � Water � Carbonic acidCarbonic acid dissolves limestone.

Hans Strand/Stone



pH Scale The strength of acids and basesis measured on the pH scale with a range of

0 to 14. On this scale, 0 is extremely acidic, 14 is extremely basic oralkaline, and 7 is neutral. Most minerals are more soluble in acidicsoils than in neutral or slightly alkaline soils. Different plants growbest at different pH values. For example, peanuts grow best in soilsthat have a pH of 5.3 to 6.6, while alfalfa grows best in soils having apH of 6.2 to 7.8.

Oxygen Oxygen also causes chemical weathering. Oxidation(ahk sih DAY shun) occurs when some materials are exposed tooxygen and water. For example, when minerals containing ironare exposed to water and the oxygen in air, the iron in the min-eral reacts to form a new material that resembles rust. One com-mon example of this type of weathering is the alteration of theiron-bearing mineral magnetite to a rustlike material calledlimonite, as shown in Figure 7. Oxidation of minerals gives somerock layers a red color.

How does oxygen cause weathering?

Effects of ClimateClimate affects soil temperature and moisture and also affects the

rate of mechanical and chemical weathering. Climate is the patternof weather that occurs in a particular area over many years. In coldclimates, where freezing and thawing are frequent, mechanicalweathering rapidly breaks down rock through the process of icewedging. Chemical weathering is more rapid in warm, wet climates.High temperatures tend to increase the rate of chemical reactions.Thus, chemical weathering tends to occur quickly in tropical areas.Lack of moisture in deserts and low temperatures in polar regionsslow down chemical weathering.

Figure 7 Iron-containing mineralslike the magnetite shown here canweather to form a rustlike materialcalled limonite. Explain how this is similar to rustforming on your bicycle chain.

Magnetite Limonite

Observing Chemicaland MechanicalWeatheringProcedure1. Collect and rinse two hand-

fuls of common rock orshells.

2. Place equal amounts of rockinto two plastic bottles.

3. Fill one bottle with water tocover the rock and seal witha lid.

4. Cover the rock in the secondbottle with lemon juice andseal.

5. Shake both bottles for tenminutes.

6. Tilt the bottles so you canobserve the liquids in each.

Analysis1. Describe the appearance of

each liquid.2. Explain any

differences.

(tl)Craig Kramer, (tr)A.J. Copley/Visuals Unlimited, (bl br)John Evans

SECTION 1 Weathering 187

Effects of Rock Type Rock type also can affect the rate ofweathering in a particular climate. In wet climates, for example,marble weathers more rapidly than granite, as shown in Figure 8.

The weathering of rocks and the process of soil formationalter rock minerals so that soil minerals are mostly inheritedfrom the parent rock type. Weathering begins the process offorming soil from rock and sediment and also affects particlesize and soil texture. Recall that sand, silt, and clay simplydescribe the different particle sizes of the soil's mineral content.

Figure 8 Different types of rockweather at different rates. Inhumid climates, marble statuesweather rapidly and become dis-colored. Granite statues weathermore slowly.

Self Check1. Describe how weathering reduces the height of

mountains through millions of years.

2. Explain how both tree roots and prairie dogs mechani-cally weather rock.

3. Summarize the effects of carbonic acid on limestone.

4. Describe how climate affects weathering.

5. Think Critically Why does limestone often form cliffs in dry climates but rarely form cliffs in wet climates?

SummaryWeathering and Its Effects

• Weathering includes processes that breakdown rock.

• Weathering affects Earth’s landforms.

Mechanical Weathering

• During mechanical weathering, rock is brokenapart, but it is not changed chemically.

• Plant roots, burrowing animals, and expand-ing ice all weather rock.

Chemical Weathering

• During chemical weathering, minerals in rock dissolve or change to other minerals.

• Agents of chemical weathering include natural acids and oxygen.

6. Venn Diagram Make a Venn diagram to compare and contrast mechanical weathering and chemicalweathering. Include the causes of mechanical and chemical weathering in your diagram.

Marble statue Granite statue

earth.msscience.com/self_check_quiz(l)William Johnson/Stock Boston, (r)Runk/Schoenberger from Grant Heilman



The Nature of Soil

Figure 9 Five different factorsaffect soil formation.Explain how time influences thedevelopment of soils.

5. Amount of time rock has been weathering

3. Types of rock 4. Types of vegetation

1. Climate 2. Slope of land

Factors Affecting Soil Formation

Formation of SoilThe word ped is from a Greek word that means “ground” and

from a Latin word that means “foot.” The pedal under your foot,when you're bicycling, is named from the word ped. The part ofEarth under your feet, when you're walking on the ground, is thepedosphere, or soil. Soil science is called pedology.

What is soil and where does it come from? A layer of rockand mineral fragments produced by weathering covers the sur-face of Earth. As you learned in Section 1, weathering graduallybreaks rocks into smaller and smaller fragments. However, thesefragments do not become high-quality soil until plants and ani-mals live in them. Plants and animals add organic matter, theremains of once-living organisms, to the rock fragments.Organic matter can include leaves, twigs, roots, and dead wormsand insects. Soil is a mixture of weathered rock, decayed organicmatter, mineral fragments, water, and air.

Soil can take thousands of years to form and ranges from60 m thick in some areas to just a few centimeters thick in oth-ers. Climate, slope, types of rock, types of vegetation, andlength of time that rock has been weathering all affect the for-mation of soil, as shown in Figure 9. For example, differentkinds of soils develop in tropical regions than in polar regions.Soils that develop on steep slopes are different from soils thatdevelop on flat land. Figure 10 illustrates how soil developsfrom rock.

■ Explain how soil forms.■ Describe soil characteristics.■ Describe factors that affect the

development of soil.

Much of the food that you eat isgrown in soil.

Review Vocabularyprofile: a vertical slice throughrock, sediment, or soil

New Vocabulary

• soil • soil profile

• humus • litter

• horizon • leaching

NGS TITLE

Figure 10

VISUALIZING SOIL FORMATION

SECTION 2 The Nature of Soil 189

I t may take thousands of years to form, but soil is con-stantly evolving from solid rock, as this series of illus-trations shows. Soil is a mixture of weathered rock,

mineral fragments, and organic material—the remainsof dead plants and animals—along with water and air.

Natural acids in rainwaterweather the surface of exposed bedrock. Water canalso freeze in cracks, causing rocks to fracture andbreak apart. The inset photo shows weathered rockin the Tien Shan Mountains of Central Asia.

A Plants take root in the cracks and amongbits of weathered rock—shown in the insetphoto above. As they grow, plants, along withother natural forces, continue the process of breaking down rocks, and a thin layer of soilbegins to form.

B

As organic matter increases and underlyingbedrock continues to break down, the soil layerthickens. Rich topsoil supports trees and otherplants with large root systems.

DLike the grub in the insetphoto, insects, worms, and otherliving things take up residence amongplant roots. Their wastes, along with deadplant material, add organic matter to thesoil.

C

(t)James D. Balog, (c)Martin Miller, (b)Steven C. Wilson/Entheos, (bkgd)Stephen R. Wagner

A horizon

B horizon

C horizon

ComparingComponents of Soil

Procedure1. Complete a safety

worksheet.2. Collect a sample of soil.3. Observe it closely with a

magnifying lens.4. Record evidence of plant

and animal componentsand their activities.

Analysis1. Describe the different par-

ticles found in your sam-ple. Did you find anyremains of organisms?

2. Explain how living organ-isms might affect the soil.

3. Compare and contrast yoursample with those otherstudents have collected.

Composition of SoilSoil is made up of rock and mineral fragments, organic

matter, air, and water. The rock and mineral fragments comefrom rocks that have been weathered. Most of these fragmentsare small particles of sediment such as clay, silt, and sand.

Most organic matter in soil comes from plants. Plant leaves,stems, and roots all contribute organic matter to soil. Animalsand microorganisms provide additional organic matter whenthey die. After plant and animal material gets into soil, fungi andbacteria cause it to decay. The decayed organic matter turns intoa dark-colored material called humus (HYEW mus). Humusserves as a source of nutrients for plants. As worms, insects, androdents burrow throughout soil, they mix the humus with thefragments of rock. Good-quality surface soil has approximatelyequal amounts of humus and weathered rock material.

Water Infiltration Soil has many small spaces between indi-vidual soil particles that are filled with water or air. When soil ismoist, the spaces hold the water that plants need to grow.During a drought, the spaces are almost entirely filled with air.When water soaks into the ground, it infiltrates the pores.Infiltration rate is determined by calculating the time it takes forwater sitting on soil to drop a fixed distance. This rate changesas the soil pore spaces fill with water.

Figure 11 This soil, which developed beneath agrassy prairie, has three main horizons.Describe how the A horizon is different from theother two horizons.

(l)Bonnie Heidel/Visuals Unlimited, (r)John Bova/Photo Researchers

Soil ProfileYou have seen layers of soil if you’ve ever

dug a deep hole or driven along a road thathas been cut into a hillside. You probablyobserved that most plant roots grow in thetop layer of soil. The top layer typically isdarker than the soil layers below it. These dif-ferent layers of soil are called horizons. All thehorizons of a soil form a soil profile. Mostsoils have three horizons—labeled A, B, andC, as shown in Figure 11.

Figure 12 Leaching removesmaterial from the upper layer ofsoil. Much of this material then isdeposited in the B horizon.

Soil Fertility Plants need avariety of nutrients forgrowth. They need thingslike nitrogen, phosphorus,potassium, sulfur, calcium,and magnesium calledmacronutrients They getthese nutrients from theminerals and organic mate-rial in soil. Fertile soil sup-plies the nutrients thatplants need in the properamounts. Soil fertility usu-ally is determined in a labo-ratory by a soil chemist.However, fertility some-times can be inferred bylooking at plants. Doresearch to discover moreimportant plant nutrients.


A Horizon The A horizon is the top layerof soil. In a forest, the A horizon might becovered with litter. Litter consists ofleaves, twigs, and other organic materialthat can be changed to humus by decom-posing organisms. Litter helps preventerosion and evaporation of water fromsoil. The A horizon also is known as top-soil. Topsoil has more humus and fewerrock and mineral particles than the otherlayers in a soil profile. The A horizon gen-erally is dark and fertile. The dark color ofthe soil is caused by the humus, whichprovides nutrients for plant growth.

Since dark color absorbs solar energymore readily, soil color can greatly affect soiltemperature. Darker color also may indicatea higher content of soil moisture. Soil mois-ture and soil temperature are important indetermining seed germination for plants andthe vitality of decomposing organisms.

B Horizon The layer below the A horizon is the B horizon,also known as subsoil. Because less organic matter is added tothis horizon, it is lighter in color than the A horizon and con-tains less humus. As a result, the B horizon is less fertile. The Bhorizon contains material moved down from the A horizon bythe process of leaching.

Leaching is the removal of minerals that have been dissolvedin water. The process of leaching resembles making coffee in adrip coffeemaker. In soil, water seeps through the A horizon andreacts with humus and carbon dioxide to form acid. The aciddissolves some of the minerals in the A horizon and carries thematerial into the B horizon, as shown in Figure 12.

How does leaching transport material fromthe A horizon to the B horizon?

C Horizon The C horizon consists of partially weatheredrock and is the bottom horizon in a soil profile. It is often thethickest soil horizon. This horizon does not contain muchorganic matter and is not strongly affected by leaching. It usu-ally is composed of coarser sediment than the soil horizonsabove it. What would you find if you dug to the bottom of theC horizon? As you might have guessed, you would find rock—the rock that gave rise to the soil horizons above it. This rockis called the parent material of the soil. The C horizon is thesoil layer that is most like the parent material.


Soil Structure Individual soil particles clump together.Examine soil closely and you will see natural clumps called peds.Soil structure affects pore space and will affect a plant's ability topenetrate roots. Figure 13 shows four classes of soil structure.Granular structures are common in surface soils with highorganic content that glues minerals together. Earthworms, frost,and rodents mix the soil, keeping the peds small, which providesgood porosity and movement of air and water. Platy structuresare often found in subsurface soils that have been leached orcompacted by animals or machinery. Blocky structures are com-mon in subsoils or surface soils with high clay content, whichshrinks and swells, producing cracks. Prismatic structures,found in B horizons, are very dense and difficult for plant rootsto penetrate. Vertical cracks result from freezing and thawing,wetting and drying, and downward movement of water androots. Soil consistency refers to the ability of peds and soil par-ticles to stick together and hold their shapes.

Calculate Percentages

1. Calculate the percentage of sand in the sample.

2. Calculate the percentage of silt in the sample.

SOIL TEXTURE Some soil is coarse, some is fine. This propertyof soil is called soil texture. The texture of soil often is deter-mined by finding the percentages of sand, silt, and clay.Calculate the percentage of clay shown by the circle graph.

Solution

This is what you know:

This is what you needto find:

This is the procedureyou need to use:

● sand weight: 20 g

● clay weight: 15 g

● silt weight: 15 g

● total weight of the sample

● percentage of clay particles

● Add all the masses to determine the total sample mass:20 g sand � 15 g silt � 15 g clay � 50 g sample

● Divide the clay mass by the sample mass; multiply by100:15 g clay/50 g sample � 100 � 30% clay in the sample

15 gClay particles

15 gSilt particles

20 gSand particles

For more practice, visit earth.msscience.com/math_practice

prismatic blocky

platy granular

Figure 13 Four major classescharacterize soil structure.



Soil TypesIf you travel across the country, you will notice that not all

soils are the same. Some are thick and red. Some are brown withhard rock nodules, and some have thick, black A horizons. Theyvary in color, depth, texture, fertility, pH, temperature, andmoisture content. Many soils exist, as shown in Figure 14.

Soil Types Reflect Climate Different regions on Earthhave different climates. Deserts are dry, prairies are semidry, andtemperate forests are mild and moist. These places also have dif-ferent types of soils. Soil temperature and moisture contentaffect the quality of soils. Soils in deserts contain little organicmaterial and also are thinner than soils in wetter climates.Prairie soils have thick, dark A horizons because the grasses thatgrow there contribute lots of organic matter. Temperate forestsoils have less organic matter and thinner A horizons thanprairie soils do. Other regions such as tundra and tropical areasalso have distinct soils.

Figure 14 The United States hasmany different soil types. Theyvary in color, depth, texture, andfertility.Identify the soil type in yourregion.

ArcticMountainDesert

PrairieGlacialWetlands

RiverTemperateTropical

45°

40°

35°

30°

20°

60°


Self Check1. List the five factors that affect soil development.

2. Explain how soil forms.

3. Explain why A horizons often are darker than B horizonsor C horizons.

4. Describe how leaching affects soil.

5. Think Critically Why is a soil profile in a tropical rainforest different from one in a desert? A prairie?

SummaryFormation of Soil

• Soil is a mixture of rock and mineral fragments,decayed organic matter, water, and air.

Composition of Soil

• Organic matter gradually changes to humus.

• Soil moisture is important for plant growth.

Soil Profile

• The layers in a soil profile are called horizons.

• Most soils have an A, B, and C horizon.

Soil Types

• Many different types of soils occur in theUnited States.

• Climate and other factors determine the typeof soil that develops.

6. Use Statistics A farmer collected five soil samplesfrom a field and tested their acidity, or pH. His datawere the following: 7.5, 8.2, 7.7, 8.1, and 8.0.Calculate the mean of these data. Also, determine therange and median.

Other Factors Parent rockmaterial affects soils thatdevelop from it. Clay soilsdevelop on rocks like basalt,because minerals in the rockweather to form clay. Rock typealso affects vegetation, becausedifferent rocks provide differentamounts of nutrients.

Soil pH, controls manychemical and biological activi-ties that take place in soil.Activities of organisms, acidrain, or land management prac-tices could affect soil quality.

Time also affects soil devel-opment. If weathering has beenoccurring for only a short time,

the parent rock determines the soil characteristics. As weatheringcontinues, the soil resembles the parent rock less and less.

Slope also is a factor affecting soil profiles, as shown in Figure 15. On steep slopes, soils often are poorly developed,because material moves downhill before it can be weatheredmuch. In bottomlands, sediment and water are plentiful.Bottomland soils are often thick, dark, and full of organic material.

Figure 15 The slope of the landaffects soil development. Thin,poorly developed soils form onsteep slopes, but valleys oftenhave thick, well-developed soils.Infer why this is so.

earth.msscience.com/self_check_quiz


LAB 195

Soils have different amounts of different sizesof particles. When you determine how muchsand, silt, and clay a soil contains, you describethe soil’s texture.

Real-World QuestionWhat is the texture of your soil?

Goals■ Estimate soil texture by making a ribbon.

Materialssoil sample (100 g) water bottle

Safety Precautions

Procedure1. Take some soil and make it into a ball. Work

the soil with your fingers. Slowly add waterto the soil until it is moist.

2. After your ball of soil is moist, try to form athin ribbon of soil. Use the followingdescriptions to categorize your soil:a. If you can form a long, thin ribbon, you

have a clay soil.b. If you formed a long ribbon but it breaks

easily, you have a clay loam soil.c. If you had difficulty forming a long

ribbon, you have loam soil.

3. Now make your soil classification moredetailed by selecting one of these descriptions:a. If the soil feels smooth, add the word

silty to your soil name.b. If the soil feels slightly gritty, don’t add

any word to your soil name.c. If the soil feels very gritty, add the word

sandy before your soil name.

Conclude and Apply1. Classify Which texture class name did you

assign to your soil?

2. Observe Find your soil texture class nameon the triangle above. Notice that the cor-ners of the triangle are labeled sand, silt,and clay.

3. Determine Is your soil texture class close toone of the three corners or near the middleof the diagram? If your soil texture class isclose to a corner, which one?

4. Describe Does your soil contain mostlysand, silt, or clay, or does it have nearlyequal amounts of each? Hint: If your soilname is close to a corner, it has mostly thatsize of sediment. If your soil name is in themiddle of the triangle, it has nearly equalamounts of each sediment size.

Clay (less than 0.002 mm)

Sand(0.5 mm–2.0 mm)

Silt(0.002 mm–0.5 mm)

Clay

Clayloam

LoamSandy loam Silty loam

Sandyclay loam

Siltyclay loam

Sandyclay

Siltyclay

Soil Texture

Soil—An Important Resource While picnicking at a local park, a flash of lightning and a

clap of thunder tell you that a storm is upon you. Watching thepounding rain from the park shelter, you notice that the waterflowing off of the ball diamond is muddy, not clear. The flowingwater is carrying away some of the sediment that used to be onthe field. This process is called soil erosion. Soil erosion is harm-ful because plants do not grow as well when topsoil has beenremoved.

Causes and Effects of Soil ErosionSoil erodes when it is moved from the place where it formed.

Erosion occurs as water flows over Earth’s surface or when windpicks up and transports sediment. Generally, erosion is moresevere on steep slopes than on gentle slopes. It’s also more severein areas where there is little vegetation. Under normal condi-tions, a balance between soil production and soil erosion oftenis maintained. This means that soil forms at about the same rateas it erodes. However, humans sometimes cause erosion to occurfaster than new soil can form. One example is when peopleremove ground cover. Ground cover is vegetation that covers thesoil and protects it from erosion. When vegetation is cleared, asshown in Figure 16, soil erosion often increases.

■ Explain why soil is important.■ Evaluate ways that human activ-

ity has affected Earth’s soil.■ Describe ways to reduce soil

erosion.

If topsoil is eroded, soil becomesless fertile.

Review Vocabularyerosion: the picking up and mov-ing of sediment or soil

New Vocabulary

• no-till farming

• contour farming

• terracing

Soil Erosion

Trees protect the soil from erosion in forested regions. When forest is removed, soil erodes rapidly.

Figure 16 Removing vegetationcan increase soil erosion.

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SECTION 3 Soil Erosion 197

Agricultural Cultivation Soil erosion is a serious problemfor agriculture. Topsoil contains many nutrients, holds waterwell, and has a porous structure that is good for plant growth. Iftopsoil is eroded, the quality of the soil is reduced. For example,plants need nutrients to grow. Each year, nutrients are bothadded to the soil and removed from the soil. The differencebetween the amount of nutrients added and the amount ofnutrients removed is called the nutrient balance. If topsoilerodes rapidly, the nutrient balance might be negative. Farmersmight have to use more fertilizer to compensate for the nutrientloss. In addition, the remaining soil might not have the sameopen structure and water-holding ability that topsoil does.

Forest Harvesting When forests are removed, soil is exposedand erosion increases. This creates severe problems in many partsof the world, but tropical regions are especially at risk. Each year,thousands of square kilometers of tropical rain forest are clearedfor lumber, farming, and grazing, as shown in Figure 17. Soils intropical rain forests appear rich in nutrients but are almost infer-tile below the first few centimeters. The soil is useful to farmers foronly a few years before the topsoil is gone. Farmers then clear newland, repeating the process and increasing the damage to the soil.

Overgrazing In most places, land can be grazed with littledamage to soil. However, overgrazing can increase soil erosion.In some arid regions of the world, sheep and cattle raised forfood are grazed on grasses until almost no ground coverremains to protect the soil. When natural vegetation is removedfrom land that receives little rain, plants are slow to grow back.Without protection, soil is carried away by wind, and the mois-ture in the soil evaporates.

Figure 17 Tropical rain forestsoften are cleared by burning.Explain how this can increase soilerosion.

Topic: Land UseVisit for Web links to information abouthow land use affects Earth’s soiland about measures taken toreduce the impact.

Activity Debate with classmatesabout the best ways to protect richfarmland. Consider advantages anddisadvantages of each method.

earth.msscience.com

Soil Scientist Elvia Nieblais a soil scientist at the U.S.Environmental ProtectionAgency (EPA). Soil scien-tists at the EPA work toreduce soil erosion and pol-lution. Niebla’s researcheven helped keep ham-burgers safe to eat. How?In a report for the EPA, sheexplained how meat can becontaminated when cattlegraze on polluted soil.

Chip & Jill Isenhart/Tom Stack & Associates



Excess Sediment If soil ero-sion is severe, sediment candamage the environment. Severeerosion sometimes occurs whereland is exposed. Examples mightinclude strip-mined areas orlarge construction sites. Erodedsoil is moved to a new locationwhere it is deposited. If the sed-iment is deposited in a stream,as shown in Figure 18, thestream channel might fill.

Preventing Soil ErosionEach year more than 1.5 billion metric tons of soil are

eroded in the United States. Soil is a natural resource that mustbe managed and protected. People can do several things to con-serve soil.

Manage Crops All over the world, farmers work to slow soilerosion. They plant shelter belts of trees to break the force of thewind and plant crops to cover the ground after the main harvest.In dry areas, instead of plowing under crops, many farmersgraze animals on the vegetation. Proper grazing managementcan maintain vegetation and reduce soil erosion.

In recent years, many farmers have begun to practice no-till farming. Normally, farmers till or plow their fields one or more times each year. Using no-till farming, seen in

Figure 19, farmers leave plant stalks inthe field over the winter months. Atthe next planting, they seed crops with-out destroying these stalks and withoutplowing the soil. Farm machinerymakes a narrow slot in the soil, and theseed is planted in this slot. No-tillfarming provides cover for the soilyear-round, which reduces waterrunoff and soil erosion. One studyshowed that no-till farming can leaveas much as 80% of the soil covered byplant residue. The leftover stalks alsokeep weeds from growing in the fields.

How can farmersreduce soil erosion?

Figure 18 Erosion from exposedland can cause streams to fill withexcessive amounts of sediment.Explain how this could damagestreams.

Figure 19 No-till farmingdecreases soil erosion becausefields are not plowed.

(t)Dr. Russ Utgard, (b)Denny Eilers from Grant Heilman

SECTION 3 Soil Erosion 199

Self Check1. Explain why soil is important.

2. Explain how soil erosion damages soil.

3. Describe no-till farming.

4. Explain how overgrazing increases soil erosion.

5. Think Critically How does contour farming help watersoak into the ground?

SummarySoil—An Important Resource

• Soil erosion is a serious problem because top-soil is removed from the land.

Causes and Effects of Soil Erosion

• Soil erosion occurs rapidly on steep slopes and areas that are not covered by vegetation.

• The quality of farmland is reduced when soilerosion occurs.

Preventing Soil Erosion

• Farmers reduce erosion by planting shelterbelts, using no-till farming, and plantingcover crops after harvesting.

• Contour farming and terracing are used tocontrol erosion on slopes.

6. Communicate Do research to learn about the differ-ent methods that builders use to reduce soil erosionduring construction. Write a newspaper articledescribing how soil erosion at large construction sitesis being controlled in your area.

Reduce Erosion on Slopes On gentleslopes, planting along the natural contoursof the land, called contour farming,reduces soil erosion. This practice, shownin Figure 20, slows the flow of water downthe slope and helps prevent the formationof gullies.

Where slopes are steep, terracing oftenis used. Terracing (TER uh sing) is amethod in which steep-sided, level toppedareas are built onto the sides of steep hillsand mountains so that crops can begrown. These terraces reduce runoff bycreating flat areas and shorter sections ofslope. In the Philippines, Japan, China, andPeru, terraces have been used for centuries.

Reduce Erosion of Exposed Soil A variety of methodsare used to control erosion where soil is exposed. During theconstruction process water is sometimes sprayed onto bare soilto prevent erosion by wind. When construction is complete,topsoil is added in areas where it was removed and trees areplanted. At strip mines, water flow can be controlled so thatmost of the eroded soil is kept from leaving the mine. Aftermining is complete, the land is reclaimed. This means that steepslopes are flattened and vegetation is planted.

Figure 20 This orchard wasplanted along the natural con-tours of the land.Summarize the benefits of usingcontour farming on slopes.

earth.msscience.com/self_check_quizGeorg Custer/Photo Researchers


Design Your OwnDesign Your Own

WEATHERING CHALK


Goals■ Design experiments to

evaluate the effects ofacidity, surface area,and temperature onthe rate of chemicalweathering of chalk.

■ Describe factors that affect chemicalweathering.

■ Explain how the chem-ical weathering of chalkis similar to the chemi-cal weathering of rocks.

Possible Materialspieces of chalk (6)small beakers (2)metric rulerwaterwhite vinegar (100 mL)hot platecomputer probe for

temperature*thermometer*Alternate materials

Safety Precautions

Wear safety goggles whenpouring vinegar. Be carefulwhen using a hot plate andheated solutions.

WARNING: If mixing liquids, always add acid to water.

Real-World QuestionChalk is a type of limestone made of the shells of microscopic organisms. Thefamous White Cliffs of Dover, England, are made up of chalk. This lab will help you understand how chalk can be chemi-cally weathered. How can you simulatechemical weathering of chalk?

Form a HypothesisHow do you think acidity, surface area, and temperature affect the rate of chemical weathering of chalk? What happens to chalk in water? What happens to chalk in acid (vinegar)? How will the size of the chalk pieces affect the rate of weathering?What will happen if you heat the acid? Make hypotheses to supportyour ideas.

(t)George H. Harrison from Grant Heilman, (b)Bob Daemmrich

Compare your results with those of yourclassmates. How were your data similar?How were they different? For more help,refer to the Science Skill Handbook.

LAB 201

Test Your HypothesisMake a Plan1. Develop hypotheses about the effects of acidity, surface area, and

temperature on the rate of chemical weathering.

2. Decide how to test your first hypothesis. List the steps needed totest the hypothesis.

3. Repeat step 2 for your other two hypotheses.

4. Design data tables in your Science Journal. Make one for acidity, onefor surface area, and one for temperature.

5. Identify what remains constant in your experiment and what varies.Change only one variable in each procedure.

6. Summarize your data in a graph. Decide from reading the ScienceSkill Handbook which type of graph to use.

Follow Your Plan1. Make sure your teacher approves your plan before you start.

2. Carry out the three experiments as planned.

3. While you are conducting the experiments, record your observationsand complete the data tables in your Science Journal.

4. Graph your data to show how each variable affected the rate ofweathering.

Analyze Your Data1. Analyze your graph to find out which substance—water or acid—

weathered the chalk more quickly. Was your hypothesis supported by your data?

2. Infer from your data whether the amount of surface area makes adifference in the rate of chemical weathering. Explain.

Conclude and Apply1. Explain how the chalk was chemically

weathered.

2. How does heat affect the rate of chemical weathering?

3. What does this imply about weathering in the tropics and in polar regions?

KS Studios

Leslie Marmon Silko, a woman of Pueblo, Hispanic,and American heritage, explains what ancient Pueblopeople believed about the circle of life on Earth.

You see that after a thing is dead, it dries up. Itmight take weeks or years, but eventually if youtouch the thing, it crumbles under your fingers. Itgoes back to dust. The soul of the thing has longsince departed. With the plants and wild game thesoul may have already been borne back into bonesand blood or thick green stalk and leaves. Nothing iswasted. What cannot be eaten by people or in someway used must then be left where other living crea-tures may benefit. What domestic animals or wildscavengers can’t eat will be fed to the plants. Theplants feed on the dust of these few remains.

. . . Corn cobs and husks, the rinds and stalks andanimal bones were not regarded by the ancient peopleas filth or garbage. The remains were merely resting ata mid-point in their journey back to dust. . . .

The dead become dust . . . . The ancient Pueblopeople called the earth the Mother Creator of all

things in this world. Her sister, the Corn mother,occasionally merges withher because all . . . green life rises out of the depths of the earth.

Rocks and clay . . . becomewhat they once were. Dust.

A rock shares this fatewith us and with animalsand plants as well.

Respond to the Reading1. What one word is repeated throughout

this passage?2. What effect does the repetition of this

word have on the reader?3. Linking Science and Writing Using

repetition, write a one-page paper on how to practice a type of soil conservation.

This chapter dis-cusses how weath-

ered rocks and mineral fragments combinewith organic matter to make soil. Silko’swriting explains how the ancient Pueblopeople understood that all living matterreturns to the earth, or becomes dust. Linessuch as “green life rises out of the depths ofthe earth,” show that the Pueblo peopleunderstood that the earth, or rocks and min-eral fragments, must combine with livingmatter in order to make soil and supportplant life.

UnderstandingLiteratureRepetition The recurrence of sounds,words, or phrases is called repetition.What is Silko’s purpose of the repeateduse of the word dust?


Landscape, History, and the Pueblo Imagination

by Leslie Marmon Silko

Larry Hamill

Copy and complete the following concept map about weathering.

Weathering

1. Weathering helps to shape Earth’s surface.

2. Mechanical weathering breaks apart rockwithout changing its chemical composition.Plant roots, animals, and ice wedging areagents of mechanical weathering.

3. Chemical weathering changes the chemical composition of rocks. Naturalacids and oxygen in the air can cause chemical weathering.

The Nature of Soil

1. Soil is a mixture of rock and mineral frag-ments, organic matter, air, and water.

2. A soil profile contains different layers thatare called horizons.

3. Climate, parent rock, slope of the land, typeof vegetation, and the time that rock hasbeen weathering are factors that affect thedevelopment of soil.

Soil Erosion

1. Soil is eroded when it is moved to a newlocation by wind or water.

2. Human activities can increase the rate ofsoil erosion.

3. Windbreaks, no-till farming, contour farm-ing, and terracing reduce soil erosion onfarm fields.

CHAPTER STUDY GUIDE 203

Type ofWeathering

Plant acid

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earth.msscience.com/interactive_tutor(l)Tom Bean/DRK Photo, (r)David M. Dennis/Earth Scenes


Fill in the blanks with the correct vocabularyword or words.

1. _________ changes the composition ofrock.

2. _________ forms from organic matter suchas leaves and roots.

3. The horizons of a soil make up the ________.

4. _________ transports material to the Bhorizon.

5. _________ occurs when many materials con-taining iron are exposed to oxygen and water.

6. _________ means that crops are plantedalong the natural contours of the land.

7. _________ is the pattern of weather thatoccurs in a particular area for many years.

Choose the word or phrase that best answers thequestion.

8. Which of the following can be caused byacids produced by plant roots?A) soil erosionB) oxidationC) mechanical weatheringD) chemical weathering

Use the graph below to answer question 9.

9. The above graph shows the percentage of clay in a soil profile at varying depths.Which depth has the highest amount of clay?A) 25 cm C) 50 cmB) 150 cm D) 100 cm

10. Which of the following is an agent ofmechanical weathering?A) animal burrowingB) carbonic acidC) leachingD) oxidation

11. In which region is chemical weatheringmost rapid?A) cold, dry C) warm, moistB) cold, moist D) warm, dry

12. What is a mixture of rock and mineral frag-ments, organic matter, air, and water called?A) soil C) horizonB) limestone D) clay

13. What is organic matter in soil?A) leaching C) horizonB) humus D) profile

14. What is done to reduce soil erosion onsteep slopes?A) no-till farmingB) contour farmingC) terracingD) grazing

Perc

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Clay Abundance

Depth (cm)

204 CHAPTER REVIEW

chemical weatheringp. 185

climate p. 186contour farming p. 199horizon p. 190humus p. 190ice wedging p. 184leaching p. 191litter p. 191

mechanical weatheringp. 183

no-till farming p. 198oxidation p. 186soil p. 188soil profile p. 190terracing p. 199weathering p. 182

earth.msscience.com/vocabulary_puzzlemaker


15. Predict which type of weathering—mechan-ical or chemical—you would expect to havea greater effect in a polar region. Explain.

16. Recognize Cause and Effect How does soil erosion reduce the quality of soil?

17. Concept Map Copy and complete the con-cept map about layers in soil.

18. Recognize Cause and Effect Why do rows oftrees along the edges of farm fields reducewind erosion of soil?

19. Form a Hypothesis A pile of boulders lies atthe base of a high-mountain cliff. Form ahypothesis explaining how the pile of rockmight have formed.

20. Test a Hypothesis How would you test yourhypothesis from question 19?

21. Identify a Question Many scientists are con-ducting research to learn more about howsoil erosion occurs and how it can bereduced. Write a question about soil ero-sion that you would like to research. Withyour teacher’s help, carry out an investiga-tion to answer your question.

22. Design a Landscape Find a slope in your areathat might benefit from erosion mainte-nance. Develop a plan for reducing ero-sion on this slope. Make a map showingyour plan.

23. Describing Peds Natural clumps of soil arecalled peds. Collect a large sample of top-soil. Describe the shape of the peds. Sketchthe peds in your Science Journal.

CHAPTER REVIEW 205

Soil

A horizon

has layers called

which include

earth.msscience.com/chapter_review

Use the illustration below to answer questions24–26.

24. Fertilizer Nutrients A bag of fertilizer is labeledto list the nutrients as three numbers. The num-bers represent the percentages of nitrogen,phosphate, and potash in that order. What arethe percentages of these nutrients for a fertil-izer with the following information on thelabel: 5-10-10?

25. Fertilizer Ratio The fertilizer ratio tells youthe proportions of the different nutrients in a fertilizer. To find the fertilizer ratio, divideeach nutrient value by the lowest value.Calculate the fertilizer ratio for the fertilizer in question 24.

26. Relative Amounts of Nutrients Which nutrientis least abundant in the fertilizer? Whichnutrients are most abundant? How manytimes more potash does the fertilizer containthan nitrogen?

Matt Meadows


Record your answers on the answer sheet providedby your teacher or on a separate sheet of paper.

Use the photo below to answer question 1.

1. Which method for reducing soil erosion isshown on the hillsides above?A. no-till farming C. contour farmingB. terracing D. shelter belts

2. Which of the following terms mightdescribe a soil’s texture?A. red C. porousB. coarse D. wet

3. Which soil horizon often has a dark colorbecause of the presence of humus?A. E horizon C. B horizonB. C horizon D. A horizon

4. Which of the following is an agent ofchemical weathering?A. ice wedgingB. burrowing animalsC. carbonic acidD. growing tree roots

5. Which of the following might damage asoil’s structure?A. a gentle rain C. earthwormsB. organic matter D. compaction

6. In which of the following types of rock arecaves most likely to form?A. limestone C. graniteB. sandstone D. basalt

7. Which of the following is most likely tocause erosion of farmland during a severedrought?A. water runoffB. soil creeping downhillC. windD. ice

Use the table below to answer questions 8–10.

8. According to the table, which horizon inthis soil has the lowest percentage of sand?A. A horizon C. C horizonB. B horizon D. R horizon

9. Which of the following is the R horizon?A. topsoil C. bedrockB. humus D. gravel

10. Which of the following is the best descrip-tion of the soil represented by the table?A. sandy C. clayeyB. silty D. organic

206 STANDARDIZED TEST PRACTICE

Come Back To It Never skip a question. If you are unsure of ananswer, mark your best guess on another sheet of paper andmark the question in your test booklet to remind you to comeback to it at the end of the test.

Texture Data for a Soil Profile

Horizon Percent

Sand Silt Clay

A 16.2 54.4 29.4

B 10.5 50.2 39.3

C 31.4 48.4 20.2

R (bedrock) 31.7 50.1 18.2

Georg Custer/Photo Researchers

STANDARDIZED TEST PRACTICE 207earth.msscience.com/standardized_test

Record your answers on the answer sheetprovided by your teacher or on a sheet of paper.

Use the illustration below to answer question 11.

11. Natural clumps of soil are called peds.Compare and contrast the different typesof peds in the sketch. Explain how thenames of the peds describe their shape.

12. Explain how caves form. What role doescarbonic acid have?

Use the diagram below to answer questions 13–15.

2003 Phosphorus Budget per AcreJones Family Farm

13. According to the diagram above, what wasthe total amount of phosphorus added toeach acre?

14. What is the total amount of phorphoruslost from each acre?

15. What is the difference between theamount of phosphorus added and theamount of phosphorus lost?

Record your answers on a sheet of paper.

16. Describe ways that humans affect Earth’ssoil. How can damage to soil be reduced?

17. How does weathering change Earth’s surface?

18. How does no-till farming reduce soil erosion?

19. How does time affect soil development?

20. How does humus form? What does itform from?

Use the graph below to answer questions 21–23.

21. During which month was the surface soilwarmest? During which month was itcoldest? Explain.

22. During which month was the soil at 30 cmwarmest? During which month was itcoldest? How is this different than surfacesoil?

23. Why didn’t soil temperature vary at adepth of 4 m?

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Variation of a Soil’s Temperaturewith Time of Year

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16.2 kg

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Fertilizer

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weathering and soil -...

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