Chapter 11: Weathering and Soil - Wikispaces · 2012-08-16 · SECTION 1 Weathering 327 Chemical Weathering The second type of weathering,chemical weathering,occurs when chemical

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sections

1 Weathering

2 The Nature of SoilLab Soil Texture

3 Soil ErosionLab Weathering Chalk

Virtual Lab How are materi-als from Earth broken down?

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

Write a poem about a tor. Usewords in your poem that rhyme with the word tor.Science Journal

Weathering and Soil

Sunshine State Standards—SC.D.1: The student recognizes that processes in the lithosphere,atmosphere, hydrosphere, and biosphere interact to shape the Earth.

Andrew Brown, Ecoscene/CORBIS

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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. Complete a safety worksheet.

2. 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.

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

4. 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.

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

6. 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.

7. 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 fl6.msscience.com

LA.A.1.3.4

SC.D.1.3.1

Andrew Brown, Ecoscene/CORBIS

http://fl6.msscience.com

324 CHAPTER 11 Weathering and Soil

Weathering and Its EffectsCan you believe that tiny moss plants, earthworms, and even

oxygen in the air can affect solid rock? These things weaken andbreak apart rock at Earth’s surface. Surface processes that workto break down rock are called weathering.

Weathering breaks rock into smaller and smaller particles,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 virtu-ally all mechanical and chemical processes in the soil, includingthe ability to hold moisture and nutrients.

Over millions of years, weathering has changed Earth’s surface. The process continues today. Weathering wears moun-tains down to hills, as shown in Figure 1. Rocks at the top ofmountains are broken down by weathering, and the sedimentis moved downhill by gravity, water, and ice. Weathering alsoproduces strange rock formations like those shown at thebeginning of this chapter. Two different types of weathering—mechanical weathering and chemical weathering—worktogether 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.

Weathering

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

Benchmarks—SC.D.1.3.1 (pp. 324–329): The student knows that mechanical and chemical activitiesshape and reshape the Earth’s land surface by eroding rock and soil in some areas and depositing themin other areas, sometimes in seasonal layers; SC.D.1.3.4 Annually Assessed (pp. 324–329): The studentknows the ways in which plants and animals reshape the landscape.

Also covers: SC.D.1.3.5 (pp. 324, 326, 328)

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

FCAT Vocabulary

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.

325

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.

325

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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 smaller andsmaller pieces. These small pieces have more surfacearea than the original rock body, as shown in Figure 5.As the amount of surface area increases, more rock isexposed to water and oxygen. This speeds up a differ-ent type of weathering, called chemical weathering,which continues to reduce the particle size of sedi-ments from a coarse 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 327

Chemical WeatheringThe second type of weathering, chemical weathering, occurswhen chemical processes dissolve or alter the minerals in rocksor change 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. Overmany thousands of years, carbonic acid has weathered so muchlimestone that caves have formed, as shown in Figure 6.

Chemical weathering also occurs when naturally formedacids come in contact with other types of rocks. Over a longtime, the mineral feldspar, which is found in granite, some typesof sandstone, and other rocks, is broken down into a clay min-eral called kaolinite (KAY oh luh nite). Kaolinite clay is commonin some soils. Clay is an end product of weathering.

How does kaolinite clay form?

Plant Acids Some roots and decaying plants give off acidsthat 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 fl6.msscience.com for Weblinks to information aboutchemical weathering.

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

Carbon dioxide � Water � Carbonic acidCarbonic acid dissolves limestone.

LA.B.2.3.4

Carbon dioxide � Water � Carbonic acidCarbonic acid dissolves limestone.

Hans Strand/Stone


MagnetiteLimonite


pH Scale The strength of acids andbases is measured on the pH scale with a

range of 0 to 14. On this scale, 0 is extremely acidic, 14 isextremely basic or alkaline, and 7 is neutral. Most minerals aremore soluble in acidic soils than in neutral or slightly alkalinesoils. Different plants grow best at different pH values. Forexample, peanuts grow best in soils that have a pH of 5.3 to 6.6,while alfalfa grows best in soils having a pH 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 the air, the iron in themineral reacts to form a new material that resembles rust. Onecommon example of this type of weathering is the alteration ofthe iron-bearing mineral magnetite to a rustlike material calledlimonite, as shown in Figure 7. Oxidation of minerals givessome rock 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 thepattern of weather that occurs in a particular area over manyyears. In cold climates, where freezing and thawing are frequent,mechanical weathering rapidly breaks down rock through aprocess of ice wedging. Chemical weathering is more rapid inwarm, wet climates. High temperatures tend to increase the rateof chemical reactions. Thus, chemical weathering tends to occurquickly in tropical areas. Lack of moisture in deserts and lowtemperatures in polar regions slow down chemical weathering.

Figure 7 Iron-containing miner-als like the magnetite shown herecan weather to form a rustlikematerial called limonite.

Observing Chemicaland MechanicalWeatheringProcedure1. Collect and rinse two

handfuls of common rockor shells.

2. Place equal amounts of rockinto two plastic bottles.

3. Fill one bottle with waterto cover the rock and sealwith a lid.

4. Cover the rock in the sec-ond bottle with lemonjuice and seal.

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.

SC.D.1.3.1SC.H.1.3.5

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

SECTION 1 Weathering 329

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 processes of soil formation alterrock minerals so that soil minerals are mostly inherited from theparent rock type. Weathering begins the process of forming soilfrom rock and sediment and also affects particle size and soil tex-ture. Sand, silt, and clay simply refer to different particle sizes ofthe 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?

SC.D.1.3.1

SC.D.1.3.4

SC.D.1.3.5

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

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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, comes 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.

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

Benchmarks—SC.D.1.3.4 Annually Assessed (pp. 330–336): The student knows the ways in whichplants and animals reshape the landscape (e.g., bacteria, fungi, worms, rodents, and other organ-isms add organic matter to the soil, increasing soil fertility, encouraging plant growth, and strength-ening resistance to erosion).

Also covers: SC.D.1.3.5 (pp. 330, 334)

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 331

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 live among plant roots.Their wastes, along with dead plant material, add organic matter to the soil.

C

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

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 and air. When soilis moist, the spaces hold the water that plants need to grow.During a drought, the spaces, or pores, are almost entirely filledwith air. When water soaks into the ground, it infiltrates thepores. Infiltration rate is determined by calculating the time ittakes for water sitting on soil to drop a fixed distance. This ratechanges as the soil pore spaces fill with water.

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. Thesedifferent layers of soil are called horizons.All the horizons of a soil form a soil profile.Most soils have three horizons—labeled A,B, and C, as shown in Figure 11.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 components ortheir 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.

SC.D.1.3.4

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

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

A Horizon The A horizon is the top layer ofsoil. In a forest, the A horizon might be cov-ered with litter. Litter consists of leaves, twigs,and other organic material that eventually canbe changed to humus by decomposing organ-isms. Litter helps prevent erosion and evapo-ration of water from the soil. The A horizonalso is known as topsoil. Topsoil has morehumus and fewer rock and mineral particlesthan the other layers in a soil profile. The Ahorizon generally is dark and fertile. The darkcolor of the soil is caused by the humus, whichprovides nutrients for plant growth.

Since dark color absorbs solar energy morereadily, soil color can greatly affect soil temper-ature. Darker color also may indicate highersoil moisture. Soil moisture and soil temper-ature are important in determining seed germination for plants and the vitality ofdecomposing organisms.

B Horizon The layer below the A horizon isthe B horizon. Because less organic matter isadded to this horizon, it is lighter in color than the A horizonand contains less humus. As a result, the B horizon is less fertile.The B horizon contains material moved down from the A hori-zon by the process of leaching.

Leaching is the removal of minerals that have been dissolvedin water. In soil, water seeps through the A horizon and reactswith humus and carbon dioxide to form acid. The acid dissolvessome of the minerals in the A horizon and carries the materialdown into the B horizon, as shown in Figure 12.

How does leaching transport material from theA horizon to the B horizon?

C Horizon The C horizon consists of partially weathered rockand is the bottom horizon in a soil profile. It is often the thick-est soil horizon. This horizon does not contain much organicmatter and is not strongly affected by leaching. It usually is com-posed of coarser sediment than the soil horizons above it. Whatwould you find if you dug to the bottom of the C horizon? Asyou might have guessed, you would find rock—the rock thatgave rise to the soil horizons above it. This rock is called the par-ent material of the soil. The C horizon is the soil layer that ismost like the parent material.

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

Soil Fertility Plants need a variety of nutrients forgrowth. They need thingslike nitrogen, phosphorous,potassium, sulfur, calcium,and magnesium calledmacronutrients. They getthese nutrients from theminerals and organic mate-rial in soil. Soil fertility usually is determined in a laboratory by a soilchemist. However, fertilitysometimes can be inferredby looking at plants. Doresearch to discover moreimportant plant nutrients.


LA.A.2.3.5


Soil Structure Individual soil particles clump or bindtogether. Examine soil closely and you will see natural clumpscalled peds. Soil structure affects pore space and will affect aplant’s ability to penetrate roots. Figure 13 shows four classes ofsoil structure. Granular structures are common in surface soilswith high organic content that glues minerals together.Earthworms, frost, and rodents mix the soil, keeping the pedssmall, which provides good porosity and movement of air andwater. Platy structures are often found in subsurface soils thathave been leached or compacted by animals or machinery.Blocky structures are common in subsoils or surface soils withhigh clay content, which shrinks and swells, producing cracks.Prismatic structures, found in B horizons, are very dense anddifficult for plant roots to penetrate. Vertical cracks result fromfreezing and thawing, wetting and drying, and downward move-ment of water and roots. Soil consistency refers to the ability ofpeds and soil particles 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.

SolutionThis 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 by 100:15 g clay/50 g sample � 100 � 30% clay in the sample

15 gClay particles

15 gSilt particles

20 gSand particles

platy granular

prismatic blocky

Figure 13 Four major classescharacterize soil structure.

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Math Practice

MA.E.3.3.1

MA.E.3.3.1



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 are thinner than soils in wetter climates. Prairiesoils have thick, dark A horizons because the grasses that growthere contribute lots of organic matter. Temperate forest soilshave less organic matter and thinner A horizons than prairiesoils. Other regions also have distinct soils such as the per-mafrost soils of the tundra and laterite soils of tropical areas.

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 weather-ing continues, 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.

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SC.D.1.3.5

SC.D.1.3.4

MA.E.1.3.2


LAB 337

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 ProblemWhat is the texture of your soil?

Goals■ Determine soil texture by making a ribbon.

Materialssoil sample (100 g) water bottle

Safety PrecautionsComplete a safety worksheet before you begin.

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

Benchmark—SC.D.1.3.5: The student understands concepts of time and size relating tothe interaction of Earth’s processes (e.g., lightning striking in a split second as opposed tothe shifting of the Earth’s plates altering the landscape, distance between atoms meas-ured in Angstrom units as opposed to distance between stars measured in light-years).

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.

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.

Benchmarks—SC.D.1.3.1 (pp. 338, 340): The student knows that mechanical and chemical activitiesshape and reshape the Earth’s land surface by eroding rock and soil in some areas and depositing themin other areas, sometimes in seasonal layers; SC.D.1.3.4 Annually Assessed (pp. 338–341): The studentknows the ways in which plants and animals reshape the landscape.

Also covers: SC.D.1.3.5 (pp. 338–340), SC.H.1.3.3 (p. 339), SC.H.1.3.4 Annually Assessed (pp. 342–343),SC.H.1.3.5 Annually Assessed (pp. 342–343), SC.H.1.3.6 (pp. 339, 344)

Review Vocabularyerosion: the picking up and

moving of sediment or soil

New Vocabulary

• no-till farming

• contour farming

• terracing

FCAT Vocabulary

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Bra

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SECTION 3 Soil Erosion 339

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 fl6.msscience.com 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.

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.

LA.C.3.3.3

LA.B.2.3.4

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 monitored, managed, and protected. People can do severalthings to conserve 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.

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

Annually AssessedBenchmark Check

SC.D.1.3.4 Why is it importantto leave vegetation on crop fieldsafter harvesting the crop?

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

SECTION 3 Soil Erosion 341

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.

More Section Review fl6.msscience.com

SC.D.1.3.4

SC.D.1.3.4

Georg Gerster/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)250-mL beakers (2)metric rulerwaterwhite vinegar (100 mL)hot plate250-mL graduated cylindercomputer probe for

temperature

Safety Precautions

Complete a safety work-sheet before you begin.

WARNING: When mixingacid and water, always addacid to water.

Real-World ProblemChalk 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 that explain your predictions.

Benchmark—SC.D.1.3.1: The student knows that mechanical and chemical activitiesshape and reshape the Earth’s land surface . . . ; SC.D.1.3.5; SC.H.1.3.4; SC.H.1.3.5;SC.H.1.3.7

(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 343

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 to testthe 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

LA.E.1.3.3

LA.B.2.3.1

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 345

Type ofWeathering

Plant acid

is

agent of

agent of

agent of

occursby

occursby

occursby

is

Oxygen Animals

Chemical

Interactive Tutor fl6.msscience.com(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

ent

clay

(%)

203040

10

25 50 1000 75 125 150

Clay Abundance

Depth (cm)

346 CHAPTER REVIEW

chemical weatheringp. 327

climate p. 328contour farming p. 341horizon p. 332humus p. 332ice wedging p. 326leaching p. 333litter p. 333

mechanical weatheringp. 325

no-till farming p. 340oxidation p. 328soil p. 330soil profile p. 332terracing p. 341weathering p. 324

Vocabulary PuzzleMaker fl6.msscience.com

FCAT Vocabulary

SC.D.1.3.4

SC.D.1.3.1

SC.D.1.3.4


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 347

Soil

A horizon

has layers called

which include

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?

Chapter Review fl6.msscience.com

SC.D.1.3.5

SC.D.1.3.4

SC.D.1.3.1

MA.A.3.3.2

MA.A.3.3.2

MA.A.3.3.2

Matt Meadows


348 FLORIDA

a How does the soil profile shown belowdiffer from the soil profile in most deserts?

A. In a desert, the A horizon is likely tobe much thinner.

B. In a desert, the B horizon is likely tocontain more humus.

C. In a desert, the A horizon is likely tobe thicker than the C horizon.

D. In a desert, the B horizon is likely tobe thinner than the A horizon.

s If an iron-containing mineral is exposedto rain, a rust-like material forms on itssurface. Which best explains this?

F. chemical weathering involving carbondioxide and water

G. chemical weathering involving oxygenand water

H. mechanical weathering caused bystrong winds

I. mechanical weathering caused by rain

C horizonC horizonC horizon

A horizon

B horizon

d Which area is likely to be most affected bysoil erosion?

A. a steep slope after a fire burned all the vegetation

B. a section of low-elevation tropical rainforest

C. a meadow with several kinds of grass

D. a hillside that has been terrace-farmed

f Which statue is likely to be affected themost by chemical weathering?

F. a marble statue in a cool, dry climate

G. a granite statue in a cool, dry climate

H. a marble statue in a warm, wet climate

I. a granite statue in a warm, wet climate

g Which method of reducing soil erosion onhillsides is shown in the diagram below?

A. terracing

B. shelter belts

C. no-till farming

D. contour farming

SC.D.1.3.1

SC.D.1.3.1

SC.D.1.3.1

SC.D.1.3.1

SC.D.1.3.4

FloridaFloridachapter chapter

The assessed Florida Benchmark appears above each question.Record your answers on the answer sheet provided by your teacher or on a sheet of paper.

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Georg Gerster/Photo Researchers

FCAT Practice fl6.msscience.com

h What is the main reason that plants bene-fit from the presence of decaying organicmaterial in the soil?

F. It adds nutrients to the soil.

G. It encourages mechanical weathering.

H. It speeds the rate of evaporation fromsoil.

I. It protects the plants from harmfulinsects.

j The table below shows the percentages ofsand, silt, and clay in each horizon of asoil profile.

Particles of clay have a diameter of lessthan 0.002 mm, particles of silt have adiameter between 0.002 mm and 0.05mm, and particles of sand have a diameterbetween 0.05 mm and 2 mm. What per-centage of the soil in Horizon C is com-posed of particles with a diameter of lessthan 0.05 mm?

Texture Data for a Soil Profile

HorizonPercent

Sand Silt Clay

A 16.2 54.4 29.4

B 10.5 50.2 39.3

C 31.4 48.4 20.2

k Describe how plants and animals can contribute to the mechanical weathering of rock.

l A farmer tried a new method of farmingin a cornfield. The table shows how thefarmer treated the field each season.

PART A Explain how this method offarming helps decrease erosion.

PART B Describe two ways this practicecan help farmers in the longterm.

Work Done Each Season

Fall/Winter Spring Summer

Harvested crops; leftstalks in field

Planted seedswithout plowingsoil

Watered andfertilized plantswhen necessary

READINQUIREEXPLAIN

READINQUIREEXPLAIN

FCAT PRACTICE 349

SC.D.1.3.4

SC.A.1.3.1

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SC.D.1.3.4

Write Clearly Write your explanations neatly in clear, conciselanguage. Use only the space provided in the Sample AnswerBook.

FCAT PracticeFCAT Practice FC

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Chapter 11: Weathering and Soil - Wikispaces · 2012-08-16 · SECTION 1 Weathering 327 Chemical Weathering The second type of weathering,chemical weathering,occurs when chemical

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