SECTION 2 Crops and Soil€¦ · Agriculture: Traditional and Modern The basic processes of farming include plowing, fertilization, irrigation, and pest control. Traditionally, plows

384 Chapter 15 Food and Agriculture

The Earth has only a limited area of land that can beused to grow crops. As our human population continues to grow,the amount of arable land per person decreases. In this section,you will learn how food is produced, how arable land canbecome degraded, and how we can ensure that we will continueto grow the crops we need in the future.

Agriculture: Traditional and ModernThe basic processes of farming include plowing, fertilization,irrigation, and pest control. Traditionally, plows are pushed bythe farmer or pulled by livestock. Plowing helps crops grow bymixing soil nutrients, loosening soil particles, and uprootingweeds. Organic fertilizers, such as manure, are used to enrich thesoil so that plants grow strong and healthy. Fields are irrigatedby water flowing through ditches. Weeds are removed by handor machine. These traditional techniques have been used sincethe earliest days of farming, centuries before tractors and pesti-cides were invented.

In most industrialized countries, the basic processes offarming are now carried out using modern agricultural methods. Machinery powered by fossil fuels is now used toplow the soil and harvest crops, as shown in Figure 7.Synthetic chemical fertilizers are now used instead of manureand plant wastes to fertilize soil. A variety of overhead sprin-klers and drip systems may be used for irrigation. And syn-thetic chemicals are used to kill pests.

arable land,Objectives� Distinguish between traditional

and modern agricultural tech-niques.

� Describe fertile soil.� Describe the need for soil conser-

vation.� Explain the benefits and environ-

mental impacts of pesticide use.� Explain what is involved in inte-

grated pest management.� Explain how genetic engineering is

used in agriculture.

Key Termsarable landtopsoilerosiondesertificationcompostsalinizationpesticidebiological pest controlgenetic engineering

S E C T I O N 2

Crops and Soil

Figure 7 � In modern agriculture,machinery is used to do much of thework previously performed byhumans and animals.

Copyright© by Holt, Rinehart and Winston. All rights reserved.

Section 2 Crops and Soil 385

Numbers of Organisms in Average Farm Soil

Organisms Quantity

Insects 23 million per hectare

All arthropods 725 million(including per hectareinsects)

Bacteria 2.5 billion per gram

Algae 50,000 per gram

Earthworms 6 million per hectare

Table 2 �

Note: One hectare equals about 2.47 acres.Source: US Department of Agriculture.

Surface litter fallen leavesand partially decomposedorganic matter

Topsoil organic matter, living organisms, androck particles

Zone of leaching dis-solved or suspended mate-rials moving downward

Subsoil larger rock parti-cles with organic matter,and inorganic compounds

Rock particles rock that has undergone weathering

Bedrock solidrock layerRhizobium bacteria

produce fixed nitrogen.

Ants and earthwormsbreak up and aerate the soil.

Bacteriaand fungidecomposeorganic matter.

Figure 8 � Soil is made of rock parti-cles, air, water, and dead and livingorganisms. The number and charac-teristics of the soil layers may be dif-ferent in different types of soil.

Fertile Soil: The Living EarthSoil that can support the growth of healthy plants is called fertilesoil. Plant roots grow in the surface layer of soil, which isusually richer in organic matter than the subsoil is. Fertile topsoilis composed of living organisms, rock particles, water, air, andorganic matter, such as dead or decomposing organisms.

Most soil forms when rock is broken down into smaller andsmaller fragments by wind, water, and chemical weathering.Chemical weathering happens when the minerals in the rock reactchemically with substances such as water to form new materials.Temperature changes and moisture cause rock to crack and breakapart, which creates smaller particles on which the seeds of pio-neer plants fall and take root. It can take hundreds or even thou-sands of years for these geological processes to form a fewcentimeters of soil.

Other processes also help to produce fertile topsoil. The rockparticles supply mineral nutrients to the soil. Fungi and bacterialive in the soil, and they decompose dead plants as well asorganic debris and add more nutrients to the soil. Earthworms,insects, and other small animals help plants grow by breaking upthe soil and allowing air and water into it. One way to tellwhether soil is fertile is to see if it contains earthworms. Table 2lists some of the organisms that live in fertile soil.

As you can see in Figure 8, several layers of soil lie under thetopsoil. The bottom layer is bedrock, which is the solid rock fromwhich most soil originally forms.

topsoil,


Soil Erosion: A Global Problemis the wearing away of rock or soil by wind and water. In

the United States, about half of the original topsoil has been lostto erosion in the past 200 years. Figure 9 shows potential soilerosion worldwide. Without topsoil, crops cannot be grown.

Almost all farming methods increase the rate of soil erosion. Forexample, plowing loosens topsoil and removes plants that hold thesoil in place. The topsoil is then more easily eroded by wind or rain.

Land DegradationLand degradation happens when human activity or naturalprocesses damage the land so that it can no longer support thelocal ecosystem. In areas with dry climates, desertification canresult. is the process by which land in arid orsemiarid areas becomes more desertlike because of human activ-ity or climatic changes. This process is causing some of our arableland to disappear.

Desertification has happened in the Sahel region of northernAfrica. In the past, people who lived in the drier part of the Sahelgrazed animals, whereas people who lived in areas of the Sahel withmore rainfall planted crops. The grazing animals were moved fromplace to place to find fresh grass and leaves. The cropland wasplanted for only a few years, and then the land was allowed to liefallow, or to remain unplanted, for several years. These methods offarming and grazing allowed the land to adequately support thepeople in the Sahel. But the population in the region has grown,and the land is being farmed, grazed, and deforested faster than itcan regenerate. Now, too many crops are planted too frequently,and fallow periods are being shortened or eliminated. As a result,the soil is losing its fertility and productivity. Because of overgraz-ing, the land has fewer plants to hold the topsoil in place. So, largeareas have become desert and can no longer produce food.

Desertification

Erosion


QuickLABPreventing Soil Erosion Procedure1. Obtain three trays, and fill one

with sod, one with topsoil, andone with a type of mulch, suchas hay.

2. Place each tray at an angle bycreating a surface that resem-bles a hill by using doorstop-pers and textbooks. Place alarge bowl at the bottom ofeach tray to catch the runoff.

3. Sprinkle 2L of water slowly oneach tray to simulate heavyrainfall.

4. Use a scale to weigh the runoffof soil and water that collectedin each bowl.

Analysis1. Which tray had the most soil

erosion and water runoff?Which tray had the least? Why?What does this lab demonstrateabout soil erosion?

Figure 9 � Soil erosion is one of themost serious environmental problemsthe world faces. This map shows thevulnerability of soils worldwide toerosion by water.


Soil ConservationThere are many ways of protecting and managing topsoil andreducing erosion. Soil usually erodes downhill, and many soil con-servation methods are designed to prevent downhill erosion, asshown in Figure 10. Building soil-retaining terraces across a hillsidemay be cost-effective for producers of valuable crops, such as winegrapes and coffee. On gentler slopes, contour plowing is used. Thismethod includes plowing across the slope of a hill instead of upand down the slope. An even more effective method of plowing isleaving strips of vegetation across the hillside instead of plowingthe entire slope. These strips catch soil and water that run downthe hill. Still, many areas of land that have hills are not suited tofarming, but may be better used as forest or grazing land.

In traditional farming, after a crop is harvested, the soil isplowed to turn it over and bury the remains of the harvestedplants. In no-till farming, shown in Figure 11, a crop is harvestedwithout turning over the soil. Later, the seeds of the next crop areplanted among the remains of the previous crop. The remains ofthe first crop hold the soil in place while the new crop develops.No-till farming saves time compared with conventional methods.This method can also reduce soil erosion to one-tenth of the ero-sion caused by traditional methods. However, no-till farming maynot be suitable for some crops. Other disadvantages to thismethod can include soil that is too densely packed and lower cropyields over time.


Figure 10 � Terracing (left) keepssoil in multiple, small, level fields.Contour plowing (right) follows the natural contours of the land.Both methods prevent soil erosionby keeping water from runningdirectly downhill.

Figure 11 � This farmer is practic-ing no-till farming. The tractor plantsa new crop by poking seeds into the soil through the remains of theold crop.

www.scilinks.orgTopic: Soil ErosionSciLinks code: HE4100


Enriching the SoilSoil was traditionally fertilized by adding organic matter, such asmanure and leaves, to the soil. As organic matter decomposes, itadds nutrients to the soil and improves the texture of the soil.However, inorganic fertilizers that contain nitrogen, phosphorus,and potassium have changed farming methods. Without these fer-tilizers, world food production would be less than half of what itis today. Over the past 50 years, the use of such fertilizers hasincreased rapidly, as shown in Figure 12. If erosion occurs inareas where the soil has been fertilized with inorganic chemicals,fertilizers and pesticides may pollute waterways.

A modern method of enhancing the soil is to use both organicand inorganic fertilizers by adding compost and chemical fertilizersto the soil. is partly decomposed organic material.Compost comes from many sources. For example, you can buycomposted cow manure in a garden store. Also, many cities andindustries now compost yard waste and crop wastes. This compostis sold to farmers and gardeners, and the process is saving costlylandfill space.

SalinizationThe accumulation of salts in the soil is known as(SAL uh nie ZAY shuhn). Salinization is a major problem in placessuch as California and Arizona, which have low rainfall and naturally salty soil. In these areas, irrigation water comes fromrivers or groundwater, which is saltier than rainwater. Whenwater evaporates from irrigated land, salts are left behind.Eventually, the soil may become so salty that plants cannot grow.

Irrigation can also cause salinization by raising the groundwaterlevel temporarily. Once groundwater comes near the surface, thegroundwater is drawn up through the soil like water is drawn upthrough a sponge. When the water reaches the surface, the waterevaporates and leaves salts in the soil. Salinization can be slowed ifirrigation canals are lined to prevent water from seeping into thesoil, or if the soil is watered heavily to wash out salts.

salinization

Compost


1. Explain the differences between traditional andmodern farming methods.

2. Describe the structure and composition of fertilesoil.

3. Explain why the presence of plants helps preventsoil erosion.

4. Explain why soil conservation is an important agri-cultural practice.

CRITICAL THINKING5. Inferring Relationships Study the graph in Figure

12. What do you think might have happened to foodproduction between 1990 and 1995?

6. Applying Ideas Erosion is a natural process. Whyhas it become such a serious environmental problem?Write a paragraph that explains your reasoning.

WRITING SKILLS

S E C T I O N 2 Mid-Section Review

GeologyConnection to

Soil Formation Over TimeMost rock breaks down into finerparticles over time and changesfrom gravel to sand to clay. Youcan tell the age of soil by lookingat its rock particles. Young soil issandy or gravelly, and it fallsapart when you squeeze it inyour hand. Older soils containclay, and damp clay staystogether in lumps when yousqueeze it in your hand.

World Fertilizer Use, 1950–2000

Am

oun

t us

ed

(in

mill

ion

s o

f m

etri

c to

ns)

Year

Source: UN Food and Agriculture Organization.

2000199019801970196019500

40

80

120

160

Figure 12 � The use of inorganicfertilizers has increased dramaticallyworldwide since 1950.


Pest ControlIn North America, insects eat about 13 percent of all crops.Crops in tropical climates suffer even greater insect damagebecause the insects grow and reproduce faster in these climates.In Kenya, for example, insects destroy more than 25 percent ofthe nation’s crops. Worldwide, pests destroy about 33 percent ofthe world’s potential food harvest.

As shown in Figure 13, insects are one of several types of organ-isms considered pests. A pest is any organism that occurs where it isnot wanted or that occurs in large enough numbers to cause eco-nomic damage. Humans try to control populations of many typesof pests, including many plants, fungi, and microorganisms.

Wild plants often have more protection from pests than cropplants do. Wild plants grow throughout a landscape, so pests havea harder time finding and feeding on a specific plant. Crop plants,however, are usually grown together in large fields, which pro-vides pests with a one-stop source of food. Wild plants are alsoprotected from pests by a variety of pest predators that live on ornear the plants. Some wild plants have also evolved defenses tomany pests, such as poisonous chemicals that repel pests.

PesticidesMany farmers rely on pesticides to produce their crops.

are chemicals used to kill insects, weeds, and other croppests. During the last 50 years, scientists invented many new pes-ticides. The pesticides were so effective that farmers began to relyon them almost completely to protect their crops from pests.However, pesticides can also harm beneficial plants and insects,wildlife, and even people.

Pesticides


EcofactCrop Rotation Farmers and gar-deners have known for centuriesthat you get higher yields and lesspest damage if you plant differentcrops each year on a piece of land.This method works because mostpests are specialists and will onlyeat one or a few types of plants.The tomato hornworm is an exam-ple of one of these pests. If youplant tomatoes in one place everyyear, the hornworm populationgrows rapidly and will destroy thecrop. If beans are planted in placeof the tomatoes in alternate years,the hornworms cannot find foodand die.

Figure 13 � Examples of majorcrop pests include weeds, plant-eating insects, and fungi.


Pesticide Resistance You might think that the most effectiveway to get rid of pests is to spray often with large amounts ofpesticide, as shown in Figure 14. However, over time, thisapproach usually makes the pest problem worse. Pest populationsmay evolve resistance, the ability to survive exposure to a par-ticular pesticide. More than 500 species of insects have developedresistance to pesticides since the 1940s.

Human Health Concerns Pesticides are designed to kill organisms,so they may also be dangerous to humans. For example, in someareas fruit and vegetable farmers use large amounts of pesticideson their crops. Cancer rates among children in those areas aresometimes higher than the national average, and nervous systemdisorders may be common. Workers in pesticide factories may alsobecome ill. And people who live near these factories may be endan-gered by accidental chemical leaks. People who apply pesticidesneed to follow safety guidelines to protect themselves from contactwith these chemicals.

Pollution and Persistence The problem of pesticides harmingpeople and other organisms is especially serious with pesticidesthat are persistent. A pesticide is persistent if it does not breakdown easily or quickly in the environment. Persistent pesticidesdo not break down rapidly into harmless chemicals when theyenter the environment. As a result, they accumulate in the waterand soil. Some persistent pesticides have been banned in theUnited States, but many of them remain in the environment formany years. DDT, a persistent pesticide banned in the UnitedStates in the 1970s, can still be detected in the environment andhas even been found in women’s breast milk.


Figure 14 � A cropduster sprayspesticide on a field of pineapples inHawaii. Cropdusting is an easy wayto apply pesticide to a large area.

LawConnection to

Pesticide Regulation The onlypesticides that are fully regulatedin the United States are newlyintroduced pesticides designedfor use on some food crops.Many older pesticides in usehave not been adequately testedfor toxicity and are not effec-tively regulated. According tothe National Academy ofSciences, much of the cancer riskfrom pesticides in our diet comesfrom older pesticides used onfoods such as tomatoes, po-tatoes, and oranges.


Biological Pest ControlMost farmers practice some form of pest management.

is the use of living organisms to control pests. Everypest has enemies in the wild, and these enemies can sometimes beused to control pest populations, as shown in Figure 15. One ofthe first recorded examples of biological control was in India inthe mid-1800s. American prickly pear cactus had been introducedinto India to feed insects that are used to make a valuable red dye.Because the cactus had no natural enemies in India, the cactusgrew and spread. The plants were finally controlled by the intro-duction of an American beetle that eats the cactus.

Pathogens Organisms that cause disease, called pathogens(PATH uh juhnz), can also be used to control pests. One of themost common pathogens used to control pests is the bacteriumBacillus thuringiensis (buh SIL uhs THUHR in JIEN sis), oftenabbreviated Bt. This bacterium can kill the caterpillars of mothsand butterflies that we consider to be pests.

Plant Defenses Scientists and farmers have bred plant varietiesthat have defenses against pests. For example, if you buy tomatoplants or seeds, you may see that they are labeled “VNT.” Thislabel means they are resistant to certain fungi, worms, andviruses. Examples of plant defenses include chemical compoundsthat repel pests and physical barriers, such as tougher skin.

Chemicals From Plants Another type of biological pest controlalso makes use of plants’ defensive chemicals. For example, chemi-cals found in chrysanthemum plants are now sold as pesticides.Most insect sprays that contain these chemicals are designed foruse in the home because they do not harm humans or pets. Theseproducts are biodegradable, which means that they are brokendown by bacteria and other decomposers.

pest controlBiological


FIELD ACTIVITY FIELD ACTIVITY Pest Search Make a list of thepests you can find in your area.Look for weeds and insects. Whatevidence can you find that theseorganisms are pests? You will notbe able to see pests in the soil ormicroscopic bacteria, fungi, orviruses, but you may be able tosee the damage the microscopicpests cause—black spots or deadpatches on leaves. Can you thinkof a way to decrease the damagethat is caused by these pests thatinvolves the use of biological pestcontrol? Record your observationsin your EcoLog.

Figure 15 � A parasitic wasp injectsits eggs into an aphid (left). A preda-tory mite is attacked by anotherspecies of mite (right).


Disrupting Insect Breeding Growth regulators are chemicalsthat interfere with some stage of a pest’s life cycle. If you have adog, you may feed it a pill once a month to keep it free of fleas.The pill contains a growth regulator that prevents flea eggs fromdeveloping. When a flea sucks the dog’s blood, the flea ingests thegrowth regulator. The regulator stops the flea’s eggs from de-veloping into adult fleas.

Pheromones (FER uh MOHNZ), chemicals produced by oneorganism that affect the behavior of another organism, can alsobe used in pest control. For example, female moths releasepheromones that attract males from miles away. By treatingcrops with pheromones, farmers can confuse the male mothsand interfere with the mating of the moths. Another way toprevent insects from reproducing is to make it physicallyimpossible for the males to reproduce. For example, maleinsects are treated with X rays to make them sterile and thenare released. When they mate with females, the females pro-duce eggs that do not develop.

Integrated Pest ManagementIntegrated pest management is a modern method of controllingpests on crops. The steps involved in integrated pest managementare shown in Figure 16. The goal of integrated pest managementis not to eliminate pest populations but to reduce pest damage toa level that causes minimal economic damage. A different man-agement program is developed for each crop. The program caninclude a mix of farming methods, biological pest control, andchemical pest control. Each of these methods is used at theappropriate time in the growing season. Fields are monitoredfrom the time the crops are planted. When significant pest dam-age is found, the pest is identified. Then a program to control thepest is created.


ChemistryConnection to

Organic Chemistry All foodcontains organic chemicals, butthe term organic is used differ-ently in the field of chemistrythan in agriculture. The termgenerally means “of or pertainingto living organisms.” In chem-istry, an organic chemical is anychemical compound that con-tians carbon. Most organic chem-icals are derived from livingorganisms, but chemists can nowsynthesize organic chemicals—and even invent new ones—inthe lab. In contrast, organic agri-culture is the practice of raisingcrops or livestock without usingsynthetic chemicals. Foodslabeled as organic in the grocerystore have been raised usingorganic methods.

Figure 16 � This flow diagramshows the steps involved in inte-grated pest management.


Biological methods are the first methods used to control thepest. So, natural predators, pathogens, and parasites of the pestmay be introduced. Cultivation controls, such as vacuuminginsects off the plants, can also be used. As a last resort, smallamounts of insecticides may be used. The insecticides are changedover time to reduce the ability of pests to evolve resistance.

Engineering a Better CropPlant breeding has been used since agriculture began. Farmersselect the plants that have the tastiest tomatoes and the least pestdamage. They save seeds from these plants to use in planting thenext crop. The selected seeds are more likely to contain the genesfor large, tasty fruits and for pest resistance than seeds from otherplants are.

A faster way of creating the same result is to usethe technology in which genetic material in

a living cell is modified for medical or industrial use. Geneticengineering involves isolating genes from one organism andimplanting them into another. Scientists may use genetic engi-neering to transfer desirable traits, such as resistance to certainpests. The plants that result from genetic engineering are calledgenetically modified (GM) plants.

Figure 17 shows an example of the steps used to produce aGM plant. In this case, the gene introduced into the plant is not aplant gene. It is an insecticide gene from Bt, a bacterium that pro-duces a chemical that kills plant-eating caterpillars but does notharm other insects. Plants that have the Bt gene make this insecti-cide within their leaves. Hundreds of gene transfers have nowbeen performed to create many other GM crops.

genetic engineering,


+

Scientists grow the corn cells and expose themto an antibiotic. Only those cells that have incorporated the inserted genes survive.

The surviving cells grow into corn plants. These plants produce the Bt toxin, which kills caterpillars.

The two genes are insertedinto corn plant cells.

Scientists isolate the gene from Bt that directs a cell to produce a toxin. The Bt gene is then joined to a “marker gene” that enables a cell to break down an antibiotic.

Antibiotic

Bacillusthuringiensis(Bt)

Marker gene

Toxin genefrom Bt

Figure 17 � Genetic EngineeringThis diagram shows the main stepsused to produce a genetically modi-fied plant—in this case, corn thatproduces its own insecticide.

www.scilinks.orgTopic: GeneticEngineeringSciLinks code: HE4047


Implications of Genetic Engineering In the United States, wenow eat and use genetically engineered agricultural productsevery day. Many of these products have not been fully tested fortheir environmental impacts, and some scientists warn that theseproducts will cause problems in the future. For example, genesare sometimes transferred from one species to another in thewild. Suppose a corn plant that was genetically engineered to beresistant to a pesticide were to pass the resistance genes to a wildplant. That wild plant might be a pest that could not be killed bythat pesticide.

Sustainable AgricultureFarming that conserves natural resources and helps keep the landproductive indefinitely is called sustainable agriculture. Alsocalled low-input farming, sustainable agriculture minimizes theuse of energy, water, pesticides, and fertilizers. This methodinvolves planting productive, pest-resistant crop varieties thatrequire little energy, pesticides, fertilizer, and water. Figure 18shows an experimental farm where new sustainable agriculturetechniques are being researched.

1. Define the term pest.

2. Compare the benefits and environmental impact ofpesticide use.

3. Describe how biological pest control is part of inte-grated pest management.

4. Describe how genetic engineering is used in agriculture.

CRITICAL THINKING5. Inferring Relationships Write a paragraph to

explain the similarities and differences between tradi-tional plant breeding and genetic engineering.

6. Predicting Consequences Read the description ofintegrated pest control in this section. Why do youthink this pest control technique is not practicedeverywhere? READING SKILLS

WRITING SKILLS

S E C T I O N 2 Review

EcofactNitrogen Fixation One of themost valuable families of cropplants is the legumes (LEG YOOMZ),which include peas and beans.Legumes produce higher gradeproteins than most plants produce,so legumes are part of diets inmany parts of the world. Plantinglegumes also improves the soil.Their roots have nodules contain-ing bacteria that take nitrogen gasfrom the air and that convert thenitrogen into a form other plantscan use to build proteins.

Figure 18 � At the Land Institute inSalina, Kansas, sustainable agriculturetechniques are being used to increaseseed quantity in wheatgrass (back-ground) and to increase yield in sun-flowers (foreground).

394 Chapter 15 Food and AgricultureCopyright© by Holt, Rinehart and Winston. All rights reserved.

Documents

SECTION 2 Crops and Soil€¦ · Agriculture: Traditional and Modern The basic processes of farming include plowing, fertilization, irrigation, and pest control. Traditionally, plows