Sustainable Agriculture

F or nearly four decades afterWorld War 11, U.S. agriculture wasthe envy of the world, almost

annually setting new records in cropproduction and labor efficiency . Dur-ing this period U.S . farms becamehighly mechanized and specialized, aswell as heavily dependent on fossilfuels, borrowed capital and chemicalfertilizers and pesticides. Today thesame farms are associated with de-clining soil productivity, deterioratingenvironmental quality, reduced profit-ability and threats to human and ani-mal health.Agrowing cross section ofAmerican

society is questioning the environ-mental, economic and social impactsof conventional agriculture . Conse-quently, many individuals are seekingalternative practices that would makeagriculture more sustainable .

Sustainable agriculture embracesseveral variants of nonconventionalagriculture that are often called organ-ic, alternative, regenerative, ecologicalor low-input. Just because a farm isorganic or alternative does not meanthat it is sustainable, however. For a

JOHN P . REGANOLD, ROBERT I . PAPEN-DICK and JAMES F. PARR are soil sci-entists who have studied agriculturalsustainability and organic farming formany years . Reganold teaches introduc-tory soil science and conservation andmanagement at Washington State Uni-versity and has conducted several stud-ies that compare the effects of conven-tional and organic farming methods onsoil systems . Papendick is head of theland management and water conserva-tion research unit with the USDA Agri-cultural Research Service in Pullman,Wash. He served as chairman and coor-dinator of the study Report andRecom-mendations on Organic Farming, pub-lished in 1980 . Parr is a soil-fertilityprogram leader with the USDA Agri-cultural Research Service in Beltsville,Md ., and an authority on crop-residuemanagement systems for soil and waterconservation .

Traditional conservation-minded methods combined with moderntechnology can reduce farmers' dependence on possibly dangerous

chemicals. The rewards are both environmental and financial

SCIENTIFIC AMERICAN June 1990

by John P. Reganold, Robert I . Papendick and James F. Parr

farm to be sustainable, it must pro-duce adequate amounts of high-qual-ity food, protect its resources andbe both environmentally safe and prof-itable . Instead of depending on pur-chased materials such as fertilizers, asustainable farm relies as much aspossible on beneficial natural process-es and renewable resources drawnfrom the farm itself .

Sustainable agriculture addressesmany serious problems afflicting U.S .and world food production : high ener-gy costs, groundwater contamination,soil erosion, loss of productivity, de-pletion of fossil resources, low farmincomes and risks to human healthand wildlife habitats . It is not so mucha specific farming strategy as it is asystem-level approach to understand-ing the complex interactions withinagricultural ecologies .

In 1980 the U.S . Department of Ag-riculture (USDA) estimated between20,000 and 30,000 farmers-about 1percent of the nation's total-werepracticing nonconventional agricul-ture, most of which could now betermed sustainable . Today some ex-perts estimate that the figure mayhave doubled or tripled . Farmers whopractice soil conservation and reducetheir dependence on fertilizers and

HEALTHY SOIL, essential to agricul-ture, is a complex, living medium. Theloose but coherent structure of goodsoil holds moisture and invites airflow.Ants (a) and earthworms (b) mix the soilnaturally. Rhizobium bacteria (c) livingin the root nodules of legumes (such assoybeans) create fixed nitrogen, an es-sential plant nutrient. Other soil micro-organisms, including fungi (d), actino-mycetes (e) and bacteria (f), decomposeorganic matter, thereby releasing morenutrients . Microorganisms also producesubstances that help soil particles ad-here to one another . To remain healthy,soil must be fed organic materials suchas various manures and crop residues .

pesticides generally report that theirproduction costs are lower than thoseof nearby conventional farms . Some-times the yields from sustainablefarms are somewhat lower than thosefrom conventional farms, but they arefrequently offset by lower productioncosts, which leads to equal or greaternet returns.To understand the rationale for sus-

tainable agriculture, one must graspthe critical importance of soil . Soil isnot just another instrument of cropproduction, like pesticides, fertilizersor tractors . Rather it is a complex,living, fragile medium that must beprotected and nurtured to ensure itslong-term productivity and stability.Healthy soil is a hospitable world

for growth . Air circulates through itfreely, and it retains moisture longafter a rain . A tablespoon of soil con-tains millions of grains of sand, silt

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and clay and has a vast expanse ofinternal surface area to which plantnutrients may cling . That same table-spoon of soil also contains billionsof microorganisms, including bacteria,actinomycetes, fungi and algae, mostof which are principal decomposers oforganic matter. Decomposition resultsin the formation of humus and therelease of many plant nutrients . Themicrobes also produce sticky sub-stances called polysaccharides thatglue soil particles together and helpthe soil to resist erosion .Another essential activity that takes

place in the soil is the fixation ofnitrogen. Certain bacteria in the soil orin the roots of plants (most notably

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legumes) convert atmospheric nitro-gen gas into fixed forms of nitrogenthat plants and other organisms useto make proteins. The amount of avail-able nitrogen strongly influences soilproductivity .

ne of the earliest landmarks ofthe sustainability movement

Oin the U.S . is Farmers of FortyCenturies: Permanent Agriculture inChina, Korea and Japan, by FranklinKing, published in 1911 . It documentshow farmers in parts of East Asiaworked fields for 4,000 years withoutdepleting the fertility of their soil .This text and others of the early 20thcentury focused on holistic aspects of

SOIL EROSION caused by water runoff has cut a deep gully in this field in east-ern Washington State . Even a small amount of topsoil erosion can diminish agricul-tural productivity. Conventional farming methods often increase rates of erosionby depleting the organic matter that helps to preserve soil structure. Sustainableagriculture inhibits erosion by promoting the addition of organic matter to the soiland the growth of cover crops, which prevent soil from blowing or washing away.

agriculture and the complex interac-tions within farming systems.Yet around this same time, U.S.

agriculture was in the early stagesof industrialization. New technologiesand scientific methods were devel-oped to help farmers meet the grow-ing demands of expanding urban pop-ulations . By substituting mechanicalpower for horses, for example, farm-ers could increase their grain acreageby from 20 to 30 percent, because theycould plow more ground in less timeand did not need to grow fodder .Many groups and individuals con-

tinued to believe that biology andecology rather than chemistry andtechnology should govern agriculture.Their efforts helped to give birth tothe soil conservation movement of the1930's, the ongoing organic farmingmovement and considerable relatedresearch . Nevertheless, by the 1950'stechnological advances had caused ashift in mainstream agriculture, creat-ing a system that relied on agrichem-icals, new varieties of crops and la-bor-saving, energy-intensive farm ma-chinery. This system has come to beknown as conventional farming.As pesticides, inexpensive ferti-

lizers and high-yielding varieties ofcrops were introduced, it became pos-sible to grow a crop on the same fieldyear after year-a practice calledmonocropping-without depleting ni-trogen reserves in the soil or causingserious pest problems . Farmers beganto concentrate their efforts on fewercrops . Government programs promot-ed monoculture by subsidizing onlythe production of wheat, corn and afew other major grains . Unfortunately,these practices set the stage for exten-sive soil erosion and for pollution ofwater by agrichemicals .

In the U.S . between 1950 and 1985,as a share of total production cost,the cost of interest, capital-related ex-penses and manufactured farm inputs(such as chemical fertilizers, pesti-cides and equipment) almost doubledfrom 22 to 42 percent, while labor andon-farm input expenses declined from52 to 34 percent . During most of thisperiod, relatively little research onsustainable agriculture was conduct-ed because of lack of funding andpublic interest .By the late 1970's, however, con-

cerns were mounting that rapidly ris-ing costs were endangering farmersnationwide . In response, Secretary ofAgriculture Robert S. Bergland com-missioned a study in 1979 to assessthe extent of organic farming in theU.S ., as well as the technology behindthe farming and its economic and eco-

GREEN MANURE CROPS, which are plowed under or surface-mulched instead of being harvested for sale, enrich the soiland improve future crop productivity . Legumes, such as sweet

logical impact. The study, Report andRecommendations on Organic Farm-ing, published in 1980, was basedheavily on case studies of 69 organicfarms in 23 states .The USDA report concluded that or-

ganic farming is energy-efficient, en-vironmentally sound, productive andstable and tends toward long-termsustainability. Since the report waspublished, it has stimulated interest,nationally and internationally, in sus-tainable agriculture . Its recommen-dations provided the basis for the al-ternative-agriculture initiative passedby Congress in the Food Security Actof 1985, which calls for researchand education on sustainable farmingsystems .The sustainable agriculture move-

ment received a further boost last Sep-tember when the Board on Agricultureof the National Research Council re-leased another study, Alternative Ag-riculture . Although controversial, thereport is perhaps the most importantconfirmation of the success of farmsthat rely on biological resources andtheir beneficial interactions instead ofchemicals . It found that well-managedfarms growing diverse crops with littleor no chemicals are as productive andoften more profitable than conven-tional farms . It also asserted that"wider adoption of proven alternativesystems would result in even greater

economic benefits to farmers and en-vironmental gains for the nation."

Sustainable agriculture does notrepresent a return to pre-indus-trial revolution methods ; rather

it combines traditional conservation-minded farming techniques with mod-ern technologies. Sustainable systemsuse modern equipment, certified seed,soil and water conservation practicesand the latest innovations in feedingand handling livestock . Emphasis isplaced on rotating crops, building upsoil, diversifying crops and livestockand controlling pests naturally .Whenever possible, external resour-

ces-such as commercially purchasedchemicals and fuels-are replaced byresources found on or near the farm.These internal resources include solaror wind energy, biological pest con-trols and biologically fixed nitrogenand other nutrients released from or-ganic matter or from soil reserves . Insome cases external resources maybeessential for reaching sustainability.As a result, such farming systems candiffer considerably from one anotherbecause each tailors its practices tomeet specific environmental and eco-nomic needs.A central component of almost all

sustainable farming systems is the ro-tation of crops-a planned successionof various crops growing on one field .

clover (shown being mulched), are very good green manuresbecause they contribute biologically fixed nitrogen to the soil,thus reducing the need for any synthetic nitrogen fertilizers .

When crops are rotated, the yields areusually from 10 to 15 percent high-er than when they grow in monocul-ture. In most cases monocultures canbe perpetuated only by adding largeamounts of fertilizer and pesticide .Rotating crops provides better weedand insect control, less disease build-up, more efficient nutrient cycling andother benefits.A typical seven-season rotation

might involve three seasons of plant-ing alfalfa and plowing it back into thesoil, followed by four seasons of har-vested crops : one of wheat, then oneof soybeans, then another of wheatand finally one of oats . The cyclewould then start over . The first seasonof wheat growth would remove someof the nitrogen produced by the alfal-fa ; the soil's nitrogen reserves wouldbe depleted much less by the soy-beans, which are legumes . Oats aregrown at the end of the cycle becausethey have smaller nutrient require-ments than wheat .Regularly adding crop residues, ma-

nures and other organic materials tothe soil is another central feature ofsustainable farming . Organic matterimproves soil structure, increases itswater-storage capacity, enhances fer-tility and promotes the tilth, or physi-cal condition, of the soil . The betterthe tilth, the more easily the soil canbe tilled and the easier it is for seed-

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LADYBUG BEETLES are natural predators of pea aphids (above) and other insectpests . Integrated pest management (IPM) programs now in use on many farms takeadvantage of natural predator-prey relationships or other biological-control mech-anisms to reduce the need for chemical pesticides . Farmers who practice biologi-cal control encourage the proliferation of beneficial microbes and insects, such asladybugs . They also make their fields generally inhospitable to herbivorous pests .

lings to emerge and for roots to ex-tend downward . Water readily infil-trates soils with good tilth, therebyminimizing surface runoff and soilerosion . Organic materials also feedearthworms and soil microbes .The main sources of plant nutrients

in sustainable farming systems areanimal and green manures . A greenmanure crop is a grass or legume thatis plowed into the soil or surface-mulched at the end of a growing sea-son to enhance soil productivity andtilth. Green manures help to controlweeds, insect pests and soil erosion,while also providing forage for live-stock and cover for wildlife.By raising a diverse assortment of

crops and livestock, a farm can bufferitself against economic and biologicalrisks . Diversity results from mixingspecies and varieties of crops andfrom systematically integrating crops,trees and livestock . When most ofNorth Dakota experienced a severedrought during the 1988 growing sea-son, for example, many monocroppingwheat farmers had no grain to har-vest . Farmers with more diversifiedsystems, however, had sales of theirlivestock to fall back on or were ableto harvest their late-seeded crops ordrought-tolerant varieties . A biologi-cally diverse farming system is alsoless susceptible to the economic woesof a flooded market or a fall in pricesfor a single crop .Controlling insects, diseases and

weeds without chemicals is also a goalof sustainable strategies, and the evi-dence for its feasibility is encouraging .One broad approach to limiting use ofpesticides is commonly called inte-grated pest management (IPM), which

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may involve disease-resistant crop va-rieties and biological controls (suchas natural predators or parasites thatkeep pest populations below injuriouslevels) . Farmers can also select tillagemethods, planting times, crop rota-tions and plant-residue managementpractices to optimize the environmentfor beneficial insects that control pestspecies or to deprive pests of a habi-tat. If pesticides are used as a lastresort, they are applied when pests aremost vulnerable or when any benefi-cial species and natural predators areleast likely to be harmed .In practice, ipm programs are a

mixed bag . They have dramatically re-duced use of pesticides on crops suchas cotton, sorghum and peanuts. Morethan 30 million acres (about 8 percentof U.S. farmland) is currently beingmanaged with ipm programs, resultingin annual net benefits of more than$500 million. On the other hand, ipmprograms have also been reduced to"pesticide management" for manycrops like corn and soybeans, forwhich pesticide usage has actually in-creased significantly.Biological-control techniques are

some of the best ways to control pestswithout pesticides . They have beenused for more than 100 years andhave been commercially successful incontrolling pests, especially insects, inmore than 250 projects around theworld. Yet USDA funds for studyingthem have declined .

Can sustainable farming practic-es make good on their promiseto be ecologically stable, pro-

ductive and profitable? To comparethe effects of sustainable and conven-

tional farming systems on soil pro-ductivity, one of us (Reganold), work-ing with Lloyd F . Elliott and Yvonne L .Unger of Washington State University,conducted a study of the soil on twocommercial wheat farms . One was an800-acre sustainable farm that hadbeen managed without synthetic ferti-lizers and with only limited amountsof pesticides since it was first plowedin 1909 . The other was an adjacent1,300-acre conventional farm, whichhad first been cultivated in 1908 buthad been treated with fertilizers since1948 and with pesticides since theearly 1950's . The sustainable farmused a complex crop rotation systemand practiced conservation-orientedmethods of tillage, whereas the con-ventional farm followed a simple two-year rotation. The sustainable farmalso grew legumes as a cover crop andgreen manure .

Because of the differences in farm-ing methods, the soil on the sustain-able farm contained significantly moreorganic matter, nitrogen and biologi-cally available potassium than that onthe conventional farm . It had a bettercapacity for storing nutrients, a higherwater content, a larger microorganismpopulation and a greater polysaccha-ride content . The soil also had betterstructure and tilth and 16 more cen-timeters of crop-nourishing topsoil .This topsoil difference was attributedto significantly more soil erosion onthe conventional farm.

Average yields of winter wheat peracre between 1982 and 1986 were 8percent lower on the sustainable farmthan on the conventional farm. Never-theless, the sustainable farm matchedthe wheat production average forthe region-in fact, it yielded almost13 percent more wheat than anothernearby conventional farm with similarsoils . Its ability to do so, even afteralmost 80 years of farming withoutfertilizer, may result in part from re-duced soil erosion and maintenanceof soil productivity.Although conserving soil productiv-

ity is important to farmers, most ofthem usually select an agriculturalsystem on the basis of its short-termprofitability . Until recently conven-tional systems have usually appearedto be more profitable in the short termthan sustainable ones. This assess-ment comes as no surprise, becauseresearch and USDA policy over the pastfour decades have promoted conven-tional agriculture .Yet the long-term profitability of

conventional agriculture seems ques-tionable if the environmental andhealth costs currently borne by soci-

ety are taken into account. If theseindirect costs-were factored into thecosts of conventional farm produc-tion, then sustainable systems wouldlikely prove to be more profitable andmore beneficial to society .One of the best-known studies of

the economics of sustainable agricul-ture was conducted by William Locker-etz, Georgia Shearer and Daniel H .Kohl of Washington University. Theycompared energy efficiency and cropproduction costs between numerouspairs of organic and conventionalfarms in the Midwest . Between 1974and 1978 the energy consumed toproduce a dollar's worth of crop onthe organic farms was only about 40percent as great as on the convention-al farms. Although the organic farmshad lower crop yields than the conven-tional farms, their operating costswere lower by about the same cashequivalent. As a result, the net in-comes from crop production on thetwo types of farms were about equalevery year except one .Despite these encouraging results,

some farmers who have shifted fromconventional to sustainable practiceshave experienced short-term difficul-ties. Some of the problems arose be-cause the farmers abruptly stoppedapplying pesticide and fertilizer to alltheir fields . Such radical changes cansometimes decrease yields because ofsevere weed problems, explosive in-creases in insect pests and diminishedsoil fertility that lasts a few years.Researchers at the Rodale Research

Center in Kutztown, Pa ., have inves-tigated the transition from conven-tional to sustainable farming and veri-fied that such changes are best imple-mented with caution. Even a gradualchange may involve small decreasesin crop yields while the soil establish-es a new set of chemical and biologi-cal equilibria. Farmers should changeonly one field at a time to avoid plac-ing whole farms at risk . The transitionis also smoother if they regularly addorganic matter to the soil in the formof animal or green manures .

What are the forces that inhibitfarmers from adopting sus-tainable methods? One obsta-

cle is the federal farm programs, whichgenerally support prices for only ahandful of crops . Corn and other feedgrains, wheat, cotton and soybeansreceive roughly three fourths of allU.S . crop subsidies and account forapproximately two thirds of croplanduse . The lack of price supports forother crops effectively discouragesfarmers from diversifying and rotat-

ing their crops and from plantinggreen manures . Instead it gives thempowerful incentive to practice mono-culture to achieve maximum yieldsand profits.The long-term economic benefits of

sustainable agriculture may not be ev-ident to a farmer faced with havingto meet payments on annual produc-tion loans . Many conventional farm-ers are greatly in debt, partly becauseof heavy investments in specializedmachinery and other equipment, andtheir debt constrains the shift to moresustainable methods . To date, societyhas neither rewarded farmers finan-cially nor given them other incentivesfor choosing sustainable methodsthat would benefit the public .

Then, too, there is little informationavailable to farmers on sustainablepractices . Government-sponsored re-search has inadequately explored al-ternative farming and focused insteadon agrichemically based productionmethods . Agribusinesses also greatlyinfluence research by providing grantsto universities to develop chemical-intensive technologies for perpetuat-ing grain monocultures .

Legislative support for change in theU.S. agricultural system is growing,but financial support for sustainableagricultural projects is still only asmall part of the total outlay for agri-culture . Congress appropriated $3 .9million in fiscal year 1988 and $4.45million in fiscal 1989 to implement

the research and education programson sustainable farming called for inthe Agricultural Productivity Act, onepart of the Food Security Act of 1985 .Funding in fiscal 1990 has been thesame as in the previous year-$4.45million-which is only .5 percent ofthe total usDA research and educationbudget .The program for low-input sustain-

able agriculture, or LISA, that hasemerged from this federal effort hasmany objectives : to reduce relianceon fertilizer, pesticide and other pur-chased resources to farms ; to increasefarm profits and agricultural produc-tivity ; to conserve energy and naturalresources ; to reduce soil erosion andthe loss of nutrients ; and to developsustainable farming systems .A 1988 U.S . House of Representa-

tives report, Low Input Farming Sys-tems : Benefits and Barriers, recom-mended that Congress restructure orremove some provisions in farm-sup-port programs, particularly those thatencourage greater use of agrichemi-cals and that impede the adoption oflow-input methods. Last year threecongressional bills were introduced-two in the Senate and one in the Houseof Representatives-that would allowfarmers to rotate crops and use otheralternative methods without losingfarm-support funds. All these bills arepending.

Shifting mainstream agriculture to-ward more sustainable methods will

COMBINATIONS OF CROPS, or of crops and livestock, make farms more sustainableby maintaining soil productivity and by reducing a farm's reliance on a single crop .On the farm shown above, the parallel strips of land have been planted on thecontour of the terrain with oats (yellow) or corn or alfalfa (both green) . Within eachstrip, crops rotate on a four-year cycle: corn (a one-year crop) is replaced by oats(another one-year crop), which is then replaced by alfalfa (a two-year crop). Suchrotations improve the control of weeds, insects anddiseases ; they also improve theefficiency of nutrient cycling. Contour strip-cropping greatly reduces soil erosion.

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SUSTAINABLE FARMS

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VARIABLE FIXED TOTAL TOTAL NETCOSTS COSTS INPUT CASH RETURNS

COSTS INCOME

PROFITS from sustainable farms canexceed those of conventional farms, accordingto Steven L. Kraten, formerly of Washington State University. The cash incomes peracre for the two types of farms were comparable over two years, but because theinput costs of sustainable agriculture are lower, its net returns are 22.4 percenthigher.Variable costs include those for fuel, machinery maintenance, seed, fertilizer,pesticide andlabor. Among the fixed costs are property taxes and interest on loans.

require more than new laws and regu-lations ; it will also require more re-search and public education . Universi-ties and the USDA are slowly puttingmore emphasis on sustainable agri-cultural research . A high research pri-ority is the development of specificcropping systems that produce andconsume nitrogen more efficiently. Itis essential to learn how much nitro-gen is fixed by legumes under variousconditions, as well as the optimummeans for integrating legumes intocrop rotations .

The U.S . should also step up itsresearch efforts on other topics .More must be learned about al-

ternatives to fertilizers and the cyclingof nutrients through the agriculturalecosystem . Effective strategies mustbe developed for controlling pests,weeds and diseases biologically. Thestrategies may rely on beneficial in-sects and microorganisms, allelopath-ic crop combinations (which discour-age weed growth), diverse crop mix-tures and rotations and geneticallyresistant crops. More research shouldalso be done on the relative benefits ofvarious cover crops and tillage prac-tices and on integrating livestock intothe cropping system .U.S . farmers now use only a frac-

tion of the thousands of crop spe-

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cies in existence . They may benefit byincreasing cultivation of alternativecrops such as triticale, amaranth, gin-seng and lupine, which are grown inother countries . Yet in addition to di-versification, germ plasm (seeds, rootstocks and pollen) from traditionalcrops and their wild relatives must becollected and preserved continually.Well-managed collections of germ

plasm will give plant breeders a broad-er genetic base for producing newcrops with greater resistance to pests,diseases and drought. Today much ofthe germ plasm that U.S. plant breed-ers use to improve crops comes fromdeveloping countries .New breeds of crops being devel-

oped by biotechnology, such as grainsthat fix their own nitrogen, may even-tually be included in sustainable crop-ping systems . But neither biotechnolo-gy nor any other single technology canfix all the problems addressed by abalanced ecological approach . The suc-cess of sustainable agriculture doesnot hinge on creating supercrops : thesystem works with crops that areavailable now .

Better education is as important asfurther research . Farmers need toknow clearly what sustainable agricul-ture means, and they must see proofof its profitability. The USDA and theCooperative Extension Service should

provide farmers with information thatis up-to-date, accurate, practical andapplicable to local farming conditions .Farmers and the public also need tobe better educated about the potential-ly adverse environmental and healthconsequences of the pollution createdby certain agrichemical practices .One of the most effective meth-

ods for communicating practical in-formation about sustainable agricul-ture is through farmer-to-farmer net-works, such as the Practical Farmers ofIowa. Farmers in this association haveagreed to research and demonstratesustainable techniques on their lands .They meet regularly to share infor-mation and compare results . Becausesuch networks have aroused growinginterest and proved effective, the land-grant community should try to pro-mote their development .Some scientists and environmental-

ists have recommended levying taxeson fertilizers and pesticides to offsetthe environmental costs of agrichem-ical use, to fund sustainable agricul-tural research and to encourage farm-ers to reduce excessive use of agri-chemicals. This approach is preciselyhow funding for the Leopold Centerfor Sustainable Agriculture was estab-lished by the Iowa State Legislature in1987 as part of the Iowa GroundwaterProtection Act.

Agriculture is a fundamental com-ponent of the natural resources onwhich rests not only the quality ofhuman life but also its very existence .If efforts to create a sustainable agri-culture are successful, farmers willprofit and society in general will bene-fit in many ways . More important, theU.S . will protect its natural resourcesand move closer toward attaining asustainable society .

FURTHERREADINGORGANIC FARMING IN THE CORN BELT.William Lockeretz, Georgia Shearer andDaniel H. Kohl in Science, Vol. 211, No.4482, pages 540-547 ; February 6, 1981 .TOWARD A MORE SUSTAINABLE AGRICUL-TURE . Raymond P. Poincelot. AVI Pub-lishing Co ., 1986.LONG-TERM EFFECTS OF ORGANIC ANDCONVENTIONAL FARMING ON SOIL ERO-SION . John P. Reganold, Lloyd F. Elliottand Yvonne L. Unger in Nature, Vol.330, No. 6146, pages 370-372 ; Novem-ber 26, 1987.ALTERNATIvE AGRICULTURE. National Re-search Council. National AcademyPress, 1989 .SUSTAINABLE AGRICULTURAL SYSTEMS.Edited by Clive A . Edwards, Rattan Lal,Patrick Madden, Robert H. Miller andGar House. Ankeny, Iowa, Soil and Wa-ter Conservation Society, 1990 .