Modern Agriculture: Ecological
impacts and the possibilities for truly sustainable farming
Miguel A. Altieri
Division of Insect Biology
University of California, Berkeley
Until about four decades ago, crop yields
in agricultural systems depended on internal resources, recycling of
organic matter, built-in biological control mechanisms and rainfall
patterns. Agricultural yields were modest, but stable. Production was
safeguarded by growing more than one crop or variety in space and time
in a field as insurance against pest outbreaks or severe weather. Inputs
of nitrogen were gained by rotating major field crops with legumes.
In turn rotations suppressed insects, weeds and diseases by effectively
breaking the life cycles of these pests. A typical corn belt farmer
grew corn rotated with several crops including soybeans, and small grain
production was intrinsic to maintain livestock. Most of the labor was
done by the family with occasional hired help and no specialized equipment
or services were purchased from off-farm sources. In these type of farming
systems the link between agriculture and ecology was quite strong and
signs of environmental degradation were seldom evident (1) .
But as agricultural modernization progressed,
the ecology-farming linkage was often broken as ecological principles
were ignored and/or overridden. In fact, several agricultural scientists
have arrived at a general consensus that modern agriculture confronts
an environmental crisis. A growing number of people have become concerned
about the long-term sustainability of existing food production systems.
Evidence has accumulated showing that whereas the present capital- and
technology-intensive farming systems have been extremely productive
and competitive, they also bring a variety of economic, environmental
and social problems (2) .
Evidence also shows that the very nature
of the agricultural structure and prevailing policies have led to this
environmental crisis by favoring large farm size, specialized production,
crop monocultures and mechanization. Today as more and more farmers
are integrated into international economies, imperatives to diversity
disappear and monocultures are rewarded by economies of scale. In turn,
lack of rotations and diversification take away key self-regulating
mechanisms, turning monocultures into highly vulnerable agroecosystems
dependent on high chemical inputs.
The expansion of monocultures
Today monocultures have increased dramatically
worldwide, mainly through the geographical expansion of land devoted
to single crops and year-to-year production of the same crop species
on the same land. Available data indicate that the amount of crop diversity
per unit of arable land has decreased and that croplands have shown
a tendency toward concentration. There are political and economic forces
influencing the trend to devote large areas to monoculture, and in fact
such systems are rewarded by economies of scale and contribute significantly
to the ability of national agricultures to serve international markets.
The technologies allowing the shift toward
monoculture were mechanization, the improvement of crop varieties, and
the development of agrochemicals to fertilize crops and control weeds
and pests. Government commodity policies these past several decades
encouraged the acceptance and utilization of these technologies. As
a result, farms today are fewer, larger, more specialized and more capital
intensive. At the regional level, increases in monoculture farming meant
that the whole agricultural support infrastructure (i.e. research, extension,
suppliers, storage, transport, markets, etc.) has become more specialized.
From an ecological perspective, the regional
consequences of monoculture specialization are many-fold:
- Most large-scale agricultural systems
exhibit a poorly structured assemblage of farm components, with almost
no linkages or complementary relationships between crop enterprises
and among soils, crops and animals.
- Cycles of nutrients, energy, water and
wastes have become more open, rather than closed as in a natural ecosystem.
Despite the substantial amount of crop residues and manure produced
in farms, it is becoming increasingly difficult to recycle nutrients,
even within agricultural systems. Animal wastes cannot economically
be returned to the land in a nutrient-recycling process because production
systems are geographically remote from other systems which would complete
the cycle. In many areas, agricultural waste has become a liability
rather than a resource. Recycling of nutrients from urban centers
back to the fields is similarly difficult.
- Part of the instability and susceptibility
to pests of agroecosystems can be linked to the adoption of vast crop
monocultures, which have concentrated resources for specialist crop
herbivores and have increased the areas available for immigration
of pests. This simplification has also reduced environmental opportunities
for natural enemies. Consequently, pest outbreaks often occur when
large numbers of immigrant pests, inhibited populations of beneficial
insects, favorable weather and vulnerable crop stages happen simultaneously.
- As specific crops are expanded beyond
their "natural" ranges or favorable regions to areas of
high pest potential, or with limited water, or low-fertility soils,
intensified chemical controls are required to overcome such limiting
factors. The assumption is that the human intervention and level of
energy inputs that allow these expansions can be sustained indefinitely.
- Commercial farmers witness a constant
parade of new crop varieties as varietal replacement due to biotic
stresses and market changes has accelerated to unprecedented levels.
A cultivar with improved disease or insect resistance makes a debut,
performs well for a few years (typically 5-9 years) and is then succeeded
by another variety when yields begin to slip, productivity is threatened,
or a more promising cultivar becomes available. A varietys trajectory
is characterized by a take-off phase when it is adopted by farmers,
a middle stage when the planted area stabilizes and finally a retraction
of its acreage. Thus, stability in modern agriculture hinges on a
continuous supply of new cultivars rather than a patchwork quilt of
many different varieties planted on the same farm.
- The need to subsidize monocultures requires
increases in the use of pesticides and fertilizers, but the efficiency
of use of applied inputs is decreasing and crop yields in most key
crops are leveling off. In some places, yields are actually in decline.
There are different opinions as to the underlying causes of this phenomenon.
Some believe that yields are leveling off because the maximum yield
potential of current varieties is being approached, and therefore
genetic engineering must be applied to the task of redesigning crop.
Agroecologists, on the other hand, believe that the leveling off is
because of the steady erosion of the productive base of agriculture
through unsustainable practices (3).
The first wave of environmental problems
The specialization of production units
has led to the image that agriculture is a modern miracle of food production.
Evidence indicates, however, that excessive reliance on monoculture
farming and agroindustrial inputs, such as capital-intensive technology,
pesticides, and chemical fertilizers, has negatively impacted the environment
and rural society. Most agriculturalists had assumed that the agroecosystem/natural
ecosystem dichotomy need not lead to undesirable consequences, yet,
unfortunately, a number of "ecological diseases" have been
associated with the intensification of food production. They may be
grouped into two categories: diseases of the ecotope, which include
erosion, loss of soil fertility, depletion of nutrient reserves, salinization
and alkalinization, pollution of water systems, loss of fertile croplands
to urban development, and diseases of the biocoenosis, which include
loss of crop, wild plant, and animal genetic resources, elimination
of natural enemies, pest resurgence and genetic resistance to pesticides,
chemical contamination, and destruction of natural control mechanisms.
Under conditions of intensive management, treatment of such "diseases"
requires an increase in the external costs to the extent that, in some
agricultural systems, the amount of energy invested to produce a desired
yield surpasses the energy harvested (4).
The loss of yields due to pests in many
crops (reaching about 20-30% in most crops), despite the substantial
increase in the use of pesticides (about 500 million kg of active ingredient
worldwide) is a symptom of the environmental crisis affecting agriculture.
It is well known that cultivated plants grown in genetically homogenous
monocultures do not possess the necessary ecological defense mechanisms
to tolerate the impact of outbreaking pest populations. Modern agriculturists
have selected crops for high yields and high palatability, making them
more susceptible to pests by sacrificing natural resistance for productivity.
On the other hand, modern agricultural practices negatively affect pest
natural enemies, which in turn do not find the necessary environmental
resources and opportunities in monocultures to effectively and biologically
suppress pests. Due to this lack of natural controls, an investment
of about 40 billion dollars in pesticide control is incurred yearly
by US farmers, which is estimated to save approximately $16 billion
in US crops. However, the indirect costs of pesticide use to the environment
and public health have to be balanced against these benefits. Based
on the available data, the environmental (impacts on wildlife, pollinators,
natural enemies, fisheries, water and development of resistance) and
social costs (human poisonings and illnesses) of pesticide use reach
about $8 billion each year (5). What is worrisome is that pesticide
use is on the rise. Data from California shows that from 1941 to 1995
pesticide use increased from 161 to 212 million pounds of active ingredient.
These increases were not due to increases in planted acreage, as statewide
crop acreage remained constant during this period. Crops such as strawberries
and grapes account for much of this increased use, which includes toxic
pesticides, many of which are linked to cancers (6) .
Fertilizers, on the other hand, have been
praised as being highly associated with the temporary increase in food
production observed in many countries. National average rates of nitrate
applied to most arable lands fluctuate between 120-550 kg N/ha. But
the bountiful harvests created at least in part through the use of chemical
fertilizers, have associated, and often hidden, costs. A primary reason
why chemical fertilizers pollute the environment is due to wasteful
application and the fact that crops use them inefficiently. The fertilizer
that is not recovered by the crop ends up in the environment, mostly
in surface water or in ground water. Nitrate contamination of aquifers
is widespread and in dangerously high levels in many rural regions of
the world. In the US, it is estimated that more than 25% of the drinking
water wells contain nitrate levels above the 45 parts per million safety
standard. Such nitrate levels are hazardous to human health and studies
have linked nitrate uptake to methaemoglobinemia in children and to
gastric, bladder and oesophageal cancers in adults (7) .
Fertilizer nutrients that enter surface
waters (rivers, lakes, bays, etc.) can promote eutrophication, characterized
initially by a population explosion of photosynthetic algae. Algal blooms
turn the water bright green, prevent light from penetrating beneath
surface layers, and therefore killing plants living on the bottom. Such
dead vegetation serve as food for other aquatic microorganisms which
soon deplete water of its oxygen, inhibiting the decomposition of organic
residues, which accumulate on the bottom. Eventually, such nutrient
enrichment of freshwater ecosystems leads to the destruction of all
animal life in the water systems. In the US it is estimated that about
50-70% of all nutrients that reach surface waters is derived from fertilizers.
Chemical fertilizers can also become air
pollutants, and have recently been implicated in the destruction of
the ozone layer and in global warming. Their excessive use has also
been linked to the acidification/salinization of soils and to a higher
incidence of insect pests and diseases through mediation of negative
nutritional changes in crop plants (8).
It is clear then that the first wave of
environmental problems is deeply rooted in the prevalent socioeconomic
system which promotes monocultures and the use of high input technologies
and agricultural practices that lead to natural resource degradation.
Such degradation is not only an ecological process, but also a social
and political-economic process (9) . This is why the problem of agricultural
production cannot be regarded only as a technological one, but while
agreeing that productivity issues represent part of the problem, attention
to social, cultural and economic issues that account for the crisis
is crucial. This is particularly true today where the economic and political
domination of the rural development agenda by agribusiness has thrived
at the expense of the interests of consumers, farmworkers, small family
farms, wildlife, the environment, and rural communities (10).
The second wave of environmental problems.
Despite that awareness of the impacts of
modern technologies on the environment increased, as we traced pesticides
in food chains and crop nutrients in streams and aquifiers, there are
those that confronted to the challenges of the XXI century still argue
for further intensification to meet the requirements of agricultural
production. It is in this context that supporters of "status-quo
agriculture" celebrate the emergence of biotechnology as the latest
magic bullet that will revolutionize agriculture with products based
on natures own methods, making farming more environmentally friendly
and more profitable for the farmer. Although clearly certain forms of
non-transformational biotechnology hold promise for an improved agriculture,
given its present orientation and control by multinational corporations,
it holds more promise for environmental harm, for the further industrialization
of agriculture and for the intrusion of private interests too far into
public interest sector research (11).
What is ironic is the fact that the biorevolution
is being brought forward by the same interests (Monsanto, Novartis,
DuPont, etc.) that promoted the first wave of agrochemically-based agriculture,
but this time, by equipping each crop with new "insecticidal genes",
they are promising the world safer pesticides, reduction on chemically
intensive farming and a more sustainable agriculture.
However, as long as transgenic crops follow
closely the pesticide paradigm, such biotechnological products will
do nothing but reinforce the pesticide treadmill in agroecosystems,
thus legitimizing the concerns that many scientists have expressed regarding
the possible environmental risks of genetically engineered organisms.
So far, field research as well as predictions
based on ecological theory, indicate that among the major environmental
risks associated with the release of genetically engineered crops can
be summarized as follows (12):
- The trends set forth by corporations
is to create broad international markets for a single product, thus
creating the conditions for genetic uniformity in rural landscapes.
History has repeatedly shown that a huge area planted to a single
cultivar is very vulnerable to a new matching strain of a pathogen
- The spread of transgenic crops threatens
crop genetic diversity by simplifying cropping systems and promoting
- There is potential for the unintended
transfer to plant relatives of the "transgenes" and the
unpredictable ecological effects. The transfer of genes from herbicide
resistant crops (HRCs) to wild or semidomesticated relatives can lead
to the creation of super weeds;
- Most probably insect pests will quickly
develop resistance to crops with Bt toxin. Several Lepidoptera species
have been reported to develop resistance to Bt toxin in both field
and laboratory tests, suggesting that major resistance problems are
likely to develop in Bt crops which through the continuous expression
of the toxin create a strong selection pressure;
- Massive use of Bt toxin in crops can
unleash potential negative interactions affecting ecological processes
and non-target organisms. Evidence from studies conducted in Scotland
suggest that aphids were capable of sequestering the toxin from Bt
crops and transferring it to its coccinellid predators, in turn affecting
reproduction and longevity of the beneficial beetles;
- Bt toxins can also be incorporated into
the soil through leaf materials and litter, where they may persist
for 2-3 months, resisting degradation by binding to soil clay particles
while maintaining toxic activity, in turn negatively affecting invertebrates
and nutrient cycling;
- A potential risk of transgenic plants
expressing viral sequences derives from the possibility of new viral
genotypes being generated by recombination between the genomic RNA
of infecting viruses and RNA transcribed from the transgene;
- Another important environmental concern
associated with the large scale cultivation of virus-resistant transgenic
crops relates to the possible transfer of virus-derived transgenes
into wild relatives through pollen flow.
Although there are many unanswered questions
regarding the impact of the release of transgenic plants and micro-organisms
into the environment, it is expected that biotechnology will exacerbate
the problems of conventional agriculture and by promoting monocultures
will also undermine ecological methods of farming such as rotations
and polycultures. Because transgenic crops developed for pest control
emphasize the use of a single control mechanism, which has proven to
fail over and over again with insects, pathogens and weeds, transgenic
crops are likely to increase the use of pesticides and to accelerate
the evolution of "super weeds" and resistant insect pest strains.
These possibilities are worrisome, especially when considering that
during the period 1986-1997, approximately 25,000 transgenic crop field
trials were conducted worldwide on more than 60 crops with 10 traits
in 45 countries. By 1997 the global area devoted to transgenic crops
reached 12.8 million hectares. Seventy-two percent of all transgenic
crop field trials were conducted in the USA and Canada, although some
were also conducted in descending order in Europe, Latin America and
Asia (13). In most countries biosafety standards to monitor such releases
are absent or are inadequate to predict ecological risks. In the industrialized
countries from 1986-1992, 57% of all field trials to test transgenic
crops involved herbicide tolerance pioneered by 27 corporations including
the worlds eight largest pesticide companies. As Roundup and other
broad spectrum herbicides are increasingly deployed into croplands,
the options for farmers for a diversified agriculture will be even more
The array of alternatives to conventional
Reduction and, especially, elimination
of agrochemical require major changes in management to assure adequate
plant nutrients and to control crop pests. As it was done a few decades
ago, alternative sources of nutrients to maintain soil fertility include
manures, sewage sludge and other organic wastes, and legumes in cropping
sequences. Rotation benefits are due to biologically fixed nitrogen
and from the interruption of weed, disease and insect cycles. A livestock
enterprise may be integrated with grain cropping to provide animal manures
and to utilize better the forages produced. Maximum benefits of pasture
integration can be realized when livestock, crops, animals and other
farm resources are assembled in mixed and rotational designs to optimize
production efficiency, nutrient cycling and crop protection.
In orchards and vineyards, the use of cover
crops improve soil fertility, soil structure and water penetration,
prevent soil erosion, modify the microclimate and reduce weed competition.
Entomological studies conducted in orchards with ground cover vegetation
indicate that these systems exhibit lower incidence of insect pests
than clean cultivated orchards. This is due to a higher abundance and
efficiency of predators and parasitoids enhanced by the rich floral
Increasingly, researchers are showing that
it is possible to provide a balanced environment, sustained yields,
biologically mediated soil fertility and natural pest regulation through
the design of diversified agroecosystems and the use of low-input technologies.
Many alternative cropping systems have been tested, such as double cropping,
strip cropping, cover cropping and intercropping, and more importantly
concrete examples from real farmers show that such systems lead to optimal
recycling of nutrients and organic matter turnover, closed energy flows,
water and soil conservation and balanced pest-natural enemy populations.
Such diversified farming exploit the complementarities that result from
the various combinations of crops, trees and animals in spatial and
temporal arrangements (15).
In essence, the optimal behavior of agroecosystems
depends on the level of interactions between the various biotic and
abiotic components. By assembling a functional biodiversity it is possible
to initiate synergisms which subsidize agroecosystem processes by providing
ecological services such as the activation of soil biology, the recycling
of nutrients, the enhancement of beneficial arthropods and antagonists,
and so on. Today there is a diverse selection of practices and technologies
available, and which vary in effectiveness as well as in strategic value.
The barriers for the implementation
The agroecological approach seeks the diversification
and revitalization of medium size and small farms and the reshaping
of the entire agricultural policy and food system in ways that are economically
viable to farmers and consumers. In fact, throughout the world there
are hundreds of movements that are pursuing a change toward ecologically
sensitive farming systems from a variety of perspectives. Some emphasize
the production of organic products for lucrative markets, others land
stewardship, while others the empowerment of peasant communities. In
general, however, the goals are usually the same: to secure food self-sufficiency,
to preserve the natural resource base, and to ensure social equity and
What happens is that some well-intentioned
groups suffer from "technological determinism", and emphasize
as a key strategy only the development and dissemination of low-input
or appropriate technologies as if these technologies in themselves have
the capability of initiating beneficial social changes. The organic
farming school that emphasizes input substitution (i.e. a toxic chemical
substituted by a biological insecticide) but leaving the monoculture
structure untouched, epitomizes those groups that have a relatively
benign view of capitalist agriculture. Such perspective has unfortunately
prevented many groups from understanding the structural roots of environmental
degradation linked to monoculture farming (16).
This narrow acceptance of the present structure
of agriculture as a given condition restricts the real possibility of
implementing alternatives that challenge such a structure. Thus, options
for a diversified agriculture are inhibited among other factors by the
present trends in farm size and mechanization. Implementation of such
mixed agriculture would only be possible as part of a broader program
that includes, among other strategies, land reform and redesign of farm
machinery adapted to polycultures. Merely introducing alternative agricultural
designs will do little to change the underlying forces that led to monoculture
production, farm size expansion, and mechanization in the first place.
Similarly, obstacles to changing cropping
systems has been created by the government commodity programs in place
these last several decades. In essence, these programs have rewarded
those who maintained monocultures on their base feed grain acres by
assuring these producers a particular price for their product. Those
who failed to plant the allotted acreage of corn and other price-supported
crops lost one deficit hectrage from their base. Consequently this created
a competitive disadvantage for those who used a crop rotation. Such
a disadvantage, of course, exacerbated economic hardship for many producers
(17). Obviously many policy changes are necessary in order to create
an economic scenario favorable to alternative cropping practices.
On the other hand, the large influence
of multinational companies in promoting sales of agrochemicals cannot
be ignored as a barrier to sustainable farming. Most MNCs have taken
advantage of existing policies that promote the enhanced participation
of the private sector in technology development and delivery, positioning
themselves in a powerful position to scale up promotion and marketing
of pesticides. Realistically then the future of agriculture will be
determined by power relations, and there is no reason why farmers and
the public in general, if sufficiently empowered, could not influence
the direction of agriculture along sustainability goals.
Clearly the nature of modern agricultural
structure and contemporary policies have decidedly influenced the context
of agricultural technology and production, which in turn has led to
environmental problems of a first and second order. In fact, given the
realities of the dominant economic milieu, policies discourage resource-conserving
practices and in many cases such practices are not privately profitable
for farmers. So the expectation that a set of policy changes could be
implemented for a renaissance of diversified or small scale farms may
be unrealistic, because it negates the existence of scale in agriculture
and ignores the political power of agribusiness corporations and current
trends set forth by globalization. A more radical transformation of
agriculture is needed, one guided by the notion that ecological change
in agriculture cannot be promoted without comparable changes in the
social, political, cultural and economic arenas that also conform agriculture.
In other words, change toward a more socially just, economically viable,
and environmentally sound agriculture should be the result of social
movements in the rural sector in alliance with urban organizations.
This is especially relevant in the case of the new biorevolution, where
concerted action is needed so that biotechnology companies feel the
impact of environmental, farm labor, animal rights and consumers lobbies,
pressuring them to re-orienting their work for the overall benefit of
society and nature.
Altieri, M.A. 1992. Agroecological foundations
of alternative agriculture in California. Agriculture, Ecosystems and
Environment 39: 23-53.
Altieri, M.A. 1995. Agroecology: the science
of sustainable agriculture. Westview Press, Boulder
Altieri, M.A. and P.M. Rosset 1995. Agroecology
and the conversion of large-scale conventional systems to sustainable
management. International Journal of Environmental Studies 50: 165-185.
Audirac, Y. 1997. Rural sustainable development
in America. John Wiley and Sons, N.Y.
Buttel, F.H. and M.E. Gertler 1982. Agricultural
structure, agricultural policy and environmental quality. Agriculture
and Environment 7: 101-119.
Conway, G.R. and Pretty, J.N. 1991. Unwelcome
harvest: agriculture and pollution. Earthscan Publisher, London.
Gliessman, S.R. 1997. Agroecology: ecological
processes in agriculture. Ann Arbor Press, Michigan.
James, C. 1997. Global status of transgenic
crops in 1997. ISAA Briefs, Ithaca, N.Y.
Krimsky, S. and R.P. Wrubel 1996. Agricultural
biotechnology and the environment: science, policy and social issues.
University of Illinois Press, Urbana.
Liebman, J. 1997. Rising toxic tide: pesticide
use in California, 1991-1995. Report of Californians for Pesticide Reform
and Pesticide Action Network. San Francisco.
Mc Guinnes, H. 1993. Living soils: sustainable
alternatives to chemical fertilizers for developing countries. Unpublished
manuscript, Consumers Policy Institute, New York.
Mc Isaac, G. and W.R. Edwards 1994. Sustainable
agriculture in the American midwest. University of Illinois Press, Urbana.
Pimentel, D. and H. Lehman 1993. The pesticide
question. Chapman and Hall, N.Y.
Rissler, J. and M. Mellon 1996. The ecological
risks of engineered crops. MIT Press, Cambridge.
Rosset, P.M. and M.A. Altieri 1997. Agroecology
versus input substitution: a fundamental contradiction in sustainable
agriculture. Society and Natural Resources 10: 283-295.
(1) Altieri, M.A. 1995. Agroecology: the
science of sustainable agriculture. Westview Press, Boulder
(2) Conway, G.R. and Pretty, J.N. 1991.
Unwelcome harvest: agriculture and pollution. Earthscan Publisher, London.
(3) Altieri, M.A. and P.M. Rosset 1995.
Agroecology and the conversion of large-scale conventional systems to
sustainable management. International Journal of Environmental Studies
(4) Gliessman, S.R. 1997. Agroecology:
ecological processes in agriculture. Ann Arbor Press, Michigan.
(5) Pimentel, D. and H. Lehman 1993. The
pesticide question. Chapman and Hall, N.Y.
(6) Liebman, J. 1997. Rising toxic tide:
pesticide use in California, 1991-1995. Report of Californians for Pesticide
Reform and Pesticide Action Network. San Francisco.
(7) Conway, G.R. and Pretty, J.N. 1991.
Unwelcome harvest: agriculture and pollution. Earthscan Publisher, London.
(8) Mc Guinnes, H. 1993. Living soils:
sustainable alternatives to chemical fertilizers for developing countries.
Unpublished manuscript, Consumers Policy Institute, New York.
(9) Buttel, F.H. and M.E. Gertler 1982.
Agricultural structure, agricultural policy and environmental quality.
Agriculture and Environment 7: 101-119.
(10) Audirac, Y. 1997. Rural sustainable
development in America. John Wiley and Sons, N.Y.
(11) Krimsky, S. and R.P. Wrubel 1996.
Agricultural biotechnology and the environment: science, policy and
social issues. University of Illinois Press, Urbana.
(12) Rissler, J. and M. Mellon 1996. The
ecological risks of engineered crops. MIT Press, Cambridge.
(13) James, C. 1997. Global status of transgenic
crops in 1997. ISAA Briefs, Ithaca, N.Y.
(14) Altieri, M.A. 1992. Agroecological
foundations of alternative agriculture in California. Agriculture, Ecosystems
and Environment 39: 23-53.
(15) Altieri, M.A. 1995. Agroecology: the
science of sustainable agriculture. Westview Press, Boulder
(16) Rosset, P.M. and M.A. Altieri 1997.
Agroecology versus input substitution: a fundamental contradiction in
sustainable agriculture. Society and Natural Resources 10: 283-295.
(17) Mc Isaac, G. and W.R. Edwards 1994.
Sustainable agriculture in the American midwest. University of Illinois