Section 2. Field Crops in the agro-ecosystem
Projected outcomes:
1. Students will understand how to apply ecological analysis to cropping systems.
2. Students will learn about key agro-ecological management practices, including
soil and fertility management, crop rotation, and pest management.
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Background / Lessons
Introduction
The Ground Beneath Our Feet
Ecological question 1: What are the nutrient flows in the system?
Ecological question 2: What are the sources and sinks of pollutants in the system?
Ecological question 3: What are the interactions of living organisms in the system?
Ecological question 4: What are the energy flows in the system?
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Introduction
Like natural ecosystems, agro-ecosystems are characterized by nutrient flows and cycles,
energy flows, and the interactions of living organisms with each other and the physical
environment. However, agro-ecosystems differ from natural ecosystems in two key
ways:
 First, we expect them to export particular biological goods for our use.
 Second, we deliberately manipulate them to get them to produce those goods in
abundance.
These two special qualities of agro-ecosystems in turn affect their key ecological
processes. Sustainable agriculture seeks to take advantage of ecosystem processes by
designing an agricultural system that works with them rather than against them to achieve
its production goals.
This section begins with a quick look at the role of the soil in the agro-ecosystem and
then encourages students to think about the ecology of field crop production by posing
four ecological questions.
1) What are the nutrient flows in the system?
2) What are the sources and sinks of pollutants in the system?
3) What are the interactions of living organisms in the system?
4) What are the energy flows in the system?
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The Ground Beneath Our Feet
“The maintenance of the fertility of the soil is the first condition of any permanent
system of agriculture.”
Sir Albert Howard, An Agricultural Testament, 1940, Oxford University Press, p. 1.
Every farmer knows that soils are important. The federal government has funded special
programs to help farmers protect and manage soils since the Dust Bowl of the 1930s.
One way to look at soils is as a physical medium. It needs to serve a variety of
mechanical functions: provide a substrate for plant roots to grow in, allow water to drain
so plant roots have access to oxygen, but hold on to enough water that roots have access
to water. The soil is also where plant nutrients are stored, transferred to roots, and
sometimes lost.
Many sustainable farmers think about soils as a living system. They value the
mechanical functions of soil, but they look beyond those properties to biological and
ecological services. These farmers seek to build and maintain good soil health, rather
than simply avoiding damaging the physical structure of the soil.
Suggested activity: Thinking about Soils <link>
Our understanding of soil biology is still quite rudimentary, but we are learning more and
more about how the multitude of living organisms in the soil affect soil quality and
processes and about how our actions in turn affect the life of the soil.
Key groups of soil organisms include:
Bacteria (single-celled organisms that are neither plants nor animals)
Fungi (neither plants nor animals, typically grow in long chains of cells called hyphae)
Protozoa (single-celled animals such as amoebae)
Nematodes (tiny non-segmented worms)
Arthropods (invertebrates such as insects, spiders, millipedes, etc.)
Earthworms
(Plant roots)
Each of these groups contains a wide variety of species, and the different species do very
different things. For example, one gram of soil may contain 11,000 different species of
bacteria. Some bacteria help decompose organic matter, some fix nitrogen, some prey on
living organisms causing disease, and a few bacteria photosynthesize.
Together, soil organisms perform critical ecological functions such as decomposing
organic matter, changing soil structure, moving, stabilizing, and transforming nutrients,
altering chemicals such as pesticides, and eating or helping each other.
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As you look at the four ecological questions below, keep the role of the soil and of soil
organisms in mind.
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Question 1: Where do key nutrients come from?
In a sustainable system they will be recycled on-site or generated in a renewable fashion.
Conventional and sustainable sources of three macro-nutrients for field crops
Nutrient
Conventional sources
Sustainable sources
Nitrogen
Synthesized from natural gas:
Fixed from the air by rhizobacteria
Urea, anhydrous ammonia
associated with legumes,
Manure and compost; blood meal?
Phosphorus Mined in Florida, Canada, etc. Manure, compost, bone meal ?
Potassium
Mined in Canada
Manure, compost
Sustainable practices
 Conserve soil (and nutrients) by minimizing tillage, installing terraces, shelterbelts,
filter and buffer strips, strip cropping, including perennial species and small grains in
the rotation, and using cover crops
 Use cover crops and legumes in the rotation to retain and fix nitrogen
 Recycle as many nutrients as possible on farm, including manure and crop residues
 Use local wastes rather than mined or newly synthesized fertilizers to compensate for
nutrients exported from the farm in the form of crops and unavoidable losses.
(Possible examples include manure from nearby farms, composted yard waste, and
food processing by-products. Some waste nutrient sources such as composted sewage
may pose threats for human health, such as elevated levels of heavy metals.)
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Question 2: What are the sources and sinks for pollutants in the system?
A sustainable system will minimize the amount of pollutants introduced into the
environment.
What is a pollutant? It is a chemical that is damaging to human health or the
environment.
Just as a weed is a “plant out of place,” so whether something is a pollutant depends in
part on context. For example, soil particles are a valuable resource in the crop field.
However, if those same soil particles are carried from the field to a stream or river by
erosion, they become sediment—a pollutant that can severely damage aquatic biological
communities.
Suggested activity: Resource or Pollutant <link>
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Also, whether a chemical is a pollutant may depend on concentration. For example,
nitrogen is a critical plant nutrient. However, if it is present in excess, it can be toxic to
plants or can degrade marine systems. Keep in mind, however, that there are some
chemicals that are not biologically beneficial at any level and that may pose threats to
human health or other species at levels too low to detect. For example, heavy metals
such as lead and arsenic are not necessary nutrients for people. At high levels lead
exposure can lead to death. At lower levels, the impacts of lead on human health are
reduced, but no threshold has been established below which lead is thought to have no
negative effect on human health. Lead is a common pollutant of urban soils, particularly
surrounding houses built before 1978, when the use of lead-based paint was banned in the
US. Heavy metals such as lead and arsenic are not common field crop pollutants, but
their potential presence in biosolids (otherwise known as sewage sludge) is one reason
some people are concerned about the use of biosolids for crop production.
Thus, the source of many agricultural pollutants is deliberate application of inputs such
as fertilizers and pesticides. Another source is beneficial resources such as soil or
manure, which turn into pollutants when mismanaged and displaced.
A sink is where the pollutant winds up. Surface waters, including rivers, lakes, and the
ocean, are a common sink for agricultural pollutants.
Sustainable practices
 Conserve soil by minimizing tillage, installing terraces, shelterbelts, filter and buffer
strips, strip cropping, including perennial species and small grains in the rotation, and
using cover crops
 Minimize use of pesticides through Integrated Pest Management (IPM) and store,
handle, and apply properly when used. Crop rotation, cover cropping, and cultivation
can greatly reduce weed, insect, and disease pressures (see Eight Ways to Reduce
Pesticide Use http://www.extension.iastate.edu/Publications/IPM59.pdf , Wisconsin
ICPM website http://ipcm.wisc.edu/green/Pest_management.htm#Field%20Crops, and
Iowa IPM website )

Manage manure by storing properly (including composting), not over-applying, and
applying properly (inject or incorporate liquid manure, avoid application when ground
is frozen) (see http://ipcm.wisc.edu/pubs/pdf/SeasonalGuideweb.pdf )

Minimize nitrogen losses by crediting N from legumes and manure, using the late
spring soil nitrate test to assess N needs, applying N fertilizer when and where the
crop can use it (e.g., during growing season and banding)

Apply phosphorus only when soil test levels are below high. Manure is high in P, so
avoid applying to soils very high in phosphorus. (see
http://ipcm.wisc.edu/pubs/nutrient/USP_2002.htm, pp. 15-16)
How significant are agricultural pollutants anyway?
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Agriculture is only one of many sources of pollution. Industry, urban development,
automobiles, and other sectors also cause significant amounts of pollution. The
importance of agriculture in causing pollution depends in part on where you look and
what pollutants you are looking at.
For example, if you take a water sample from the Milwaukee waterfront of Lake
Michigan or from the Mississippi River at Davenport, chances are that pollutants from
urban stormwater runoff, industrial discharges, and effluent from sewage treatment plans
will dominate your sample. On the other hand, when you look at all of Lake Michigan
and the Mississippi River, agriculture is the major source for such significant pollutants
as excess nitrogen and phosphorus, sediment, and pesticides such as atrazine.
Suggested activity: Hypoxia hearings <link>
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Question 3: How do the living organisms in the system interact?
Typically, sustainable agro-ecosystems will try to work with species interactions and will
favor species diversity.
Everything in an ecosystem affects other parts of the ecosystem. Typically, production
agriculture has focused on the negative impacts of organisms other than the crop. In this
worldview, all non-crop plants are seen as weeds that compete for water, nutrients, and
sunlight, and all non-crop animals from insects to birds and mammals are seen as useless
at best and crop-destroying pests or disease carriers at worst.
There is a lot of truth to this outlook. Weeds do compete with crop plants, and many
types of animals eat parts of the crop and can cause substantial yield losses. Agroecosystems differ from natural ecosystems in that we require them to export a good
portion of their production for off-site human consumption. So we cannot afford to give
weeds, crop predators, and diseases a free hand.
On the other hand, it turns out that many non-crop organisms benefit crop production in
a variety of ways, such as by improving nutrient cycling and availability to the crop,
eating crop pests, providing habitat for beneficial species, and reducing disease.
Practices such as heavy use of synthetic fertilizers and pesticides and mono-cropping may
harm beneficial organisms as much or more than pests.
Comparison of plant communities
Pre-agricultural / “natural”
 Mix of perennial and annual species
 Variety of communities depending on soil type, climate and micro-climate, and site
history (prairies, savannahs, wetlands, forests, etc)
 Large number of species at one site, typically little or no exposed soil year-round
 Substantial genetic variation within most species
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Conventional agriculture
 Sequential mono-cultures (usually 2 or at most 3 species in rotation, sometimes only
one species)
 Same 3 species planted on more than half of all farmland in the two states (extremely
low tolerance for weeds)
 Only annual row crops in Iowa, lots of bare soil exposed for 6 months of the year or
more on nearly all acres in row crops
 Little genetic variety within the species
Sustainable agriculture in Wisconsin and Iowa
 Sequential mono-cultures (usually 3 to 6 species in rotation)
 6 or more species in common use; strong interest in adopting more crops (low
tolerance for weeds)
 Annual row crops rotated with perennials and small grains. Moderate adoption of
cover crops. Bare soil exposed for 6 months on a substantial percentage of field crop
acreage
 Interest in increasing genetic variety within crop species, but variety is only slightly
higher than for conventional agriculture at present. In traditional agriculture regions in
the developing world genetic variation is extremely high, but under great pressure.
 Restoration of complex natural or partly natural plant communities around crop fields
Other biological components of the agro-ecosystem: soil biota, including fungi, animals,
bacteria, insects, birds, mammals. We know that agricultural management changes the
distribution of soil organisms, but we do not yet have enough information to know details
of the changes or what impacts many of the changes have. See Soil Biology Primer online: http://soils.usda.gov/sqi/soil_quality/soil_biology/soil_biology_primer.html
Sustainable practices:
 Increase diversity of crop rotation, both in terms of more species and more different
types (e.g., row crops, perennials, small grains)
 Use cover crops (for detailed information on cover crop uses and benefits, see
Managing Cover Corps Profitably at http://www.cacscw.org/ .)
 Plant hedgerows, shelterbelts, riparian buffers, field borders to provide habitat for
beneficial insects and for native plants and animals
 Maintain and use landraces and other non-standard crop varieties
 Avoid synthetic fertilizers and pesticides, which can negatively affect soil biota and
wildlife
 Minimize tillage and build organic matter in the soil, which tends to encourage greater
diversity of soil biota
Suggested activity: Diversity in natural habitats <link>
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Question 4: What are the energy flows?
Sustainable agro-ecosystems rely more on solar energy rather than on fossil fuels.
Sustainable systems minimize energy waste.
Sentence about fossil fuel use of industrial ag…
Sustainable practices
 Minimize use of nitrogen fertilizer and pesticides (see specific strategies
recommended in the pollution prevention segment)
 Recycle nutrients and resources on the farm
 Minimize transportation costs by using feed on-farm or selling to and buying from
local sources
 Allow animals to graze (including strip-grazing of standing crops) to avoid energy use
involved in harvesting, drying, and storing feed crops. In fact, if the crop will be used
to feed livestock, consider switching from row crops to pasture. (See energy data
from the Wisconsin Cropping Systems Trials, especially Table 3, at
http://www.wisc.edu/cias/wicst/pubs/energy.htm.)
 Use renewable energy sources such as wind, solar, or biomass-fueled power
Suggested activity: How much oil are we eating? <link>
More than half of the energy in our food system is used not on the farm, but in
transportation, processing, storage and packaging, and home cooking.
Sustainable practices for the consumer
 Buy local foods, when possible
 Avoid excess packaging
 Use energy-efficient appliances and techniques when possible
 Use renewable energy sources, if possible (solar and wind power)
 Consider eating lower on the food chain or sticking to grass-fed meat and dairy
products (most of the food energy contained in grain is used by livestock to sustain
their own life and only a small amount is stored as meat. Thus it takes 4 lbs of corn to
produce 1 lb of pork and 10 lbs of corn to produce 1 lb of beef)
http://www.organicconsumers.org/corp/family_farm_wind_power.cfm is an article about
the economic benefits to farmers of allowing wind generation of electricity on their land.
(Check following ref. Energy: Its Use and The Environment by Roger A. Hinrichs, 1996)
Biofuels: good or bad?
According to Amory and Hunter Lovins and Marty Bender, a program to make liquid
fuels (gasohol or biodiesel) from biomass must adhere to four principles if it is to be
sustainable:
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1.
The land comes first. All operations must be based on a concern for soil fertility and
long_term environmental compatibility
2. Efficiency is vital. Both the vehicle for which the fuel is intended and the means of
converting the biomass into fuel must be efficient.
3. Wastes are the source. Use farming and forestry wastes as the principal feedstocks,
no crop should be grown just to make fuels.
4. Sustainability is a goal. The program should be a vehicle for the reform of currently
unsustainable farming and forestry practices.
Cited from Amory B. Lovins, L. Hunter Lovins, and Marty Bender, “Energy and
Agriculture,” in Meeting the Expectations of the Land ed. Wes Jackson, Wendell Berry,
and Bruce Colman. 1984. San Francisco: North Point Press, p. 80.
Suggested activity: is ethanol sustainable? <link>
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Conclusion
Jules Pretty sums up the agro-ecological principles of sustainable agriculture this way:
It makes the best use of nature’s goods and services while not damaging the
environment. Sustainable farming does this by integrating natural processes,
such as nutrient cycling, nitrogen fixation, soil regeneration and natural pest
control, within food production processes. It also minimizes the use of nonrenewable inputs that damage the environment or harm the health of farmers and
consumers.
From Agri-Culture: Reconnecting People, Land and Nature, London: Earthscan
Publications Limited, 2002, p. 56.
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