Section C: Field Crops in the Agro-ecosystem
- Projected Outcomes
- Background / Lessons
- 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
- Career Pathway content standards
- Students will understand how to apply ecological analysis to cropping systems.
- Students will learn about key agro-ecological management practices, including soil and fertility management, crop rotation, and pest management.
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.
- What are the nutrient flows in the system?
- What are the sources and sinks of pollutants in the system?
- What are the interactions of living organisms in the system?
- What are the energy flows in the system?
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.
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.)
- (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.
Short videos on soil quality, including a number of mini case studies http://vimeo.com/channels/raythesoilguy/page:1 Make sure you check out pages 2-4 as well as the first page.
As you look at the four ecological questions below, keep the role of the soil and of soil organisms in mind.
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:
Urea, anhydrous ammonia
|Fixed from the air by rhizobacteria associated with legumes,
Manure and compost; blood meal?
|Phosphorus||Mined in Florida, Canada, etc.||Manure, compost, bone meal ?|
|Potassium||Mined in Canada||Manure, compost|
- 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.)
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.
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.
- 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 IPM powerpoint, IPM powerpoint notes, Wisconsin Integrated Pest and Crop Management, 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/download/pubsNM/SeasonalGuide_FINAL.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 Understanding Soil Phosphorus (A3771) pp. 15-16)
How significant are agricultural pollutants anyway?
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.
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. Agro-ecosystems 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
- 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 on-line.
- 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 Community Action Coalition for South Central Wisconsin, Inc.
- 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
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.
It is difficult to get recent detailed information about energy use in modern agriculture, but some analysts estimate that on average it takes between 5 and 10 units of fossil fuel energy to produce 1 unit of food energy consumed in the United States. Since fossil fuels are not renewable, this situation is not sustainable. However, certain farming and food system practices can provide substantial savings in fossil energy use.
- 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.
- Minimize trips across the field, including tillage.
- Make sure equipment is properly maintained and adjusted.
- Use renewable energy sources such as wind, solar, or biomass-fueled power.
For more information on energy use on farm, see Iowa State University Extension publications, including
Tracking the Energy Use on Your Farm
Tractor Maintenance to Conserve Energy
How Much Energy is Being Used on Your Farm?
Limiting Field Operations
The eXtension website also includes great farm energy resources at http://www.extension.org/pages/26649/introduction-to-farm-energy
University of Wisconsin Extension’s Bioenergy Training website includes a unit on Farm Energy Conservation and Efficiency.
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. Energy Use in the US Food System pp. 20-22
Sustainable practices for the consumer
- Buy local foods, when possible.
- Avoid excess packaging.
- Minimize waste. When you throw food away, not only is all the energy to grow it and get it to your plate wasted; it also takes energy to haul it away and put it in a landfill.
- 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 about 4 lbs of corn to produce 1 lb of pork and 10 lbs of corn to produce 1 lb of beef)
Saving Family Farms–Ethanol Production or Wind from Organic Consumers Organization Power provides information about the economic benefits to farmers of allowing wind generation of electricity on their land.
Biofuels: good or bad?
The costs and benefits of producing transportation fuel (ethanol) from farm crops are hotly debated. Advocates claim that biofuels such as corn ethanol can significantly reduce our dependence on petroleum and reduce pollution. Opponents claim that the energy gains from ethanol are minimal at best, may increase total greenhouse gas emissions and other pollutants, and direct research and policy away from more viable energy solutions.
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:
- The land comes first. All operations must be based on a concern for soil fertility and long-term environmental compatibility
- Efficiency is vital. Both the vehicle for which the fuel is intended and the means of converting the biomass into fuel must be efficient.
- Wastes are the source. Use farming and forestry wastes as the principal feedstocks, no crop should be grown just to make fuels.
- 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.
After several years of intensive biofuels development, a group of 75 scientists from 21 countries came up with surprisingly similar recommendations for sustainable biofuels use, including:
- Many of the adverse effects of biofuels on the environment could be reduced by using best agricultural management practices, if production is kept below sustainable production limits, although choice of feedstocks and the overall demand for biofuel and level of production remain critical.
- In general, biofuels made from organic waste are environmentally more benign than those from energy crops. Using biomass primarily for material purposes, reusing and recycling it, and then recovering its energy content can gain multiple dividends.
- The first steps towards sustainable energy and resource management should aim for significant reductions on the demand side, with greater conservation and improved efficiency. Government mandates and economic incentives aimed at expanding biofuel production should be coupled with policies that manage the overall demand for energy.
Quoted from the executive summary of “Rapid Assessment on Biofuels and the Environment: Overview and Key Findings” by Robert W. Howarth et al.: http://www.greenchoices.cornell.edu/downloads/energy/biofuels/Rapid_Assessment_Biofuels.pdf
A case study of high school students who are converting a waste stream to an energy source: Our Cooking Oil Started an Alternative Energy Revolution
A TED Talk on cellulosic biofuels by a Wisconsin teacher: http://ed.ted.com/lessons/biofuels-and-bioprospecting-for-beginners-craig-a-kohn
An article on government policy promoting biofuels: http://www.choicesmagazine.org/choices-magazine/submitted-articles/the-renewable-fuel-standard–where-do-we-go-from-here
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 non-renewable 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.
Career Pathway content standards
|Projected Outcome||National Agricultural Education Standards
Performance Element or
(in this section)
|1. Identify the components of a healthy soil and including key nutrients.||ESS.03.02 Apply soil science principles to environmental service systems.
PS.02.02 Determine the influence of environmental factors on plant growth.
|2. Identify sources and sinks for pollutions in a soil system and brainstorm sustainable practice solutions.||AS.08 Analyze environmental factors associated with animal production.
ESS.03.03 Apply hydrology principles to environmental service systems.
|3. Describe how living organisms in a soil system interact and provide examples of sustainable practices that encourage greater diversity.||ESS.03.02 Apply soil science principles to environmental service systems.||C-4|
|4. Compare energy flows of conventional farming to sustainable farming practices.||ESS.01 Use analytical procedures to plan and evaluate environmental service systems.
ESS.05.01 Compare and contrast the impact of conventional and alternative energy sources on the environment.
|5. Apply ecological analysis to cropping systems.||PS.03.04 Apply principles and practices of sustainable agriculture to plant production.||C|
|6. Provide examples of key agro-economical management practices.||NRS.02.06 Apply ecological concepts and principles to natural resource systems.||C-3|
|7. Recognize controversies of agricultural pollution and design a way to address this issue.||CS.01 Acquire the skills necessary to positively influence others.
ESS.02 Assess the impact of policies and regulations on environmental service systems.
|8. Discuss how the production of ethanol affects our transportation needs.||ABS.06.01 Conduct appropriate market and marketing research.
ESS.01.01 Analyze and interpret samples.
CS.03.02 Decision making – analyze situations and execute an appropriate course of action.
NRS.05 Use effective methods and venues to communicate natural resources processes to the public.