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Sustainable agriculture integrates three main goals: environmental stewardship, farm profitability, and prosperous farming communities. These goals have been defined by a variety of disciplines and may be looked at from the vantage point of the farmer or the consumer.
"It's easy to understand why key individuals and organizations in agriculture have flocked to this term. After all, who would advocate a 'non-sustainable agriculture?'" - Charles A. Francis. 
Additional recommended knowledge
Sustainable agriculture refers to the ability of a farm to produce food indefinitely, without causing irreversible damage to ecosystem health. Two key issues are biophysical (the long-term effects of various practices on soil properties and processes essential for crop productivity) and socio-economic (the long-term ability of farmers to obtain inputs and manage resources such as labor).
The physical aspects of sustainability are partly understood (Altieri 1995). Practices that can cause long-term damage to soil include excessive tillage (leading to erosion) and irrigation without adequate drainage (leading to accumulation of salt in the soil). Long-term experiments provide some of the best data on how various practices affect soil properties essential to sustainability.
While air and sunlight are generally available in most geographic locations, crops also depend on soil nutrients and the availability of water. When farmers grow and harvest crops, they remove some of these nutrients from the soil. Without replenishment, the land would suffer from nutrient depletion and be unusable for further farming. Sustainable agriculture depends on replenishing the soil while minimizing the use of non-renewable resources, such as natural gas (used in converting atmospheric nitrogen into synthetic fertilizer), or mineral ores (e.g., phosphate). Possible sources of nitrogen that would, in principle, be available indefinitely, include:
The last option was proposed in the 1970s, but would be well beyond the capability of current (2007) technology, even if various concerns about biotechnology were addressed. Sustainable options for replacing other nutrient inputs (phosphorus, potassium, etc.) are more limited.
In some areas, sufficient rainfall is available for crop growth, but many other areas require irrigation. For irrigation systems to be sustainable they must be managed properly (to avoid salt accumulation) and not use more water from their source than is naturally replenished, otherwise the water source becomes, in effect, a non-renewable resource. Improvements in water well drilling technology and the development of submersible pumps have made it possible for large crops to be regularly grown where reliance on rainfall alone previously made this level of success unpredictable. However, this progress has come at a price, in that in many areas where this has occurred, such as the Ogallala Aquifer, the water is being used at a greater rate than its rate of recharge.
Socioeconomic aspects of sustainability are also partly understood. Regarding nonindustrialized farming, the best known analysis is Netting's (1993) study on smallholder systems through history.
Given the finite supply of natural resources, agriculture that is inefficient may eventually exhaust the available resources or the ability to afford and acquire them. It may also generate negative externality, such as pollution as well as financial and production costs. Agriculture that relies mainly on inputs that are extracted from the earth's crust or produced by society, contributes to the depletion and degradation of the environment. Despite this continuing practice, unsustainable agriculture continues because it is financially more cost-effective than sustainable agriculture in the short term.
In an economic context, the need for the farm to generate revenue depends on the extent to which it is market oriented and on government subsidy. The way that crops are sold must be accounted for in the sustainability equation. Fresh food sold from a farm stand requires little additional energy, aside from that necessary for cultivation, harvest, and transportation (including consumers). Food sold at a remote location, whether at a farmers' market or the supermarket, incurs a different set of energy cost for materials, labour, and transport.
To be sold at a remote location requires a complex economic system in which the farm producers form the first link in a chain of processors and handlers to the consumers. This practice allows greater revenue because of efficient transport of a large number of items, but because it produces externalities and relies on the use of non-renewable resources, shipping, processing, and handling, it is not sustainable. Moreover, such a system is considered vulnerable to fluctuations, such as strikes, oil prices, and global economic conditions including labour, interest rates, futures markets, and farm product prices.
In Third World agriculture, much of what is known about the social components of sustainability comes from anthropologist Robert Netting's work. In Smallholders, Householders: Farm Families and the Ecology of Intensive, Sustainable Agriculture, he defines an important cross-cultural pattern of high-labor, high-production cultivation exemplified East Asian paddy rice cultivators, African cultivators such as the Kofyar, alpine peasants, and Mesoamerican farmers of raised fields. One key to socio-economic sustainability in such systems is that these farmers systems provide for much of their own subsistence and also participate in the market.
From a system's view, the gain and loss factors for sustainability can be listed. The most important factors for an individual site are sun, air, soil and water as rainfall. These are naturally present in the system as part of the larger planetary processes and incur no costs. Of the four, soil quality and quantity are most amenable to human intervention through time and labour. (The economic input depends solely on the price of labour and cost of machinery used).
Natural growth and outputs are also subject to human intervention. What grows and how and where it is grown are a matter of choice. Two of the many possible practices of sustainable agriculture are crop rotation and soil amendment, both designed to ensure that crops being cultivated can obtain the necessary nutrients for healthy growth.
Monoculture, a method of growing only one crop at a time in a given field, is a very widespread practice, but there are questions about its sustainability, especially if the same crop is grown every year. Growing a mixture of crops (polyculture) sometimes reduces disease or pest problems (Nature 406:718, Environ. Entomol. 12:625) but polyculture has rarely, if ever, been compared to the more widespread practice of growing different crops in successive years crop rotation with the same overall crop diversity. For example, how does growing a corn-bean mixture every year compare with growing corn and bean in alternate years? Cropping systems that include a variety of crops (polyculture and/or rotation) may also replenish nitrogen (if legumes are included) and may also use resources such as sunlight, water, or nutrients more efficiently (Field Crops Res. 34:239).
Some pesticides, though sometimes useful in the short term, can harm the soil food web, a complex ecology of micro-organisms in soil that helps sustain the plant from the roots down. Experiments comparing plants grown in soil compared to plants grown through hydroponics have shown a thirty-three percent higher growth rate when there are beneficial soil microorganisms available.
Certain pesticides synthesized by chemical companies can impart a sometimes fatal toxicity to humans, livestock and insect pollinators, such as bees and butterflies, which may be necessary for plant success. Without insect pollinators, farm labor must be expended to manually pollinate each plant. Crops such as cacao beans and vanilla are examples of crops requiring highly labor-intensive practices in the absence of natural pollinators.
Throughout history, farmers seeking to grow crops usually confine themselves to growing only the fastest and most productive plants. Such practices can result in growing crops without the genetic diversity found in wildlife. Without such diversity in the genes, crops may become more susceptible to disease and crop failure. The Great Irish Famine (1845-1849) is a well-known example of the dangers of monocultural and mono-varietal crop cultivation.
Many scientists, farmers, and businesses have debated how to make agriculture farming sustainable. One of the many practices includes growing a diverse number of perennial crops in a single field, each of which would grow in separate season so as not to compete with each other for natural resources. This system would replicate the biodiversity already found in a natural environment, resulting in increased resistance to diseases and decreased effects of erosion and loss of nutrients in soil. Nitrogen fixation from legumes, for example, used in conjunction with plants that rely on nitrate from soil for growth, will allow the land to be reused annually. Legumes will grow for a season and replenish the soil with ammonium and nitrate, and the next season other plants can be seeded and grown in the field in preparation for harvest. This method is considered to require a minimal amount of outside resources.
In practice, there is no single approach to sustainable agriculture, as the precise goals and methods must be adapted to each individual case. There may be some techniques of farming that are inherently in conflict with the concept of sustainability, but there is widespread misunderstanding on impacts of some practices. For example, the slash-and-burn techniques that are the characteristic feature of shifting cultivators are often cited as inherently destructive, yet slash-and-burn cultivation has been practiced in the Amazon for at least 6000 years (Sponsel 1986); serious deforestation did not begin until the 1970s, largely as the result of Brazilian government programs and policies (Hecht and Cockburn 1989).
There are also many ways to practice sustainable animal husbandry. Some of the key tools to grazing management include fencing off the grazing area into smaller areas called paddocks, lowering stock density, and moving the stock between paddocks frequently.,
What if a farm is able to "produce perpetually", yet has negative effects on environmental quality elsewhere? Most people concerned with sustainability take a global view, so they try to avoid negative off-farm impacts. For example, over-application of synthetic fertilizer or animal manures can pollute nearby rivers and coastal waters. On the other hand, if crop yields are too low, because of soil exhaustion of nutrients or reduced ability to retain water, farmers would need to access new lands for agriculture, leading to the decimation of the rainforest, draining wetlands, etc.
There has been considerable debate about which form of human residential habitat may be a better social form for sustainable agriculture. Generally, it is thought that village communities can improve sustainability in that such communities tend to provide a cooperative environment that supports farming.
Many environmentalists pushing for increased population density to preserve agricultural land point out that urban sprawl is less sustainable and more damaging to the environment than living in the cities where cars are not needed because food and other necessities are within walking distance. However, others have theorized that sustainable ecocities, or ecovillages which combine habitation and farming with close proximity between producers and consumers, may provide greater sustainability.
The use of available city space (e.g., rooftop gardens and community gardens) for cooperative food production is another way to achieve greater sustainability.
One of the latest ideas in achieving sustainable agricultural involves shifting the production of food plants from major factory farming operations to large, urban, technical facilities called vertical farms. The advantages of vertical farming include year-round production, isolation from pests and diseases, controllable resource recycling, and on-site production that eliminates the need for transportation costs. While a vertical farm has yet to become a reality, the idea is gaining momentum among those who believe that current sustainable farming methods will be insufficient to provide for a growing global population.
Universities with Sustainable Agriculture Programs
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sustainable_agriculture". A list of authors is available in Wikipedia.|