Although the word originally meant the act of cultivating fields, i.e. growing crops, it has now been extended to include all aspects of growing crops and raising livestock. Indeed, the definition is now flexible enough to include such contrasting activities as intensive animal production in feedlots or barns, ranching, horticulture and the complex small-scale subsistence farming which characterizes parts of the humid tropics. The common element is conscious management of a biological resource. Although agriculture can be differentiated from Forestry on the basis of the time-scale of decision-making, these terms represent the ends of a continuum that includes agroforestry. Because of this diversity, it is important to recognize that the environmental impact varies with the type of agriculture. Classifications can be based on the type and number of components (specialized arable, mixed livestock-arable, etc.), the degree to which the farm is integrated into the global market for agricultural produce (subsistence farming, cash cropping, etc.) or the level of inputs, such as fertilizer, that are applied (organic farming, high-input systems, etc.). If external inputs alone are considered, high-input farms are typical open systems.
Agriculture produces most of the world's foodstuffs together with raw materials for the fibre and vegetable oil industries. The three most important crops, on the basis of edible dry matter production, are wheat, rice and maize. Cattle are by far the most important class of livestock in terms of biomass. More than half the world's working population is still employed in agriculture and the associated industries. When permanent pasture is included, almost 40% of the world's land area is devoted to agriculture, of which about one-quarter is arable, i.e. cropped. These figures, however, disguise considerable variation from continent to continent and from country to country (see Table A1). The area of agricultural, and particularly arable, land per person is declining as the world population increases more rapidly than the agricultural land. Between 1977 and 1992, there was a net increase in the world's agricultural land area of just less than 5% (destruction of forests accounted for about 60% of this). This figure, however, represents a balance between gains from cultivating new land and losses from urbanization, erosion, desertification and salinization.
The origins of settled agriculture are complex and the subject of debate. What is clear is that the domestication of plants and animals has taken place over a period of more than 12000 years. During this time, agricultural systems have evolved which take advantage of the potential of the land while seeking to overcome the limiting factors, which may be climatic, edaphic or socio-economic in origin. The dawn of scientific agriculture and the start of major intensification are often considered to have coincided following the discoveries made in the 19th century by agricultural chemists in western Europe. However, many effective agricultural systems were developed long before then. For example, 2000 years ago, when the tribes of north-west Europe were fighting each other, sustainable irrigation-based agricultural systems had already been developed in south-east Asia and in Central America. By the close of the 12th century AD, the Arab agronomists of Andalucia in Spain had developed rules for solving many of the management problems associated with Mediterranean agriculture. In recent times many indigenous systems have been displaced by intensive agricultural operations tied closely to the global market economy. There is now a growing realization that a considerable store of valuable indigenous knowledge is in danger of being lost.
A wide variety of agricultural techniques to grow crops and raise livestock have evolved over the centuries. These techniques include mechanization, pest and disease control, plant and animal nutrition, and the breeding of better types of crops and livestock. Successful innovations have spread widely and rapidly around the world even though subsistence farmers cannot afford to take large risks and must do their utmost to protect the land resource on which the livelihood of their family depends.
Agriculture affects the environment in a number of ways. Direct agricultural pollution has caused serious local damage in some circumstances although its global importance has probably been overestimated. The risk of damage is greatest when high inputs of persistent or soluble agrochemicals are applied to a permeable soil overlying a water table that is near the surface. In these cases, persistent pesticides, nitrates and organic wastes can pollute watercourses and aquifers. Where surface run-off is significant, eroded soil particles can also contaminate watercourses. Agronomists and soil scientists know how to minimize these problems but the introduction of control measures is difficult where there is either no political will to make changes or the farmers are poor and do not own their land. Agroecosystems tend to show less biodiversity than the natural systems they replace, particularly since weed species and many invertebrates and microorganisms are targeted for control. However, this reduction is not inevitable and many diverse and highly prized landscapes are the product of management for extensive agriculture. Although individual fields are often monocultures of single crop cultivars, farms usually contain fields with different crops. Continuous culture of the same crop year after year in the same field is largely restricted to those parts of the world where the climate inhibits the build-up of pests and diseases. Continuous culture may, for example, be ecologically appropriate in areas where there is a pronounced dry season after harvest.
Agriculture is important both as a source and a sink for greenhouse gases. Ignoring fossil fuel inputs, agriculture has a broadly neutral effect on the Carbon dioxide (CO2) flux at the Earth's surface. However, burning forest to create new agricultural land and cultivation of Prairie and steppe soils, which are high in organic matter, has led to the release of large amounts of CO2. Methane (CH4) is emitted by ruminant animals—although these losses are partly offset by a reduction in the number of wild ruminants—and from rice paddies where the source is the anaerobic fermentation of organic matter. The emission of nitrous oxide (N2O), which is formed from the natural processes of nitrification and denitrification, is increased when land is cultivated for arable agriculture.
A more subtle effect of agriculture is on the regional energy balance. The replacement of natural vegetation by farmland is often marked by an increase in both the proportion of Solar radiation reflected (albedo) and the latent heat flux (evapotranspiration) although it is difficult to generalize to the effect on regional climate.
Sustainability (see Sustainable development) is an increasingly important issue for agriculture. Soil erosion can be a major problem in parts of the world with a dry climate and soils of low organic matter. It is caused by the use of practices which are inappropriate for the soil type and climatic zone. Some areas have been permanently damaged. Water has long been recognized as a major limiting factor to agricultural production. About one-third of the Asian crop area is irrigated, as are about 10% of the crops in the rest of the world. Barring major changes in climate, some of these systems are sustainable indefinitely. However, others such as those on the High Plains of the USA and in parts of Saudi Arabia depend on water from aquifers that are being depleted faster than they can be recharged. Elsewhere there is competition for water between agriculture and an increasingly urbanized population. Areas unsuitable for arable cropping may still support grazing by livestock, provided that the stocking density is balanced against the carrying capacity of the land.
Current intensive European and North American farming depends on a continual use of external inputs such as fertilizer and pesticides, although techniques such as integrated pest management are being developed to reduce the need for such inputs. Estimates of typical values of inputs are given in Table A2. Current intensive agricultural systems are characterized by a large fossil fuel input, mainly to support mechanization. However, the energy input to agriculture often represents only a small part of the total energy required to produce food, the major inputs occurring in the transport, processing and packaging sectors. Considerable research has been carried out into the feasibility of producing ethanol, rape methyl-ester and other biofuels that could minimize the fossil fuel energy subsidy to agriculture. However, the land needed to produce energy crops will be unavailable for food production. One-quarter of the farm's arable area might be needed to produce all the farm's energy requirements, as was the case in the days of horse power. [G.R.]Table A1Proportional world agricultural areas by continent. Relative areas are given as a proportion of the total land area. Agricultural land includes arable land, land under permanent crops, such as olives, and permanent pasture. The Russian Federation is included with Europe. The data are for 1993 and have been calculated from FAO (1994) 1993 FAO Production Yearbook, Vol. 47. Food and Agriculture Organization of the United Nations, Rome.
|Africa||North and Central America||South America||Asia||Europe||Oceania|
|Relative agricultural area||0.37||0.29||0.35||0.47||0.46||0.57|
|Relative arable area||0.06||0.12||0.06||0.16||0.26||0.06|
|Agricultural land per person (ha)||1.5||1.4||2.0||0.4||0.4||17.4|
|Arable land per person (ha)||0.23||0.60||0.31||0.13||0.24||1.86|
Table A2Selected inputs for two countries with contrasting agricultural economies. These data have been presented per hectare of cropped land. The definition of agricultural workers excludes people employed in the ancillary industries. Note that the figure for nitrogen consumption in The Netherlands has been inflated by heavy applications to grassland. The support energy figures, which include the energy costs of fertilizer and machinery manufacture, are estimates.
|Agricultural workers per hectare||Tractors per hectare||Nitrogen fertilizer use (kg ha−1 year−1)||Support energy (MJ ha−1 year−1)|
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