AGRICULTURAL TECHNOLOGY

application of techniques to control the growth and harvesting of animal and vegetable products. Additional reading Factors in soil preparation are analyzed by William R. Gill and Glen E. Vanden Berg, Soil Dynamics in Tillage and Traction (1967); Milton A. Sprague and Glover B. Triplett (eds.), No-Tillage and Surface-Tillage Agriculture (1986), a review of these alternatives to traditional plowing; Ronald E. Phillips and Shirley H. Phillips (eds.), No-Tillage Agriculture: Principles and Practices (1984); and Samuel L. Tisdale et al., Soil Fertility and Fertilizers, 5th ed. (1993).Various cropping systems are described in John Vandermeer, The Ecology of Intercropping (1989); and Charles A. Francis (ed.), Multiple Cropping Systems (1986). Hubert Martin and David Woodcock, The Scientific Principles of Crop Protection, 7th ed. (1983), focuses on pest control. Regional variations in farming technique are presented by K.G. Brengle, Principles and Practices of Dryland Farming (1982); Hans Ruthenberg et al., Farming Systems in the Tropics, 3rd ed. (1980); and L.V. Crowder and H.R. Chheda, Tropical Grassland Husbandry (1982). James Sholto Douglas, Advanced Guide to Hydroponics, new ed. (1985); and Howard M. Resh, Hydroponic Food Production, 4th ed. (1989), treat this specialized technique.Weather information is available in Weekly Weather and Crop Bulletin, published by the U.S. Dept. of Commerce, Weather Bureau. Studies of agricultural meteorology include Rudolf Geiger, The Climate Near the Ground (1965; originally published in German, 4th ed., 1961), a classic text; Jen-hu Chang, Climate and Agriculture (1968); Robert H. Shaw (ed.), Ground Level Climatology (1967); and Norman J. Rosenberg, Blaine L. Blad, and Shashi B. Verma, Microclimate, 2nd ed. (1983). George W. Cox and Michael D. Atkins, Agricultural Ecology (1979), analyzes world grain and vegetable production systems, with an emphasis on the influence of weather. David J. Briggs and Frank M. Courtney, Agriculture and the Environment (1985), describes temperate agricultural practices and systems and their impact on the environment, with examples from Britain. Mervyn L. Richardson, Chemistry, Agriculture, and the Environment (1991), focuses on pesticide and fertilizer pollution from both crop and livestock production. Pollution's effect on agriculture is reported in James J. MacKenzie and Mohamed T. El-Ashry (eds.), Air Pollution's Toll on Forests and Crops (1989). Robert E. Stewart The Editors of the Encyclopdia Britannica Factors in cropping Cropping systems The kind and sequence of crops grown over a period of time on a given area of soil can be described as the cropping system. It may be a pattern of regular rotation of different crops or one of growing only one crop year after year on the same area. Crop rotation Early agricultural experiments showed the value of crop rotations that included a legume sod crop in the regular sequence. Such a system generally maintains productivity, aids in keeping soil structure favourable, and tends to reduce erosion. Alfalfa, sweet clover, red clover, and Ladino clover are considered effective for building up nitrogen. Some legumes, however, do not leave nitrogen behind in the soil because it is deposited as protein in the harvested seed; soybeans are an example. Turning under the top growth of a legume aids in adding nitrogen. Though yields of grains are higher when they are rotated with legumes, it is difficult to determine how much of the improvement depends on the nitrogen added by the legume and how much on improved soil structure or fewer insects and disease. The determination of the best rotation depends upon whether the crops compete with each other (i.e., if growing one crop lowers the yield of its successor) or complement each other; and the output of one crop on a given acreage leads to increased output of the other. This desirable complementary relationship exists only when one crop or soil-management practice concurrent with it provides nutrient or conditions required by the other crop. In this circumstance, grasses and legumes may complement grains or row crops by furnishing nitrogen, controlling erosion and pests, and improving soil structure to such an extent that greater production is achieved. The reverse can also occur; in certain prairie soils, continuous growing of deep-rooted legumes depletes soil moisture, and subsequent forage yield is improved by frequent plowing of the sod and planting of corn. In high-rainfall or irrigated areas, forage stands deteriorate from winter killing, disease, or grazing, to a point where a year of grain in the rotation allows an improved stand of forage later. Fallow (idle) land is complementary to wheat and other small grains in subhumid areas such as the Great Plains of the United States; such rotation is quite beneficial to wheat yield. Complementary relationships between crops can be terminated by the application of the physical law of diminishing returns, however, and give way to competition. Both long-range and short-range profits motivate the farmer as cropping systems are examined in relationship to soil erosion. Excessive loss of soil to streams, rivers, and reservoirs is unacceptable to public policy as well as economically damaging to the farmer, and crop rotations that promote erosion are minimized. Soil losses are least from fields in continuous sod and most from continuous row crops. If row crops are grown in rotation with sod, the erosive susceptibility of row crops is reduced over a period of time. Peanuts (groundnuts), potatoes, tobacco, cotton, sugar beets, and some vegetables, and similar row crops that require frequent cultivation (intertillage) and leave minimal post-harvest residue are most likely to permit serious erosion. Less erosive are row crops such as corn (maize), sugarcane, and grain sorghum, which require less cultivation and leave more residue. Small grains such as wheat, oats, barley, and rye usually permit less erosion than the row crops. Among sod crops, grasses or grasslegume mixtures are less erosive than pure stands of legumes such as alfalfa. Fortunately, cropping systems that tend to control soil erosion usually tend also to give better yields than systems that promote excessive erosion. This results from increased availability of water to the plants and increased amounts of nutrients, which in erosive systems are washed away and lost. Regional variations in technique Dryland farming Dryland farming refers to production of crops without irrigation in regions where annual precipitation is less than 20 inches (500 millimetres). Where rainfall is less than 15 inches (400 millimetres) per year, winter wheat is the most favoured crop, although spring wheat is planted in some areas where severe winter killing may occur. (Grain sorghum is another crop grown in these areas.) Where some summer rainfall occurs, dry beans are an important crop. All dryland crop yield is mainly dependent on precipitation, but practices of soil management exert great influence on moisture availability and nutrient supply. Where rainfall exceeds 15 inches (380 millimetres), the variety of crop possibilities is increased. In areas of favourable soils and moisture, seed alfalfa is grown, as is barley. Some grass seed may be grown, particularly crested wheat grass of various types. Fallow system and tillage techniques Dryland farming is made possible mainly by the fallow system of farming, a practice dating from ancient times. Basically, the term fallow refers to land that is plowed and tilled but left unseeded during a growing season. The practice of alternating wheat and fallow assumes that by clean cultivation the moisture received during the fallow period is stored for use during the crop season. Available soil nitrogen increases and weeds are controlled during the fallow period. One risk lies in the exposure of soil while fallow, leaving it susceptible to wind and water erosion. Modern power machinery has tended to reduce this risk. Procedures and kinds of tillage that are comparatively new have proved effective in controlling erosion and improving water intake. Moldboard and disk plows are being replaced with chisels, sweeps, and other tools that stir and loosen the soil but leave the straw on the surface. Where the amount of straw or residue remaining from the previous crop is not excessive, this trashy fallow system works well, and tillage implements are designed to increase its effectiveness. Contour tillage helps to prevent excessive runoff on moderate slopes. Broad terraces can aid in such moisture conservation. Steeper slopes are planted to permanent cover. Compacted zones at a depth of five to eight inches (13 to 20 centimetres) can be caused by tillage. As such zones interfere with storage of moisture, they can be controlled by growing deep-rooted alfalfa at intervals, or the compacted zone can be broken by fall tillage with chisels or sweeps set to a depth just below the zone of compaction. Such deep tillage will result in reduced runoff and deeper moisture penetration. When using power machinery in dryland farming, the timing of operations is important. The soil is broken in the fall or early spring before weeds or volunteer grain can deplete the moisture. Use of a rod weeder or similar equipment during fallow can control the weeds. Planting is timed to occur during the short period in fall or spring when temperature and moisture are favourable. The effects of pollution Practically all forms of technology exact a certain price in environmental damage; agriculture is no exception. Agriculture in turn is sometimes damaged by undesirable by-products of other technologies (see also pollution: The pollution of natural resources). Air has physical properties and a chemical composition that are vital parameters of life for both plants and animals. Temperature, water vapour, movement, oxygen, and carbon dioxide in the atmosphere have a direct effect on food and fibre production. Air quality is changed by introduction of contaminants into it, and agricultural activities using such air may be affected adversely. Damage to plants by air pollutants is related to meteorological conditions, particularly temperature inversions in the atmosphere. Air pollution Air pollution damage to agriculture For more than a century air pollution has affected agriculture. Burning coal and petroleum produce sulfur oxides. Fluorides result from smelting and glass and ceramic manufacture. Rising levels of ammonia, chlorine, ethylene, mercaptans, carbon monoxide, and nitrogen oxides are found in the air. Motor vehicles and growing population produce photochemical air pollution affecting not only the urban concentrations but also the contiguous rural areas. The mixture of pollutants from all sources, including agriculture, has released a host of contaminants into the air, such as aldehydes, hydrocarbons, organic acids, ozone, peroxyacetyl nitrates, pesticides, and radionuclides. The effect of these pollutants on food, fibre, forage, and forest crops is variable, depending on concentration, geography, and weather conditions. Damage to crops by air pollution, of course, brings economic loss as well. The effects of air pollution on plants and animals may be measured by the following factors: (1) interference with enzyme systems; (2) change in cellular chemical constituents and physical structure; (3) retardation of growth and reduced production because of metabolic changes; (4) acute, immediate tissue degeneration. Pollutants that enter the air from sources other than agriculture and that produce plant response are classified as: (1) acid gases; (2) products of combustion; (3) products of reactions in the air; and (4) miscellaneous effluents. The factor of weather Weather information The interaction of weather and living systems is a basic aspect of agriculture. Although great strides in technology have resulted in massive production increases and improved quality, weather remains an important limiting factor. Though man is not yet able to change the weather, except on a very small scale, he is capable of adjusting agricultural practices to fit the climate. Thus, weather information is of utmost importance when combined with other factors, such as knowledge of crop or livestock response to weather factors; the farmer's capability to act on alternative decisions based on available weather information; existence of two-way communication by which specific weather forecasts and allied information can be requested and distributed; and the climatic probability of occurrence of influential weather elements and the ability of the meteorologist to predict their occurrence. Other weather-research benefits Apart from the many applications of weather forecasting to current problems, meteorological research may benefit agriculture in at least three other ways: (1) improved planning of widescale land usage depends partly on detailed knowledge of plant-climate interactions; radiation, evapotranspiration, diurnal temperature range, water balance, and other parameters are measured and analyzed before a plan realizing maximum economic benefit for a given area is prepared; (2) agronomic experiments are combined with climatological documentation to obtain the greatest scientific and technological return; (3) problems of irrigation, row spacing, timing of fertilizer application, variety selection, and transplanting can best be solved with the aid of climatic environmental data; cultural practices related to artificial modification of microclimates should be based on research knowledge rather than personal judgment.