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Nitrogen Use and Behavior in Crop Production

出版	Agricultural Experiment Station, College of Agriculture, University of Illinois at Urbana-Champaign, 1979
URL	http://books.google.com.hk/books?id=Q5OhbA99DVMC&hl=&source=gbs_api
EBook	FULL_PUBLIC_DOMAIN

註釋Most plants absorbmore nitrogen than any other nutrient. Because the amount needed is so large and easily be lost from many soils, nitrogen is usually the most limiting nutrient for plant growth. Although about 79 percent of the atmosphere is nitrogen, only nitrogen-fixingplants such as legumeswith their associated bacteria are able to use this abundant source. The nonleguminous grain crops must receive supplemental nitrogen to produce satisfactory yields. Until the last few decades the supply of available nitrogen in the soil was increased primarily by legumes and manure. These sources should be used when economically feasible, but many important grain-producing areas of the world must now rely on commercial fertilizer nitrogen. For economic reasons researchers and growers have been interested for many years in improving yields from each unit of nitrogen. Recently, however, the efficient use of nitrogen has become an environmental issue as well, because high nitrate concentrations in water may be harmful to humans, especiali infants, and to livestock. If plants absorb more of the addedfertilizer nitrogen, then less is likely to leach from fields into drinking water. Improving nitrogen efficiency has also become crucial in order to conserve dwinling supplies of natural gas, which is used in large quantities to manufacture nitrogenfertilizers.