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Our Good Earth The future rests on the soil beneath our feet.  Soil A Dirty Word By Brad Scriber and Emily Krieger, National Geographic staff First things first: Soil is more than just dirt. While "dirt" suggests something that's dead, soil is very much alive: a complex stew of living organisms and decaying organic matter mixed with pebbles, sand, clay, silt, air, water, and nutrients that supports life literally from the ground up. Essential organisms digest dead matter and transform the nutrients into a form plants can use. These nutrients keep plants healthy enough to perform photosynthesis—a process that converts the sun's radiant energy into a form that all plant, animal, and fungal life depends on. Without plants to make this transformation, we would starve. And just as we depend on plants, plants depend on good soil. The mix and amount of nutrients determines what will grow in any given soil. Carnivorous plants evolved in part because flora growing in nutrient-poor environments needed a way to supplement their diets. Humans enhance soil quality with fertilizers, both organic and synthetic. Nitrogen, one of the most essential nutrients and a basic component of protein, must be "fixed" (chemically combined with another element) or released from decaying matter by organisms before plants can use it. Symbiotic bacteria that live on the root nodules of legumes—soybeans, peas, alfalfa—fix nitrogen gas, producing compounds plants can use. Planting legumes in crop fields in alternate years boosts available nitrogen levels and varies the demand on soil nutrients. The pH of soil is also important. Plants generally prefer earth that is alkaline; too much acidity stunts the growth of microbes, including those that fix and release nitrogen. 相关链接National Geographic Cover Story on Soils (September 2008) ：http://ngm.nationalgeographic.com/2008/09/soil/mann-text http://www.namipan.com/d/187fe762276ac4f9a027ff13bffb5e78e46ef46a157d0a00 [ Last edited by wbaishi on 2009-2-22 at 17:38 ] |
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