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White Clover (Trifolium repens)
Known as the white or Dutch clover, Trifolium repens was introduced to North America from Europe as a forage crop and is now commonly found as a weed along pathways, roadsides, and in lawns. In similarity with the other clovers in the Trifolium genus, white clover is an excellent soil builder, particularly for nitrogen-depleted soil.
View a second image of a white clover (Trifolium repens).
Able to reseed itself under favorable conditions, this short-lived perennial clover grows rapidly and spreads vegetatively via stolons. Intolerant of droughty soils, T. repens has a relatively shallow root system. Dutch clover prefers well-drained, fertile soils and cool, moist weather. Soil pH between 6 and 7 is optimal for the plant, and it is highly tolerant of grazing and mowing. Pure stands of white clover are usually not planted because of their low growth habit and low yields. However, when mixed in pastures with grasses, high-quality range and hays are produced because the clover fixes nitrogen for the grasses' usage.
Technically, it is not this member of the pea family (Leguminosae) that actually fixes the nitrogen in the soil, but rather the symbiotic bacteria (Rhizobium species) found living in its root nodules. Visible nodules form when bacteria infect a growing root hair. The mutualistic relationship works well, with the white clover supplying carbon dioxide and other simple carbon compounds to the bacteria, and the bacteria converting atmospheric nitrogen into a form that the plant host can metabolize. The decomposition of roots and leaves of host plants provides soil nitrogen for the surrounding area. Legumes and their symbionts are important because although nitrogen gas is plentiful in the air, it is very stable and rarely combines directly with other atoms. The nitrogen-fixing bacteria convert the gas into ammonia, which is directly taken in by the host's roots. Other species of nitrogen-fixing bacteria exist that are free-living, and that perform the same conversion of nitrogen into ammonia. Still another category of bacteria known as nitrifying bacteria can convert ammonia to nitrates and nitrites, which are assimilated readily by plant roots. The reduced nitrogen compounds are waste products of the nitrifying bacteria, which metabolize ammonia in order to obtain hydrogen that they require for the fixation of carbon.
Cynthia D. Kelly, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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