The rhizosphere comprises the small zones immediately surrounding plant roots in the soil, in which the kinds, numbers and activities of microorganisms differ from that in the bulk soil (Gregorich et al., 2001). Included are the microorganisms and the organic and inorganic materials released by the roots themselves. The rhizosphere is thus a poorly defined zone around any root, commonly extending out no more than 1 or 2 mm but occasionally as far as 20 mm. The name was originally proposed in 1904 by Hiltner for the volume of soil near legume roots in which the growth of heterotrophic bacteria might be stimulated by the nitrogen released from nodules (Neal et al., 1970). The meaning has since been expanded to cover roots of all plants.
The root surface and adhering soil particles are called the rhizoplane. It is rather distinct as compared to the rhizosphere. It is where microorganisms invade and modify the viscous mucilaginous coatings on the epidermal surfaces of roots.
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