The soil fauna may be concentrated in the rhizosphere. Ingham et al. (1985) found up to 70% of the bacterial- and fungal-feeding nematodes in the 4-5% of the total soil that was rhizosphere, namely the amount of soil 1-2 mm from the root surface (the rhizoplane). Griffiths and Caul (1993) found that nematodes migrated to packets of decomposing grass residues, where there were considerable amounts of labile substrates and microbial food sources. They concluded that nematodes seek out these "hot spots" of concentrated organic matter and that protozoa do not. Nematodes also move and occur vertically in soils. In deserts, nematodes are associated with plant roots to depths of 15 m as are mites and other biota (Freckman and Virginia, 1989).
Wallace (1959) noted that movement of nematodes was optimum when soil pores were half drained of free water. Using pressure plates, Demeure et al. (1979) showed that nematode movement was found to cease and anhydrobiosis to begin when the water film thickness surrounding a soil particle is between 6 and 9 monomolecular layers of water. This is equivalent to soil pores being completely drained of free water. However, a nematode species from desert habitats tolerated drier soils with less pore water than a species from a tropical habitat. Elliott et al. (1980) noted that the limiting factor for nematode survival often hinges on the availability and size of soil pore necks, which enable passage between soil pores. Yeates et al. (2002) measured the movements, growth, and survival of three genera of bacterial-feeding soil nematodes in undisturbed soil cores maintained on soil pressure plates. Interestingly, the nematodes showed significant reproduction even when diameters of water-filled pores were approximately 1 pm.
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