So far, our description of the soil habitat summarized physical and chemical parameters that interact with soil biology. Although it may be obvious that soil temperature, water content, organic matter content and quality change over time, it is less obvious that the physical structure itself is not static. Fluctuations in water content change the size of pore spaces and displace peds. Flow of gravitational water, for instance after a heavy rain, can carry clays, silt and fine sand, and alter pore sizes and structure. Freeze-thaw cycles in cold weather also physically displace peds. Thus the pore reticulum is continuously changing, simply by physical forces. The interstitial living organisms also disturb peds and mineral particles. The cilia of motile protists is strong enough to displace small particles. The passage of larger species, such as nematodes and microarthropods, creates tunnels (micropores) and contributes to continually changing the reticulum and maintaining tortuosity. Larger species obviously create even larger disturbances (macropores), for instance the burrowing of an animal or elongation of large roots. In soils that are biologically inactive, over time and with gravity, the fraction of pore spaces decreases with compaction, and the bulk density increases. The overall effect of the biological and physical disturbance causes a mixing of the soil, which contributes to moving microdetritus and SOM into deeper soil horizons, maintains the pore spaces, and increases drainage and aeration.
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