Ask Mr. Smarty Plants

Using the standard method of testing specified by the Natural Resource Conservation Service (NRCS), part of the US Department of Agriculture (USDA), most soils do indeed yield pH values in the range of 5.0 to 8.0 or slightly higher, although soils in some parts of the world commonly have higher or lower values.  This pH range is related to: 1) the stability of common soil-forming minerals; 2) a soil's capacity to retain and release moisture and nutrients; 3) the processes by which certain hazardous materials become concentrated in the soil; 4) the viability of many disease organisms (especially fungi and bacteria) affecting particular plants; etc.  It is for this host of reasons that soil pH is an important factor in plant health.

Few plants grow in soils with pH values far outside of this "optimal" range, although there are a few specialized plants and plant communities that require somewhat higher or lower pH conditions.  I doubt, however, that there are any vascular plants that normally live and reproduce in soils with pH values below approximately 3.0 or above approximately 9.0, although some non-vascular plants may tolerate values slightly outside of this range.  Some "extremophyle" microbes (certain bacteria and photosynthesizing cyanobacteria, etc.) and even a few invertebrate animals live in waters with pH values of 2.0 or below or above 9.0.

There are, of course, regional and local variations in soil pH, which are primarily related to the nature of the underlying bedrock and sediment, seasonal variations in precipitation and temperature, soil drainage (i.e., persistence of water saturation), etc.  The pH values for soils in any given area therefore generally fall within a relatively narrow range.  Naturally, the plant life native to each area is adapted to grow in soils with those particular pH values, at least in part.

Here is a list from Colorado State University of acid-loving plants, some of which have ranges down to pH 4 and more information from University of Hawaii about why acid soils are not favorable for plants.

You can find more information by googling "acid tolerant plants" and/or "alkali tolerant plants".