The pH of the soil is described as alkaline (pH > 7) or acidic (pH < 7).  Variations in pH affect the availability of certain nutrients needed by plants and therefore will affect their growth.  Some plants grow better in acidic soil while others prefer a more basic soil (see link below).  When evaluating soil, pH is one of the factors looked at as there are additives that can be mixed with the soil to adjust the pH either up or down.  Care must be taken in what additives are used as some additives (i.e. ammonium) can also result in a decrease in the pH of the soil.  When soil has a pH below 4, most plants can't survive as the high acid content will destroy the cell membranes within the plant. 

The affect on a particular set of plants will vary depending on the type of plant and whether it prefers a more acidic soil or a more alkaline (basic soil).  Some plants will flourish at a pH of 4, while others will not survive.