Soil pH is often referred to as how "sweet" or "acid" a soil is with pH 7 being neutral. Gardeners often use a pH meter, or mail soil samples to a soil lab for testing for pH. Soil in areas with considerable rainfall tend to revert back to an 'acid' condition and should be re-tested every 5 years. Soil in "bottom ground" near streams tends to be "sweeter" and is less likely to need an application of lime to raise the soil pH into a desirable range for plant growth.
When the soil pH is too "acidic" (aka "sour" with a low pH) or "alkaline" (aka "sweet" with a high pH) nutrients present in the soil become locked-up or unavailable to plants. Correcting the pH has the same effect as applying fertilizer in some cases, since it "unlocks" plant nutrients already present in the soil.
< 5.5 = Strongly acid
5.5 - 5.9 = Medium acid
6.0 - 6.4 = Slightly acid
6.5 - 6.9 = Very slightly acid
7.0 = Neutral
7.1 - 7.5 = Very slightly alkaline
7.6 - 8.0 = Slightly alkaline
8.1 - 8.5 = Medium alkaline
> 8.5 = Strongly alkaline
Most plants grow best within a pH range of 6.5 to 7.2 (7.0 is neutral) The best way to find out what your soil needs is by sending a sample to a soil lab. Penn State Extension sells soil test kits with instructions for use.
The soils we've tested in the South Hills of Pittsburgh, and Washington County, over the past 20 years, tend to need applications of both lime and phosphorus. The primary exception would be soils located in "bottom ground" (near a stream or river) which usually don't require lime. Of those soils requiring lime, dolomitic lime (which is higher in magnesium) usually fulfills the test recommendations best.