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The pH Factor is simply a way of expressing the amount of acidity/alkalinity of the soil.  The scale runs from 0 to 14; with 7 representing a neutral soil (numbers below 7 indicate the degree of acidity; numbers above 7 indicate the degree of alkalinity).  When conditions of pH become too extreme, chemical changes occur which prevent the nutrients from being used by the plants.  Most garden plants do best in a slightly acid to neutral soil; pH 6.5-7.  Finished compost generally tends to be slightly acidic  (although this can be influenced by the materials used).  When compost is added to the soil, it acts as a chemical buffer, increasing the plants tolerance to pH; plants will thrive in a broader pH range; ph 6-8.  Adding compost to a slightly alkaline soil will bring it towards neutral.

To correct an overly acid soil requires the addition of lime; but here we encounter an area of controversy, as regards composting.  Some authorities advise adding some lime to the compost; the reasoning being that the microorganisms thrive best under slightly acidic conditions.  However, it has been shown by repeated research that adding lime to a compost pile results in serious losses of nitrogen (through volatilization as ammonia).  If you decide that your conditions warrant the addition of lime, and you think you might want to run the lime through your compost;
1) be sure to maintain enough nitrogen in the pile (expecting some losses), and
2) don't use quicklime (hydrated), as it tends to overdose with alkaline substances.

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