This abbreviation is short for the potency, or concentration, of hydrogen. It is used to measure the acidity or alkalinity of soil, compost or water.
You can test the soil with a with a simple kit, which gives a pH reading from 0 to 14. A value of pH 7.0, that of pure water, is neutral. Higher readings indicate higher alkalinity, or levels of lime. Values lower than 7.0 show increasing acidity.
Garden soil usually ranges from pH 4.5 to pH 8.0. Few plants will grow well at more extreme values. Most prefer soil that is between pH 6.5 and 7.0, but ericaceous plants need soil that is ideally below pH 6.0.