The term "pH" refers to the acidity or alkalinity of soil. Soil with a pH below 7.0 is considered to be neutral. A lower pH means that your soil is acidic, while a higher pH indicates alkaline soil. If you are unsure of your soil's pH, get a soil test through your local University Extension Office; this test also tells you the levels of various nutrients in your soil.
Many of the most commonly grown garden vegetables prefer slightly acid soil with a pH between 5.5 and 7.0. These include beans, broccoli, cabbage, carrots, celery and cucumbers, as well as peppers, squash, sweet corn, tomatoes and turnips.
A pH between 5.5 and 6.5, which is moderately acidic, is ideal for endive, parsley, rhubarb and soybeans. Strongly acidic soil, with a pH of 4.5 to 5.5, is good for growing sweet potatoes and radishes. Irish potatoes grow well with a pH between 4.8 and 5.5.
If you have very acidic soil, you can raise its pH to make it more applicable to growing vegetables. The most common way to do this is to add lime to the soil. Lime is composed of calcium carbonate, which raises pH by neutralizing acid. If you have alkaline soil and want to lower the pH, incorporate sulfur into the soil.