Get a Soil Test Kit
A soil analysis will give you the pH level of the soil and the levels of nitrogen, phosphorus and potassium, as well as the levels of critical trace elements. Call your local extension service to get a soil test kit; they'll tell you where to send it and how to get the best samples for your soil type. You can also buy test kits at your garden center; the results are quick, but they're just not as accurate as what you'll get by sending a sample off to your county lab. The soil samples can go to your local extension service for testing. You get the results back in a few weeks.