Get your soil tested

There are many reasons to get your garden soil tested. Many plants thrive in certain PHs, some do very poorly when a PH is too low or too high. By testing your soil, you will be in the know about what's in all that dirt. Then you can adjust as needed. Many fertilizers and other supplements will state right on the package the percent of nutrients included and you can adjust your soil as needed.

Here's another great reason why you should get your soil tested, and it's a bit more serious than having the best tomatoes:

For Urban Gardeners, Lead is a Concern

As we all know, lead is quite the scary substance!

Fortunately there are many ways to test your soil.
Cornell Cooperative Extensive will test your soil for you for a small fee. And here is their PH testing kit. They will even give you recommendations after they've tested your soil!
For those living outside of New York State, you may still send your soil to Cornell CE, however, they will not be able to give you recommendations because their recommendations are based on the soil composition of NYS.
I recommend doing a Google search for soil testing in your state, you're sure to come up with something more local.