Testing and appropriately applying amendments to your soil is the most important thing you can do for your garden this season. Across the US most County Extension Services offer free soil testing. I do recommend paying a little bit extra to have a nitrogen test done, as well as organic matter so you can track your progress overtime. If you have a really large garden, you can take multiple samples. However, for most home gardeners in hobbyists, one is fine. Be sure to sample only the mineral soil