How do I get my soil tested?
For home gardeners and community gardens, the University of Maryland Extension Home and Garden Information Center has a very helpful webpage on soil testing, with a video showing you how to collect a sample, a list of testing labs, and a series of FAQs.
For professional farmers, the University of Maryland Extension Agricultural Nutrient Management Program has this webpage on soil testing. In particular, I would recommend their instructions for soil sampling and their list comparing soil test labs.