Hi, and thanks for contacting Ask an Expert.
You can call the OSU Linn County Extension office 541-967-3871. They keep a list of several labs that do soil testing. We do not do soil testing for amendments but can do a pH test for you.
Five or six soil samples should be taken in various parts of your yard about 3-5 inches down. Take a small scoop of each area and mix it in a bag. You can bring it to the Linn Extension office for a free test. I do not know if all Extension offices do this. If Linn County is not your county, you could call your local Extension office and ask. Here is a link to one of our reports on how to interpret your lab results: https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/ec1478.pdf