With only a few exceptions every US state has a university based extension program that operates a soil lab. These soil testing labs can used by both commercial growers and farmers and home gardeners. Most tests are very affordable (~$20 or less) and there are instructions for how to collect and package the samples on each website.
It is common for the service to also include fertilizer or other amendment recommendations based on the test results and the crop or plants you want to grow. These are typically customized by the state (for example, here in New England, there are special requests for those interested in growing blueberries – a popular crop in the region). Your extension may offer other regionally appropriate options.
California – University of California Cooperative Extension (add your county to this search to narrow the list)
Iowa – Iowa State University closed their lab in 2017 (visit this link for alternative labs in nearby states)
Nevada – University of Nevada Reno – Environmental Soil Analysis Laboratory (this will forward you to nearby state labs)
Rhode Island – University of Rhode Island Soil Program (Use the program at UMass)
Updated April 2022