At the NOFA conference, one of the many great exhibitors was a Pennsylvania company, Lancaster Agriculture Products. They are very knowledgeable about soils and one key service is soil testing. They had a refractometer there (measures specific gravity) but I did not get back to the booth before they left to learn more about that.
They suggest oats as a winter cover crop because they pick up more phosphorus and other minerals that some other plants will. They die in the winter, making it easier to plant in the spring.
If you have horses, or farm, or if you are buying your meat from a farmer, suggest using this company. Stephen Stoltzfus and his son, Reuben, started this renewable agriculture business in 1994 in a small corner of a building on the family farm.