African Garden + [soil]

Do I Truly Need a Soil Test?

Gardeners are advised strongly to have their soil tested. This advice I have persistently ignored. However, while poking around in the reference section of my local library, I came across a soil survey of my county done in 1970. Each part of the county was photographed from the air, and then soil maps were made from these photos. Because this was a reference book, I could not check it out. Here is a copy of my part of the soil map (the original was color).

The red X marks my spot. According to this map, my soil is "Markham silt loam, 1 to 4 percent slope[]." "Markham soil" "consists of deep, nearly level to strongly sloping, well drained to moderately well drained soils that form in thin silty deposits and the underlying calcareous glacial till of silty clay loam texture... The native vegetation consisted of grass and hardwood trees." (Read savanna.) "These soils are medium acid... [with] a somewhat clayey subsoil. The available moisture capacity is high." The "1 to 4 percent slope[]" sites are "generally at the top of moranic ridges." (Not to be confused with moronic ridges.)
With the Spring planting season immanent, the time for having a soil test done will soon arrive. I have worked with this soil for almost 15 years, so I have learned how plants grow in it and how much moisture it needs. Will a laboratory soil test tell me any more than I've already learned about my soil from the soil survey book? Is having a soil test done worth the bother?