Has anyone found a soil test kit for home gardeners that they believe is reasonably reliable?

And does anyone know "how" such home kits test for the basic elements? I know it depends on some kind of chemical in the kit reacting with soil samples, but I'm just curious about how they give results.

I am thinking about a soil kit that would be supplemental to the lab-done soil tests that many of us get done. If reliable, it could be used after the growing season to ascertain what nutritional elements are low or out of balance. Or it could be used on the soil of a specific part of the garden where you plan to plant heavy feeders.

Any recommendations?