The go to sources are Steve Solomon’s book The Intelligent Gardener and Michael Astera’s The Ideal Soil both available via Amazon.com. Steve’s book should also be available in local bookstores.
The gist of soil testing and its rational are summarized at Growabundant.com, a website operated by Steve Solomon’s co-author. Both Steve Solomon and Michael Astera recommend a Melich III test from Logan Labs.
Logan Labs soil test is now $25. Logan provides soil sampling instructions here. You send a copy of this worksheet in with your soil sample and a check for $25 or more if you are getting multiple samples. Growabundant.com has a series of short essays to get you started.
Subscribe to Organicalc Organicalc is a function on the Growabundant website that allows you (for $9.50) per year to input the results of your Logan Labs test. Organicalc will then give you a recipe for your garden’s custom fertilizer.
There is another essay that tells how to apply the amendments. Applying amendments
Amendments can be easily mixed in an Oddjob Mixer.