But in the long run, how you increase the fertility of your soil organically is to keep feeding it lots of organic materials. All the compost you can make and keep your soil well mulched all the time. I use a green/brown mix for mulch just like I do for the compost pile: fall leaves + grass clippings, pulled weeds + shredded paper, etc. I think it breaks down faster that way, doesn't mat down, and it is a complete soil food. Keep adding more as it breaks down. If you are really working on enriching your soil, you can add well aged, composted manure a few weeks before you plant and at the end of the season.
Since I don't buy any commercial additives, I don't bother with soil tests - wouldn't change what I do anyway. Nitrogen is the most transient of the nutrients, so the one you most need to focus on when you are doing it my way.