All good advise on testing. I’ll throw my 2 cents in too…
In general there are several things you want to know about your soil: levels of nutrients (macro and micro), the availability of those nutrients, texture/composition of your soil, and organic matter are the major ones.
When it comes to testing, there are several different “camps” or philosophic/scientific approaches. The standard soil tests will give you NPK values which correspond to the total amount there (not necessarily the readily available amounts) and pH. That’s a good basis, and will tell you in very general terms where you stand with your soil. It isn’t necessarily a guarantee that the NPK that test shows will be available to your plants, that will depend upon soil temps, bio activity of the soil, how the nutrients are bound, and some other things, but at least you’ll know its there, and perhaps more importantly it will help prevent you wasting time and money adding more of anything which may already be there in excess. If you go thru your county extension agent, these tests are often relatively cheap or free in some areas.
Albrecht (former head of a state Ag school’s soils dept) came up with a “theory” of good/ideal soil based upon cation exchange capacity (CEC). It took a while to gain some acceptance, but now most labs will give you CEC numbers if you ask (and pay). To condense his life’s work into a sentence or two, Albrecht found productive soils had a certain ratio of CEC values (Ca 60%, Mg 15% etc). Those labs take a look at your CEC test numbers and give recommendations not only for the current crop, but also to push your soil’s CEC numbers in the right direction. They use the same conventional testing methods and so also give you total nutrient numbers not necessarily available numbers. One advantage of the Albrecht method tests is the lab I have used includes quite a few micro nutrient tests in their basic test, so it actually ends up far less expensive than going with the extension lab and paying extra for the micro tests. Albrecht was also the first (AFAIK) that said Ca was an important nutrient in its own right, and not just something you use to adjust the pH; an approach I agree with.
Reams was a somewhat controversial figure in Ag (and other fields), but he adopted a soil testing system which uses very mild extracting solutions for the tests. Similar to what plant roots excrete. So these tests tend to show numbers which are more in line with what the plant can extract from your soil (what’s really available). These numbers are useful as a comparison to the numbers from conventional tests which show you the total amount available in the soil. By comparing the two, you can get an idea of what nutrients you may need to supply even though it appears that there is plenty in your soil because it may not be that available to your plants. Reams labs also generally include some micro nutrient tests in their basic fees.
One other thing to be aware (cautious) of, some companies which do testing also sell fertilizer and supplements. I tend to avoid those labs, because I want numbers not just info on which of their products they want me to buy.
Another thing to consider in all this is that there is also foliar feeding, which is a way to give your plants nutrients directly (spraying dilute nutrient solutions on the leaves) as well as adding nutrients to the soil. This can be very useful for acute deficiencies where you don’t have the time for a nutrient to be broken down in the soil and made available, as well as just for a boost in non-critical situations.
There are also some simple tests you can do at home. The shaking some soil in a jar of water and letting it settle will tell you quite a bit about the makeup of your soil from a structural POV (see online for specific instructions).
Well this post is quite a bit longer than I thought it would be, hope I haven’t lost you in all this. As you can see there is quite a bit of info out there on soils, with several “schools” of thought on how to do it right. But perhaps the most useful thing you can do is get your soil tested, whichever test(s) you want. Just be aware of the limitations of the testing lab/philosophy you choose. With your test numbers, you will have an idea of where your soil is at, and what it needs more of (and what it doesn’t). Try out the different approaches and see which ones work for you.