I came across this excellent video of how to use a refractometer in the garden, and it confirms what I have been finding out in the garden without the use of any scientific instruments.
First is that ammonia nitrogen is far better for plants than nitrate nitrogen. He suggests that the ammonia to nitrate ratio in fertilizers should be in a ratio of at least 3 to 1, which means that if you are going to apply one measure of ammonium nitrate, you had better add to it two measures of straight ammonia. But my practice has been to cut out the nitrate altogether. I use only manure, manure tea, or urine in my fertilizers. I apply them as root drenches and as foliar feeds, and that along with copious amounts of wood chip mulch is all that I need to keep my plants green and healthy. Well, they look healthy, but I won't know if I have achieved that 12 degrees Brix level that he refers to until I get a refractometer and start juicing up some leaves.
I can understand why reduced nitrogen (ammoniacal) is better for plants than oxidized nitrogen (as nitrate): plants, just like us humans, need anti-oxidants to stay healthy. Proteins, the building blocks of life, all contain nitrogen in reduced form, so the nitrogen has to enter the anabolic process as ammonia, and if it shows up as nitrate, well, that adds another step (reduction) before it can be utilized.
Nitrate seems like it should go in the same class as other strong oxidizing agents, such as bleach and peroxide -- a little goes a long way. While organisms, plants included, use such strong oxidizing agents to defend themselves from bacterial invaders, it is probably best to generate them in situ at the site of an infection, than it is to be drenched in them. If there is no infection for the nitrate to kill by oxidizing, it is going to put that oxidizing potential to work in ways that stress the plant.
Another reason to prefer ammonia over nitrate is that nitrate is very soluble, meaning that it can easily leach from a field after a heavy rain, while ammonia will chelate with any metal ion it can find in the soil. Much better to add nitrogen to soil in the reduced form, where it will stay until it is needed (and if need be, get oxidized to nitrate in the process), than to add it as nitrate and hope it won't wash off or burn anything while it is waiting to get taken up.
I'm glad he mentioned boron at the end. I'm still working on the pound of boric acid I bought four years ago, that's how little it takes to keep the garden happy. One gram in a 5 gallon bucket of compost tea is plenty to keep the boron levels where they need to be. I'm glad that plants can tolerate this element in its oxidized form well; I can't imagine what it would be like trying to apply reduced forms of boron like diborane. That would be some bizarro science fiction type world indeed!