My flourish nitrogen bottle says it provides nitrogen in both the nitrate form and the plant preferred ammonium form. The solution is derived from potassium nitrate, urea (iminium salt).
It also says that because one half of the nitrogen in flourish nitrogen is from nitrate you can get a reasonable estimate of nitrogen levels by doubling a nitrate reading, or, you can use seachem's multitest: Total Nitrogen (coming soon) to accurately measure total nitrogen while distinguishing between nitrate, ammonia and organic nitrogen.
Personally I just add all the nutrients in the proper doses twice a week regardless what my test kit is showing me. I do a 50% WC weekly. My nitrates stay between 20-50 ppm. My test kit is not calibrated, I more so use test kits to give me a general idea of what my tank is doing. I use the tank appearance itself to judge if there is a problem.
I'm growing aquarium plants emersed and I'm trying to figure out how to fertalize them correctly. My set up don't have a filter or anything to provide nitrates. And I'm getting blue green algae growing on the substate. I was told to increase and maintaine potasium nitrate at 20ppm. I'm not sure how to do it.
If you read the sticky on dosing methods, and follow the EI dosing amounts you will do pretty well even for emersed plants, which I am assuming have their roots in the water. If not, just do the same, and dose each plant separately a few ml of a mix of water and KNO3.
You can also use a scale to 2-3 decimal places and do this also.
VERY few aquarist bother to do this, but plenty of hobbyists tell others to test these parameters, generally forgetting, or simply not knowing how to calibrate or even that they need to, they just assume the readings are correct.
Getting aquarist to do this step is like pulling teeth as you can image, particularly with folks not familiar with KNO3 etc and all the other stuff.
Water changes are much easier to explain and get folks to do, so that and experience knowing when to do a water change is a much easier sell to planted aquarists. Then you dose known amounts(nothing runs out), and change the water(nothing builds up).