Low tech tanks and high tech tanks have one concept in common:
Supply the plants with the nutrients they need in quantities that work. The target here is like hitting the side of a barn.
With both systems you have a starting point:
Tap water- Read the report your water company produces, or have your well checked- remember the problem Diana W had with zinc. If you know the water varies, then test that before each water change.
Substrate- If you set up a reasonably rich substrate with good CEC then fertilizing is a lot easier. The substrate starts off the tank with reasonable levels of most things, and before it is exhausted you are dosing to refill it.
Fish food- If you have livestock, and are feeding them this is a good source of most nutrients for a low tech level of tank. Not enough for a high tech tank. I found that my heavily stocked tanks were short on K and Fe first, as well as C. Those were the first things I started dosing.
If you find that contributions from these sources are plenty then do not dose more of that element.
If you find the supply of something is anywhere near the bottom of what will work, then dose it. This is common to both high tech and low tech tanks.
For low tech tanks 'dosing' might simply be a water change once a month. Or maybe it does involve adding fertilizers or carbon.
For high tech tanks it probably means dosing pretty much all the nutrients.
EI is one method of dosing. It hits the 'side of the barn' target just fine, then clears the tank of excess weekly. As you get to using the system you will adjust as needed. You might look at test results to see if you are using too much, or you will look at the plants to see deficiency. When I started using it I still had heavily stocked tanks, so I adjusted the KNO3 way down, but had to add KH2SO4 in large amounts. Now that I have a lot fewer fish I am swinging back the other way.
Here is how I decided how much to use:
N- based on NO3 test.
P- I have a phosphate test, and it shows a little color. Before I had the test I equated the P with the N. If fish food was supplying lots of N, it was also supplying lots of P.
K- Holes in the leaves show up when I skimp on the K. New growth comes out without holes when I dose more. My K test is the plants.
Ca, Mg: Tap water has plenty, per GH test. Add Seachem equilibrium or Barr's GH Booster for hard water tanks, but this is for the fish, not the plants. Plants never took so much Ca or Mg that the tests showed a difference. When I dosed GH per EI recipe the test climbed. My assumption: Fish food and water changes supplied enough Ca and Mg.
Fe: This is one of the first things I needed to dose even running low tech tanks. So I keep on dosing a little extra with the CSM+B.
Dose CSM+B per EI. and I know there is plenty of everything except, maybe, iron.
Somewhere I read dose micros, but just test for Fe. When the Fe tests in the right range, assume the rest of the micros are OK. This works as long as your substrate or water does not have weird levels of something. My Fe test never showed me any color that was on the chart. It was so obviously green, when the chart when through grey and red. So I went back to looking at my plants. They were rich green, and the reds were as red as my low light would let them be.