“N” refers to nitrogen. The P, K, and any other mineral in the formula is totally irrelevant when discussing N.
Fertilizer formula is irrelevant. It tells you the concentration of the ingredients in weight percent. A 20-10-10 has double the concentration of a 10-5-5, so you simply need to use half as much per feeding to give the plant the same nutrition.
Likewise, ppm N merely specifies the concentration of nitrogen you are supplying in your solution.
In addition to concentration, you need to consider frequency of application and extent of exposure in your thinking.
We have no idea how much of what we apply is actually absorbed by the plant, but can reasonably assume it is the same every time we feed - assuming it’s done the same each time. Based upon that, if you feed twice a week, you’re giving the plant double the mass of nitrogen that you would if you fed once a week. AND, if you apply a 50 ppm N solution, the plant can reasonably be expected to absorb twice as much nitrogen as it would if fed a 25 ppm N solution at the same frequency.
Then there’s the exposure.
If you feed a vanda growing bare root in a basket, the time it has to capture nutrients is only as long as you’re wetting the roots. At the other end of the spectrum is hydroponic growing, where the roots are always wetted by the nutrient solution.
If you want the vanda and hydro-grown plant to get the same mass of nitrogen, it is going to need a much more concentrated solution, applied frequently.
As to your flushing with pure water question, recognize that plants can only take up nutrients in solution, so by doing thorough flushes, you’re replacing a nutrient solution with one containing no nutrients, as they all have been removed. (I’m disregarding the nutrients accumulated in the medium, as there’s not much you can do about that, and it is never completely removed.)
For plants needing a “winter rest”, stop feeding.
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