Comfrey makes a great fertilizer. I wouldn't burn the leaves, they will rotten easily in water and this creates a liquid fertilizer. See here: https://www.thespruce.com/comfrey-plants-make-great-free-fertilizer-1402191
People do use dead or discarded plants as a source of nutrients for the soil (rather than for plants directly) AFTER they've been composted - in other words, the best use of soft plant material is on or in a compost pile or heap. The resulting compost, once its ready, is a great soil conditioner, and frequent applications of such material increases soil bio ...
There is a big difference in general purpose NPK fertilizer and the one specially designed for Hydroponics. The N(Nitrogen) in NPK fertilizer mostly comes from Urea or Ammoniacal form. It is designed to decompose with time and release nitrite which is later converted to nitrate by bacteria. Thus it is more suitable for use in soil.
Plants readily absorb ...
From a science-based approach, here are three sources from Dr Linda Chalker-Scott from Washington State University Extension that will give you more information. Note that she's approaching these topics from an urban gardening (in the ground, not in pots) perspective.
On Phosphorus in the soil
On organic material to soil ratios
On foliar kelp extracts. Note ...
So going by the Triple Mix method. 1/3 Top Soil, 1/3 Compost Manure, 1/3 Peat Moss.
So now I know a 1:1 ratio of compost manure and Top Soil is what's needed.
Now to figure out nitrogen and Phosphorus.
I think I understand potassium, it makes the soil less acidic.