Well... I looked up a recipe for 'comfrey tea' and it's just 20 day anaerobic rotting under water... I've never made it but I can tell you that 'any other plant' rotted the same way smells just as sweet. I know it because buckets of weeds, or kelp, plus rain, plus neglect... whew.
I'm sorry though, I have to take issue with the evaluation that comfrey "is not any better than any other plant". That would lead a person to believe that all plants have the same NPK values and therefore have identical fertilizer value... and this isn't true.
Take a look at this database, the NPK of everything (well... not by a long shot, but a start
)
https://www.thenutrientcompany.com/n...able-database/
For example of ordinary leafy plant material, nettles and comfrey are listed next to each other (in the 100 items list) and have unique values for NPK. There's more N in nettles and more K in comfrey. I know my friends used to fill a barrel with water in the spring and toss nettles in there, which they used to water the crops. Maybe nettles were better than comfrey for that application because of the higher N. Maybe comfrey is touted for tomatoes because K is important for fruit quality. Maybe comfrey tea is better in the UK because the soil leans toward K deficiency. Maybe some other plant entirely would serve you better in the Argentina pampas.
What I'm getting at, is that saying that comfrey is no better than any other, is like saying that all fertilizers regardless of their NPK are no better than others. That doesn't sound like science to me.
Doesn't even sound like gardeners, who do argue mightily about which fertilizer formula is the best!!
Is it conceivably possible that the rotting process in 'tea' making could reduce every plant material to the ho-hum balanced 4-4-4 or similar value usually given for "garden compost" ? I'm not sure why or how that could be, if a single plant material with specific NPK ratio is exclusively used.
Losses due to aromatic volatiles?
You'd expect different 'teas' to smell different, in that case.
Incidentally, blackstrap molasses has been recommended as another source of K and trace minerals for liquid ferts. I use it to balance the N-heavy fish emulsion. And guess what... it smells pretty good, compared to plain fish.