Well, there are ways of knowing though. There are existing analyses of plants, you can get (or deduce) data from.
Examples for NPK:
Potatoe Vines (dried): 0.6/0.16/1.6
Soybean Hay: 1.5-3.0/0/1.2-2.3
Orange Skins: 0/3.0/27.0
Banana Residues (ash): 1.75/0.75/0.5
Apple Leaves: 1.0/0.15/0.4
Coffee Grounds: 2.0/0.36/0.67
Grapefruit Skins (ash): 0/3.6/30.6
There are ways to get to it, but it's not an easy task.
The easiest and most useful application of compost tea in my opinion is the use as complementary foliar spray.
I always and only use self made
nutrients (based on commonly used raw materials) and they are extremely cost effective with around 3-4 US$ per 1000 Liter of solution. And with self calculated and tested formulas, I have exactly what I want and need. A batch of 8 Liter of concentrate (lasting for 1000-1200 Liter of solution) takes me 1 hour of work, preps and cleaning included.
