Great article from the-organic-gardener.com regarding Comfrey.
The Hack: make a concentrated liquid containing all of the big 3 nutrients: nitrogen, phosphorus, and potassium, in substantial amounts. NPK= 8 : 2.6 : 20.5 %.
“…Place the plant’s leaves into a large container, preferably one with a tap or hole at the bottom, and a tight lid at the top to exclude water and flies as well as keep any smell inside. Fill the container with leaves e.g. an empty Can-O-Worms container will do the job, filling the bottom module with tap, as well as the upper modules. Cut the leaves before the flower buds appear and before the ageing leaves develop infections. (I normally exclude rust infected leaves if I’m going to give any of the liquid away, otherwise rust is simply ‘part and parcel’ of the herb’s life.)
A block of wood and a brick could be placed on top of the pile to press it firmly and gently down, (without crushing). Fresh Comfrey leaves contain more nitrogen than farmyard manure and a black liquid smelling of ammonia will soon collect in the bottom. It is drained off into a screw topped collecting bottle. …”
Be sure to dilute it by adding 15-20 parts water before using, bringing it down to 0.5 : 0.4 : 3.8. – a high potash feed