You can make comfrey and nettle teas but you are never sur of their exact strength. Using a commercial liquid feed you know your plants are getting exactly the right combination on NPK, in addition a lot of them have seaweed in them.

A carrier bagful of comfrey and /or nettles in a sack placed in a bucket of water. A bag of sheep / chicken poo in a bucket. Then after a couple of weeks, use as 1 litre goo with 9 litres water. I haven't bought tom feed for years now and never have had a problem.

...but be prepared for the smell!!

Indeed!