Cane, Comfrey is a plant that has a strong deep root system that pulls nutrients from waydownunderthere. Mostly it pulls up Potassium, the K part of that NPK numbers on fertilizer boxes.
After the Comfrey plant pulls up that K a good amount of it is stored in the plant. So, when Comfrey is a good part of your compost heap, you have a compost heap rich in Potassium.
Comfrey is like a living miner with its roots. It mines that K from farther down than most vegetables can go to get it.
Fodder Radish is another living miner good to grow as a compost crop. It picks up a good mix of nutrients from waydownunder. Alfalfa is probably the best miner for nutrients, but Alfalfa has some contoversy about being used too much in compost. Some say it has some chemical that causes reduced growth of other plants, some disagree with that.
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Stinging Nettle has a different reason for being added to compost. Supposedly it has a property about it that causes compost to biodegrade more rapidly.
I fully understand Bob's feelings about stinging nettle. But Bob, if you are at the burn pile with your nuclear powered torch ready to incinerate that vermin nettle @#$^^ stuff, may as well put on the gloves, grab some snips, get a bucket, and cut a bucket of the ^$%^#(& nettle vermin's $%&& leaves, and give them a good curse while sticking them on top of a pile of fresh lawn cuttings, a volcanic caldra on top of it, add some more curses, then put a pile of fresh manure over that, and then another 3 foot high pile of lawn clippings over that. Make a nice wide fire line around it for if it catches. If it does, good. If not, that's good too! Either way, those #$&^*& vermin nettle leaves will help you while they thought they'd get you. It's a win win!!! Won't it be great to do a win win on those #$^$%&%#^*%^## vermin nettles? Beat them twice!

<<< Stinging Nettles wars! Beat them twice. Kill them, and then make them help you! insert evil laughter here!