All these weeds have taken nutrients from our garden soil simply to grow and reproduce, even more they have manufactured more nutrients from the sun adding generously to the nutrients they consumed.
A few of the more well known ones ---
Dandelion - rich in iron, sodium, potash, phosphate.
Chamomile rich in iron, calcium, phosphate, sulphur.
Yarrow - rich in iron, calcium, potash, phosphate, nitrogen,sulphur.
Stinging nettle - rich in iron, nitrogen.
Comfrey - rich in nitrogen and potash.
like a horse and carriage compost and weeds belong together, you can even make a compost activator from then for free.
Mineral rich herbs like nettles, dandelion, chicory, and salad burnet will also act as compost activators.
Comfrey is an excellent addition to the compost heap. It has a carbon/nitrogen ratio very close to that of manure. chamomile, yarrow and valerian also act as as ‘compost plants’.
perhaps instead of weeding I am gathering valuable nutrients to return to the soil ? Perhaps instead of doing battle with weeds I might manage them .... hmmmmn.... if I am vigilant perhaps i can harvest the nutrients before they seed, human nature being what it is there will be some escapees to continue the cycle 
ops :lol actually there are at least 60 different species of dandelion and some say up to 235 species recorded in the UK! 
