It's certainly true that stinging nettles are rich in Iron and many other useful minerals and vitamins, there are claims of many other health benefits too, but that’s beyond my expertise, they are abundant, tasty and free! Springtime is the best time to pick the young tips before they grow too fiberous, from long lived experience it’s the tiny ones that give the meanest sting so make sure your gloves are a decent thickness.
I’ve added spinach here to enhance the colour of the soup, when picking nettles you’ll more than likely come across hedge garlic and ground elder growing alongside them in the hedgrows, by all means add these to the mix too if you know what to look for.