Isn't the idea that you remove the nickel and then go back to farming "normal" crops? So this is a one off way to mine without destroying the environment. And maybe a way to remove excess metals from soil too. Not a continuous supply.

Edit, I guess you could use this technique in waste cleanup too, so maybe once a mine stops producing you plant these trees on the site for one last load. Or you bring waste from the mine (high nickle etc) to farms like this and "replenish" the sites