http://en.wikipedia.org/wiki/Fertilizer ML take a look at this site if you get a chance, you probably already know this but I found it interesting. Now if I just knew the scarcest element. Some times the problem with great answers, I am not smart enough to know the question.
Oh, and I would say narcotic is a bit strong, food or nutrients might be a better comparison, no flame intended, JOMO, just one man's opinion.
