Why are nitrates, magnesium and phosphorus needed in plants and what are their functions?

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Moderator Note

Moved to homework help, where it will likely get more attention and participation.

http://en.wikipedia.org/wiki/Plant_nutrition

As with all homework questions, you'll need to tell us where you're at with the question before we can help you (we don't just give out answers). Do you know what sort of biological compounds have phosphorus in them, for starters? There's one in particular that is ubiquitous to most all life, which incidentally incorporates magnesium as well. Might not necessarily be the answer you're after, but it should at least get you thinking.