How does the phosphorous cycle differ from the nitrogen cycle and the carbon cycle?

A good way to answer this would be to think about what the major sinks of each resource are. For example phosphorus is mainly pooled in rocks and is released when rocks are weathered and then cycles with the death and decay of plants (etc.) that take it up.

It doesn't occur like this for the nitrogen and carbon cycles - what are their major sinks?

I've attached a pdf which outlines the cycles pretty well :)

Cr(oh)3 -> cr(3+) + 3oh-