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I've noticed that water soluble fertilizers tend to be blue. What chemicals or chemical reactions cause this color here? Is this necessary or for aesthetic purposes? I haven't seen this in organic fertilizers. I figured chemists might be able to examine the ingredients of Miracle Gro or something and tell me. I'm guessing it has something to do with copper.

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What chemicals or chemical reactions cause this color here? Is this necessary or for aesthetic purposes?

I have some Miracle Gro on hand, the water soluble kind: The blue stuff is copper sulfate, or ${\rm CuSO}_{4}$. Here is the MSDS for Miracle Gro Water Soluble All Purpose Plant Food, which lists the chemicals in the mixture.

Is this necessary or for aesthetic purposes?

It's necessary in these formulations for purposes detailed (in part) here and here. In short, it is included in these formulations because plants need copper for biosynthesis; and because it acts as a fungicide, presumably leading to a nicer ecosystem in one's lawn/garden.

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