May be an impossibly teleological question

[fivemack]

Why is it that volcanic ash makes good fertiliser?

It contains minerals pulled up from the upper mantle and pulverised into conveniently absorbable powder; but is it purely a coincidence that it tends not to contain chromium, cadmium, arsenic and heavy-metal fluorides (fluorides mentioned specifically because volcanic steam does contain non-negligible HF)?

I suppose the volcanoes that erupt great clouds of sulphur tend to be known as sulphur mines, and obviously people won't farm next to volcanoes whose outpourings make plants wither, but it seems unaccountably convenient that they so often work.

Comments

[ritaxis.livejournal.com]

I think you're looking at it from the wrong end: if you consider it as "how come so many plants thrive on volcanic ash?" instead of "how come volcanic ash is good for so many plants?" the answer is embedded in the question.

Plans thrive on volcanic ash because at various times in our history plants had volcanic ash to live in. So they evolved to make use of the minerals brought up by volcanoes.

[twoeleven.livejournal.com]

One could go even further and say it supports the "black smoker" hypothesis, that life first evolved around black smokers and other volcanic vents on the sea floor. Volcanic ash is then just a similar environment on land.

[peristaltor.livejournal.com]

This.

[kaberett.livejournal.com]

I see this and will try to put together an answer. :-)

[hairyears.livejournal.com]

The 'absence of heavy metals' part of the question interests me...

[tigerfort.livejournal.com]

I'm with ritaxis; this is the same question as "why do so many Earth species thrive in an oxygen-rich atmosphere?". When volcanic ash is what the environment presents you with on a regular basis for hundreds of millions of years, you evolve to do well on it.