[jessaustin]

They could be using iron dust because it's cheaply available as a byproduct of something else, but I would have expected e.g. silica dust to be cheaper still.

[EDIT:] thanks for the info! TFA went on and on, yet somehow omitted the explanation for iron...

[civilian]

Iron is needed for photosynthesis, and silicia is not. Iron is the key limiting factor in the growth of oceanic plankton, so it has to be iron.

Optionally, we could use volcanic ash instead. But only because it contains a lot of iron. :)

https://en.wikipedia.org/wiki/Iron_fertilization

[shiggerino]

I bet sewer sludge would be even cheaper, but that practice was banned for a reason. I fail to see how iron oxide is much better.