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

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