Nickel is a concern, but seems not to make the the whole idea untenable.
In particular, the alternative of spreading powdered olivine on cropland will require monitoring. Different sources of olivine have different amounts of nickel and chromium, and the sources with acceptable levels can be favored for that use. Use on beaches and shallows seems to tolerate more. There is no reason to do one but not the other, and dozens of other measures besides.
