For sources, try Steve's source page.Micronized Iron is an iron supplement meant for correcting iron defficiency symptoms in ornamental evergreens, shrubs and lawns (i.e. terresterial use). It is derived from Iron Oxide (the same chemical formula as rust) and contains 25% Fe. It is NOT chleated, nor is it in the proper ionic state to be useful to plants (ferrous) as it comes out of the container. However, it does seem to be useful, when it is used as Steve Pushak recommends in his article "How to Build a Soil Substrate".
The term "micronized" means that a particular material is finely ground, and micronized iron is certainly that. Not as finely ground as graphite powder, but it certainly has a huge surface area, exposing a lot of Iron to the environment. In a substrate, and in Steve's substrate recipe, the presence of a small percentage of peat moss will produce locally acidic conditions and this can change the ionic state of the micronized iron to one that the roots of the plants can absorb.
(In case you missed it, that was another of my plugs for following a recipe exactly unless you have the experience and know how to make proper substitutions. In Steve's soil recipe, micronized iron and peat work _together_ to provide useable iron for the plants. Leaving either one out or making improper substitutions won't necessarily work.)
I have a small, 20 gallon tank set up using Steve's methods and it has been running for almost a year. It seems the longer I leave it, the better this tank gets at growing plants (it had some minor problems initially, due to the "early effects" of a submerged soil). I have never been able to measure any Iron in the water column of this tank using a Lamotte iron test kit, however it does have Micronized Iron in the substrate (mixed with peat) and I have never noticed any symptoms of Iron deficiency in any of the multitude of plants which I have been able to grow in the tank. So I _assume_ that the plants are getting what they need out of the substrate (and the micronized iron).
In another 20 gallon tank which I use as a "grow out" tank, with plants in individual clay pots, a few scraggly Crypt.wendtii were placed in a shallow clay saucer with a few tablespoonfuls of soil with a sprinkling of peat and micronized iron, then covered with plain gravel. Within three months the plants had become thick, full and lush and now (eight months later) it looks like it's time to divide the group and replant them as they have run out of room to grow.