This is Milorganite's History page from their web site: http://www.milorganite.com/about/history.cfm As you can see, not word 1 mentioned about iron past or present. This specific question may be worth a call or two to their 800 number, what'ya think? Oh, sure, I'm not debating whether iron is a naturally-occurring element or not. It has it's esteemed 26th place on the periodic table. And no doubt there's X amount of digested Fe that is processed into Milorganite right along with the turds. But is that X amount sufficient to total 4%? As far as your point about relatively few people not knowing the benefits of Fe on turfgrass, I take exception to that. 'Ironite' has been seasonally marketed quite heavily nationwide the last couple of decades on radio & TV, newpapers & magazines since it left its early origin of golf course greens and entered the retail market: http://www.ironite.com/ Like Milorganite, Ironite perennially suffers from an onslaught of criticism from organic gardeners, especially, because of a problem with arsenic & lead at ppm levels. But to the point....I think that because of super-aggressive & very well-timed marketing by a namesake company like Ironite, generally more folks know about the (short-term, cosmetic) benefits of iron than you may think.