Kelp Meal is a great supplement for any animal at all. I initially mixed it with feed, but now I just put it in a tray like grit and oyster shell. It costs around $35 for a 50 pound bag from the feed store.
Think about the ocean. It's a huge repository for all the minerals on earth. Kelp is increidbly high in trace minerals which contribute to good overall health of your birds. Another thing to consider is that chickens are omnivores. I know people who give them fish meal since there are certain proteins and/or amino acids which only exist in animals. So your hens on a layer ration, getting all the protein from corn or grain, will never have those nutrients in them - unlike how they would be in nature.
It also is a great source of your Potassium if you make your own organic garden fertilizer.