A long time ago I had a milk cow & a large garden & a neighbor whom was also a close friend told me that I needed to add commercial fertilizer as well as the barnyard manure & when I asked the elderly gentleman what he used for fertilizer before commercial fertilizer was available he just smiled at me, I was 20 years younger but I was raised old school. Remember the vitamin commercials where they compared how much you needed to eat to get the same vitamins & minerals in just one of their capsules, pills, or whatever? Think old school, I have known a lot of people that were as healthy as horses that had never wasted their money on supplemental vitamins & minerals. They're for those on a restricted diet. Unless you restrict the diets of your chickens then they should do quite well foraging for themselves but supplement their feed at the end of the day with a commercial feed to insure that they found everything that they could've hoped for. Chickens, unlike people, are less likely to overeat, but like people, shouldn't go to their roost hungry.