Chicken nutrition

Because I live in rain country and have ridden and helped take care of horses, I have been looking for ways to give my chicken vitamins and minerals.
I read that they will eat anything so you have to be careful what you give them. My growing room for vegetables is limited so I will have get creative, but it was recommended to give them greens for nutrition because it is less harmful to them.
It was also recommended to sprout oats. Since I have never done this it will be interesting.  The Grange only had a 50 lbs bag so I have lots of room to make mistakes.
I  did learn that my chickens love ground up carrots.