Very occasionally, my dog eats poop that has been outside for a while. Usually it is a dried-up piece that insects have long-since abandoned. This is appalling behavior and my only way to prevent it is to be diligent about yard cleanup.
He also occasionally eats dirt. I feed him well, he gets vitamins and food, but I think he might be a little nuts.
Why is he doing this?
Like you have noted, coprophagia (the technical term for feces-eating) can, like dirt eating, be a response to a dietary need (often stuff like missing enzymes and the like in their diet). If the dog eats a dry-food only diet, they will be missing certain things that might make them more prone to seeking out poop as a way of filling that need for digestive enzymes.
It can also be a stress response - dogs who are anxious or stressed have a tendency to eat poop more often, as do dogs who have been excessively punished for making messes in the house, for example.
Attention seeking can also account for it as well - negative attention is still attention after all! I can also just be an oral fixation, which usually can be solved with giving the dog something better to chew on than poop.
There are a few things you can do to stop the behaviour. If you know the dog is eating its own poop, there are products you can add to its food to make the poop taste undesirable. You can also work with your dog to teach it a "leave it" command, but if the dog is outside without you present, this will be less effective, as then you won't be around to stop the dog from eating it. There are also things (such as bitter apple and bitter orange spray) that you can put on the poop itself to make it less interesting to the dog.
In general, the best suggestions are to keep the yard poop-free, or to train your dog to leave things alone on command.
A female dog will eat the feces of her puppies. It starts as soon as the newly born puppies start producing stool and it continues until the puppies have been left to their new owners. In case a puppy stays with its mother the poop-eating may continue for a rather long time. I've witnessed a mother-dog still eating her puppy's poop when the puppy was five months old, being almost the same size as its mother.
Reason for this is to prevent predators from finding her puppies by the smell of their poop.
Adult dogs mark their territory with urine, but also with feces. It could be that eating another dog's poop would happen in order to disclaim the other dog's territorial rights, but I have no proof to support this as a valid possibility. The other answers to your question seem to have more credibility when it comes to a dog eating old dried-up poop.
Dog poo eating is a common problem with dog owners, especially with some breeds. My sister's Labrador had the same problem and it took quite a long time (few months) to control this habit. The problem is, some dog breeds are unable to understand the difference between edible and inedible things, which is very common in Labrador, Golden Retrievers, etc. For that you need proper training for him; negative enforcement should be not used.
A few tips from my own case:
It can take time depending from dog to dog, but you required little care as you said, he eats his old poo which can be very unhealthy due to germs, etc.
There are some cases when dogs eat excrements in an almost compulsive way. You can recognize these cases because the dog spots every single piece of poop lying around and actually hurries to get there before you do. It can sometimes feel like racing a petulent child to the candy store that is always faster and always swallows treats without chewing.
These cases have nothing to do with nutritional needs and chances are high that although you try to prevent your dog eating poop, you are actually reinforcing the behavior.
Let me give you an example:
The way out of this cycle is behavior adjustment training. Basically you train a desired behavior with your dog (like obedience training or a certain game) and every time you encounter the stimulus, you initiate the desired behavior instead of the undesired poop eating. Unfortunately the topic of behavior adjustment training is too broad to discuss here, but there are many sources like books, blogs and videos out there to help you. A talk to a professional dog trainer can be very helpfull, too.
