Pup eats things in the garden,dietary deficiency?

Our 4 month old Golden Retriever pup eats everything in the garden - trees, shrubs, leaves, mud etc. None of the plants are poisonous to dogs but there is obviously a danger from sticks, branches etc. He will even leave his food and ask to go outside where he will start to eat anything in sight. Is this just a thing puppies do or is there some deficiency in his diet? He is fed on Arden Grange classic junior food as this is what he was given at the rescue centre and what they recommendeded we should continue to feed him with. He is gaining weight steadily - about 1.2kg per week - and is fit and healthy. Sometimes he eats all his food and other times he leaves some, which is taken away after half an hour. He drinks normally and we have no worries about his health except for the problems that can be caused by eating everything that grows outside. He doesnt try and eat things indoors only when outside - although he did have a go at the pot plants when we first had him he no longer bothers with them. He will also try and eat the seaweed when we are at the beach. Any help would be greatly appreciated.
Happy new year to you all.

Hi Lizziel your puppy is of course teething and will chew everthing in sight , it could be his gums are sore. Has he got lots to chew on ? Try giving him whole large carrots these can also be put in the freezer, cold things are soothing to sore puppy gums.Also try ice cubes and an old tea towel knotted and frozen. Large Nylabones are good for teething on too, you can get them on the internet.
As for food Arden grange should be ok as long as your puppy looks healthy, maybe add some finely chopped raw veg if your puppy will eat it .

Thanks for the reply. yes he has loads to chew on and this only happens when he is out in the garden or at the beach - doesn't seem to do it in the woods though where I would have thought he would make use of an abundance of trees. He now has all his new front teeth but I'm not sure how many others are expected in the rest of his ever open mouth.
We have never had a young puppy before as our last rescue dog was about 10 months old when we got him so it has all been quite new to us but we are learning fast.