I am a first year container grower and I have just emptied one of them for my first crop of potatoes, my question, Is the earth/compost any good for future growing or has all the goodness been taken out of it.
This would be impossible to determine without testing the soil - which probably isn't worth the time and trouble. Assuming it was potting soil to begin with, whatever plants were put into it would have gradually utilized the nutrients; how much is now left is anyone's guess. Having said that - I see no problem putting the old contents into a compost bin - if you have one. It might not help much, but it couldn't hurt. Otherwise, just discard it and use fresh soil for your potatoes.