"Digging" certainly takes skill. Plus, you need a tool – a spade or shovel to be exact. For our speaker, his only tool is the pen. It might not seem like much at first, but when you consider the old saying, "the pen is mightier than the sword," you just might change your mind. Though the speaker admits that maybe his pen isn't quite as great as the spades of his father and grandfather, you get the sense that by the end of the poem, he's decided to be as skilled as he possibly can with it. The grandfather and father are the very best at digging potatoes. Our speaker isn't so good at farming, but he knows that his calling is to be a writer. So why not be good at that, too?
In this poem, Heaney shows that physical strength is much more important than skill.
In this poem, Heaney shows that strength and skills come in many different forms.