By God, the old man could handle a spade. Just like his old man.
Okay, there are two things to notice here, but before we talk about them, let's get a grip on what these lines are actually saying. It seems our speaker's father was really good at digging up these potatoes, and apparently his grandfather wasn't too shabby in the fields either.
First, we've got another comment about tools and trade. The speaker seems to think the ability to work with one's hands is a good thing; he's paying his father and grandfather a compliment.
Second, check out the first two words of line 15. "By God" is an exclamation (without the exclamation point). It grabs our attention and shows us just how much enthusiasm and admiration our speaker has for his father and grandfather's skill. Not only does work seem important to our speaker, but tradition, too – or work as part of the family tradition. He comes from a long line of diggers, and he seems pretty proud.
Also, check out how these lines are their own stanza. We take that to mean they're pretty important.