Seamus Heaney's poem, "Digging," has a single central conceit (extended metaphor), which presents an analogy comparing the poet's pen to the spade of his father and grandfather.
This comparison creates a poetic meta-narrative device, wherein the metaphor Heaney is using and commenting on also functions as the active means of conveying the content of the poem.
In other words, the pen the poet uses to write the poem is doing the "digging" of exploring memory and "...going down and down" through personal history to access "the good turf" that serves as the poets essential material.
Given this reading of the poem as a meditation on personal history, we might argue that the major theme of "Digging" is the poet's relationship to his family legacy via his own craft and expertise.
The poet suggests somewhat elliptically that in his labors with the pen he will carry on a tradition of dedicated effort. He will work out ideas with the pen - and work with as much skill and determination as his father and grandfather worked the earth with a spade.
As his father "rooted out tall tops" with a shovel, so will Heaney use his pen so that "living roots awaken" and the legacy of hard work and expertise will remain unbroken. The bond of craftsmanship will remain intact.