Diction is defined as an author's choice of words within a text. IN the case of Seamus Heaney's "Digging," the author's word choice encompasses the terminology which one would associate with agriculture, war, and writing (all battles in their own sense).
For example, the words Heaney chooses to represent agriculture are spade, flower beds, potato drills, turf, sod, and dig. The words Heaney chooses to illustrate war are as follows: gun, twenty yards (a measurement used to describe distances in war/battle), boot, and levered. Finally, the single word associated with writing is simplistic: pen.
Another aspect of the author's choice of diction worth mentioning is the use of first person. By using the pronoun I, the author provides the reader with a more intimate view and connection with the speaker.