Potatoes and peat play a similar symbolic role as dirt. If the earth is the hard work on which the speaker is brought up, then the potato and peat are the rewards at the heart of it. They were what all the hard work was for. Okay, well that presents a bit of our problem for the speaker, who is not exactly harvesting potatoes or peat. He's a writer, so what tangible, "cool hardness" will he come up with when he goes to work digging with his pen? Perhaps writers dig for truth, or for a sense of family identity? Whatever you think, that's what our speaker hopes to harvest when he writes, or digs.