We discover earth in many different ways in this poem. Heaney talks about the sound, texture, and smell of it, and it seems to have left quite an impression in the speaker's mind in association with his father and grandfather. Earth, in its many different forms, seems to represent what the speaker is tied to, or rooted in, and the association of earth to his father and grandfather is undeniable, so feel free to insert any of these adages to describe their relationship: They make him feel grounded. They are the salt of the earth to him. Their family was rooted in farming. You get the picture. The earth, because it was so important to his growing up, is where the speaker feels he comes from. Let's take a closer look.
- Lines 4, 17, 22, and 26: "gravelly ground," "turf," "sods," and "soggy peat" are all different kinds and terms for dirt, or earth, and they pop up quite a lot, which tells us they're probably pretty important. But why so much dirt? You might notice that each time dirt appears, it's in the context of the father's or grandfather's hard work. The father and grandfather dig up the earth to create food and fuel, which are essential for survival. So you wouldn't be too off base to argue that earth symbolizes the source of these things, and the cause of their hard work.
- Line 27: This line looks at earth from a different angle. Instead of dirt, we're talking about roots, which dig into the dirt or earth. So if we think of the earth as a source of food, fuel, and hard work, well then these roots might be a symbol for our speaker's connection to these things, which he gains from his father and grandfather. These are the roots of the family tree we're talking about here.