I believe that one of the themes in this poem is that the world is big compost pit. When people and other living things passed they eventually comes back to the soil and become compost, nourishing the next generation.
"I will run a furrow with my plough—I will press my spade through the sod, and turn it up underneath;"
On the other hand I also believe that he tries to show us the beauty of the earth we live in. Praising the new life that happens everyday and reminding us to treasure it.
"Behold this compost! behold it well!
The delicate spear of the onion pierces upward,
The apple-buds cluster together on the apple-branches,
The resurrection of the wheat appears with pale visage out of its graves,
The tinge awakes over the willow-tree and the mulberry-tree,
The he-birds carol mornings and evenings, while the she-birds sit on their nests,
The young of poultry break through the hatch’d eggs,
The new-born of animals appear—the calf is dropt from the cow, the colt from the
mare,
Out of its little hill faithfully rise the potato’s dark green leaves,
Out of its hill rises the yellow maize-stalk—the lilacs bloom in the door-yards"