There are several symbols in the novel. I will list a few. I cannot also address imagery. This needs to be submitted as a separate posting.
The first symbol is "the road." Like The Wizard of Oz, the road is the beginning of the journey home, even though one may not be sure where or what home will be at the journey's end. The families are using the roads to leave the old and find something new and more promising. Tom travels the road to find his home again. The road, while connecting two destinations, can also be a dangerous place, even as it is today. The road provides connections and gives way to hope in this novel.
Another symbol found in the novel is bugs. Bugs are referred to several times. In terms of farming, bugs can cause great damage to crops. Bugs in the past have been known for giving life and taking it away. In the Bible, John the Baptist ate locusts and honey in the wilderness, but locusts were also one of the Biblical plagues. At one point Tom kills a grasshopper with precision and brutality. Perhaps this is symbolic of how the strong treat the weak, i.e., landowners and tenant farmers.
A third symbol is that of the farm. When the tenant farmers leave the land, the machines take over, and the men that work the land do not have the emotional connection to these farms that the families did. Without the care of the families who have lived there, the houses fall to ruin. And although the farmers try to fight this change, progress will not step aside. No thought is given to the multiple forms of life that exist on a farm. Certainly today's farmers are fighting the same battles to survive.