As Muley talks with Tom Joad and Jim Casy in Chapter Six, he expresses the relationship of the sharecroppers and the land,
"I put my han' right on the groun' where that blood [from his dead father] is still....An' I went in the room where Joe was born....An' all them things is true, an' they're right in the place they happened. Joe come to life right there....
The land is part of the family, part of the history of their lives. In a sense, it is an extension of them, as though it were a body part. The sharecroppers are born on the land, they work the land, it provides food for them, and they give new birth on this land while others die on the land. To leave the land is to lose their identity.
The first chapter introduces us to the prejudices and the commerial forces that serve to define the Joad family. As Tom makes his way back home, we are presented with an attitude from the (corporately employed) truck driver that shows us how society is moving and how the Joads are being left behind.
Steinbeck goes to great lengths to describe the "land" in chapter one, particularly the devastating effect of the environmental disaster known as the "Dust Bowl." This helps the reader get a feel for the plight of the sharecroppers and migrant workers, whose way of life of was destroyed by the effects of the Dust Bowl.
He continues to devote chapters to the land throughout the novel. The land can actually be thought of as a character in the book.