Starvation was the main reason for the migration from the Great Plains area of the United States. Because many of the farmers were sharecroppers, they had a limited amount of land on which to grow crops. As a consequence, the precaution of rotating crops to prevent deprivation of minerals from the soil did not occur. Added to this a phenomenal change in the temperatures of the Pacific Ocean as well as the Atlantic Ocean altered the Jet Stream and created "ideal" drought conditions, which led to the land drying up in areas such as Oklahoma where the Judds of Steinbeck's novel resided. With no vegetation or trees to hold down the topsoil, the land became barren, and, when the wind blew, there were terrible dust storms.
Forced from their homes by starvation and lack of clean air, many of the Oakies traveled to California, especially the Salinas Valley where they had learned that there were jobs as vegetable and grape pickers. This journey of the Oakies became one out of the metaphoric desert of Oklahoma to the Promised Land of California.
The poor farm practices mentioned above including plowing up centuries-old grass turf (the sodbusters) that had held the topsoil in place from wind and rains storms so that large wheat crops could be planted (wheat was getting great prices in the early to mid 1920s). Then when wheat prices collapsed, farms went broke and left the topsoil exposed to the elements.
So the other major reason farmers like the Joads left for somewhere else was because they could no longer be farmers - their farms were literally gone, the topsoil picked up by the ton and hurled across several states until the land left, even with water, was virtually unplantable.
The reason that the Joads and many others like them are moving from the midwest to California is that they cannot make a living in the midwest anymore. They are hoping that there will be work for them in California.
This happened for two main reasons:
- The Great Depression made it hard for farm families to keep their land. They could not sell crops for good prices and this led to them losing their land if they owed money on it.
- There were a series of dry years that (along with poor farm practices) led to huge dust storms. This also made it impossible for people to make a living farming in the "Dust Bowl."
In the book The Grapes of Wrath many families in Okalahoma suffered from the drought that caused the crops to dry up and dry. Once the crops died and dried the topsoil began to blow away from the winds that came. With no topsoil the dust blew and blew rendering the land worthless and the living circumstances unbearable.
Farmers lost their properties because they could OT make enough or any money from crops sales. They became homeless. Many had heard about the fertile soil in California and had hoped that they would find work doing what they knew best.