In The Good Earth, Wang Lung does decide to move his family south when a drought takes over the land, but this happens earlier in the novel. In Chapter 29, Wang Lung and his family have already returned to their land, and they have met prosperous harvests over the years. Wang Lung has taken Lotus as his second wife, and his sons are older--the eldest married and his wife is soon expecting a child. Wang Lung buys the House of Hwang and decides to move his family from the farm to the courts at the big house. The women continually fight, and Wang Lung wants peace, so he figures that moving them to separate courts in the House will be a good decision.
I the book The Good Earth Wang Lung had tried hard to buy up as much land as he could from others. Land is very important to him and central to the theme of the story. When he buys land but is unable to feed his family he moves with them to the city where he hopes they can get jobs to earn money.
The family moves to a city in the south where they will have to beg for food. All of this has happened because Wang Lung was getting greedy over land. He had used his family's savings to buy more land. When the drought came they could not feed themselves. The trip was necessary for their survival.