What are some motifs in The Good Earth?
One of the major ones, particularly significant given the title "The Good Earth," is the importance of land and the earth itself in people's lives. The earth is what nourishes Wang Lung and he feels that working in the fields and having that connection is absolutely vital, that people in the city without that connection have lost something very important.
Another is the disturbing power of great wealth to corrupt even the most virtuous and trustworthy people. Wang Lung sees it and is terrified by it at first, but after a little while as he grows in stature and wealth himself he begins to change as well.