"He would, he told himself, in spite of gods and drought, do that which he had determined." What did Wang Lung do? This is from chapters 5-9.
At this point in the story, Wang Lung has worked hard and scrimped and saved so that he was able to buy a small plot of land from the House of Hwang near the mote. By focusing only on this land (knowing that everything else would perish), Wang Lung again finds himself in possession of a slight excess of wealth.
Instead of hoarding or burying the silver he makes from the immediate sale of his grain, what he "had determined" to do is to obtain more land. He rushes back to the House of Hwang and offers to purchase the adjoining land between the two pieces he now owns. The House of Hwang has been slowly growing poorer and poorer, and is willing to sell at a low price.
In the end, Wang Lung's land and family also fall victim to the famine and must abandon everything and flee to the south in order to survive. With nothing but the clothes on their backs they go. But the difference between Wang Lung and others is that he is a land owner, so he will have something to return to. And when he is finally able to return, he becomes a very wealthy man.