This question goes to the very heart of this profound novel as it explores the way that the pearl, which was hoped to have done so much good for Kino and his family, actually becomes a symbol of man's greed and avarice, and brings disaster and destruction to Kino. In this sad chapter, we see the pearl bringing discord between Kino and Juana, Kino is attacked for the third time because of it and kills a man to defend the pearl, Kino's canoe that he inherited from his ancestors is destroyed and their house is ransacked and then burnt to the ground. As Juan Tomas says:
"It is the pearl... There is a devil in this pearl. You should have sold it and passed on the devil. Perhaps you can still sell it and buy peace for yourself."
However, as Kino responds later on in this Chapter, it is clear that the significance of the pearl has changed for him:
"I have it," said Kino. "And I will keep it. I might have given it as a gift, but now it is my misfortune and my life and I will keep it." His eyes were hard and cruel and bitter.
This is why the pearl has become his "soul," because on the one hand the pearl has lost Kino so much that now it is all he has left, and on the other hand he has lost himself to the pearl and to the dreams of wealth that he has become a changed man, "cruel and bitter" and one whose eyes light up when he touches the blade of a knife.