Symbolism, Imagery, Allegory
First a history lesson, brought to you by the professors at Shmoop University and the number five.
Foot binding may have started in China as early as the 10th century. No one really knows how or why it started, but the idea was that small feet were beautiful. At first, only upper-class women had their feet bound, but by the 19th century the practice spread to the lower classes.
So here's how you do it. When the girl is really young—like younger than five years old—you basically start breaking her foot. The foot is bent in half and layers of tightly bound cloth break the toes. When that's all done her feet will look like this.
Bound feet could get infected, rot, or have all kinds of other problems. They were a lot of work to create and to maintain. On top of the medical issues, women with bound feet couldn't walk more than a few steps because of the pain. However, the strange ways they walked were considered attractive.
Considering that these women could never work in the field, it makes sense that it took a long time for this practice to become popular with the lower classes; it's totally not practical. But by the time The Good Earth takes place, even poor farmers like Wang Lung knew that bound feet were sexy.
O-lan's feet are not bound, and Wang Lung isn't too happy about that, “He saw with an instant's disappointment that her feet were not bound" (1.131). Grandpa, because he's more reasonable than Wang Lung, tells him he doesn't need that kind of lady. Even though Wang Lung agrees at first, that doesn't stop him from seeking out bound feet.
No wonder he immediately notices Lotus's feet: “And if one had told him that there could be feet like these, little feet thrust into pink satin shoes no longer than a man's middle finger, and swinging childishly over the bed's edge—if anyone had told him he would not have believed it" (19.24). That bit about pink satin shoes no longer than a man's middle finger, by the way, is totally legit. Take a look at these if you don't believe us.
If we weren't sure before what bound feet meant, it's clear now. Bound feet represent beauty, and unbound feet represent ugliness. So even before he sees Lotus's tiny feet, Wang Lung starts to resent O-lan for her unbound ones. “It seemed to him that she was altogether hideous, but the most hideous of all were her big feet in their loose cotton cloth shoes, and he looked at them with anger so that she thrust them yet farther under the bench" (18.8).
In a way, this shows us Wang Lung's confusion between appearance and reality. A bound foot may seem sexy when it's in a cute pink shoe, but the physical reality underneath isn't too pretty. (Only a foot in bindings or in little shoes was considered sexy; women with bound feet weren't supposed to show men what their feet actually looked like underneath.) Lotus may seem sexy on the outside, too, but we know she's no good for Wang Lung, and vice-versa. Meanwhile, O-lan, with her unbound, dependable feet, seems like chopped liver to Wang Lung.
Now, O-lan's not dumb. She knows that only the beautiful people get love. So she binds her youngest daughter's feet without even telling Wang Lung. He doesn't even notice until she's 10 (25.28). Why does O-lan bind her feet? As the third daughter tells Wang Lung, "[M]y mother said I was not to weep aloud because you are too kind and weak for pain and you might say to leave me as I am, and then my husband would not love me even as you do not love her" (25.30).
So O-lan teaches her daughter the formula: bound feet equal beauty, and beauty equals love. Therefore, if you know your math: bound feet equal love.