110 years ago, the people of Green Lake went to both Doc Hawthorn and Sam, the onion man, for their maladies. Sam grew onions on the opposite side of the lake. He claimed that his donkey, Mary Lou, was 50 years old thanks to her steady diet of onions. Nobody was sure they believed Sam, as Sam himself was only twenty. Sam sold all manner of ointments, pastes, and lotions made out of onions. Doc Hawthorne even used one to cure baldness.
Because the residents of Green Lake go to both Sam and Doc Hawthorn, it suggests that the town as a whole has a relatively good relationship with both nature and the manmade world. Sam's potions in particular suggest that he's very tuned into nature and is able to use it to his advantage, though not exploit it.
When Katherine bought onions, she often bought one for Mary Lou, as well. One afternoon, Katherine lamented the coming rain, explaining to Sam that the schoolhouse leaked. He offered to fix it in exchange for six jars of spiced peaches. It took him six days, as he could only work in the afternoons. As a black man, he wasn't allowed to attend school—he could only fix the building. He and Katherine spoke at length while he worked, and she was surprised by his love of poetry. After the roof, Sam fixed the windows, the wobbly desk, and the crooked door. Within a few months, the schoolhouse was all fixed.
Katherine's surprise at learning about Sam's love of poetry mirrors Stanley's surprise at Zero's mathematical prowess; this suggests that these two are becoming friends in much the same way that Stanley and Zero are. Similarly, Sam's interest in poetry makes the injustice of not being able to attend school appear even worse, as he clearly has a desire to learn.
Katherine, however, was sad, as she had nothing else for Sam to fix. One afternoon as she cried and rain poured outside, she heard Sam selling onions. She raced outside, hugged Mary Lou, and told Sam her heart was breaking. Sam kissed her. Nobody else was in the street except for Hattie Parker. She whispered that God would punish them.
To a contemporary audience, Hattie Parker is intended to read as impossibly racist for her remark. Notably, however, her remark suggests that she believes that racial differences are set out and upheld by the divine, an idea that the novel will test soon.