Consider the soybean.

A legume about a centimeter in size, it is eaten from the pod as edamame or processed into tofu, soy milk and other products. But that is not why it is one of the world’s most lucrative commodities. High in fat and protein, soybeans are what much of the world’s livestock eat.

And now the humble crop is at the center of the trade war between the United States and China.

The United States sells more soybeans to China, by value, than any other single product. Last year, that amounted to more than 27 million metric tons, worth $12.8 billion, or about 9 cents of every dollar of goods the United States sold to China.

But with the enormous tariffs erected between the two countries over the past two weeks, those sales are likely to suffer soon. That is bad news for the American farmers who grow soybeans and the Chinese chicken and hog farmers who buy them — and potentially very good news for the nation ready to step in: Brazil.

American soybean farmers are worried about whether their biggest customer will keep buying. More than half of U.S. soybean exports went to China last year, but the price just went up 135 percent under the tariffs China installed in response to President Trump’s 145 percent tax on Chinese imports.

“Farmers deal with bad weather. We deal with pests. We deal with tractors breaking,” said Heather Feuerstein, who owns a farm near Grand Rapids, Mich. “That’s our lives.”