Washington (CNN) President Donald Trump's trade war made last year tough for American soybean farmers, but 2019 could be the year they really start feeling the pain -- despite Beijing's pledge to resume buying from the United States.

The amount of soybeans sitting in storage in December hit a record high of 3.7 billion bushels, according to new data from the US Department of Agriculture. That's equivalent to about 80% of the total US harvest last year

Even if China follows through on the additional purchases officials have pledged in trade negotiations, there could still be 900 million bushels for soybeans in storage at the end of this season -- more than the entire crop grown in Iowa, one of the biggest producers.

Farmers have been patient as tariffs have hurt their export markets. The Trump administration made $12 billion in aid available to farmers hurt by tariffs last year, which softened the blow. Plus, some demand for US soybeans shifted to other countries.

"So far, farmers don't feel the pinch directly. But if this lingers on, and we still have billions of bushels in extra supply, this could become more of a 2019 story," said Grant Kimberley, a soybean and corn farmer and director of market development at the Iowa Soybean Association.

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