In other words, if China doubled the amount of goods it bought from the United States starting next year, it would take nearly a decade to reach the $1.2 trillion goal.

Scott Kennedy, a China expert at the Center for Strategic and International Studies, said that purchases of that magnitude would take years to accomplish and that it is difficult for two countries to commit to such a long-term agreement given constantly changing market dynamics and technology.

“It sounds nice because there’s a ‘T’ in it,” Mr. Kennedy said, referring to the word “trillion.”

Even if China agreed to double its goods purchases, the question remains: What could it buy?

Mr. Trump suggested two commodities on Wednesday that could fit the bill: liquefied natural gas and soybeans. In both cases, China has effectively halted what had been large purchases of American exports, in the midst of the trade war.

The United States exported 1.7 million metric tons of soybeans to China over the course of a single week in November last year, data from the Agriculture Department shows. This year, the number of exports during that same week fell to zero.

Simply returning to previous levels would be a significant win for American farmers — but it would not come close to $1.2 trillion. In the 2017 fiscal year, before trade tensions flared, the United States exported about $22 billion in agricultural products to China. For the 2018 fiscal year, that fell to about $16 billion. For the 2019 fiscal year, it is projected to fall further, to $9 billion. That is a $13 billion gap from the pre-trade war levels, which, if filled with Chinese purchases, would take 92 years to add up to $1.2 trillion.

Likewise, China stopped importing any American liquefied natural gas in September, after buying 10 million cubic feet of the gas in September 2017. At this year’s prices, that was a loss of about $47 million — though overall liquefied natural gas exports continue to trend up for the United States, as countries outside of China increase their demand.