American farmers are bracing for another blow should President Trump proceed with his plan to impose steep tariffs on goods imported from Russia, raising the price of a key ingredient in crop production.

The United States imported $1.3 billion worth of fertilizer — mostly in the form of urea and urea ammonium nitrate — from Russia last year and is on a pace to bring in even more this year. Farmers of row crops like corn and soybeans, the biggest users of fertilizer, are already dealing with low sales prices and cannot afford rising input costs or a slowdown in supply.

Mr. Trump threatened this week to levy a 100 percent tariff against Russia, as well as high tariffs against countries that do business with Russia, if there isn’t a cease-fire in the war in Ukraine within 50 days. It was not clear whether fertilizer would be exempted or not, or whether Russia would face tariffs, additional sanctions or a combination of the two.

“We already know these companies that are either importing fertilizer or otherwise in the stream, they are building in the potential for disaster into the cost already,” Rob Larew, the president of the National Farmers Union, said in an interview. He said he worried that rising costs would force farmers to give up farming and sell their land.

Most farmers use fertilizers that contain one of three main elements: phosphorus, potassium and nitrogen. Russia exports of urea and urea ammonium nitrate supply nitrogen, while potash is a source of potassium.

Urea ammonium nitrate imports are under the most threat, said Allan Pickett, the head of fertilizer analysis at S&P Capital’s agribusiness unit, since the United States imports 46 percent of that type of compound from Russia.