NEW YORK—Soybean prices plunged Monday as investors bet that Argentina’s government will soon resolve a farmers’ strike over an export tax that has slowed grain shipments.

Other commodities traded mostly higher, with crude oil, gold, silver and wheat futures rising.

Argentine farmers have been on strike the past two months to in a bid to overturn a 44 percent export tax on soybeans and other products, a protest that has crippled grain exports from the world’s third-largest soybean producer after the U.S. and Brazil.

In a potential breakthrough, members of fo...