WASHINGTON — The U.S. Department of Agriculture said this morning that China bought 510,000 tonnes of soybeans from U.S. private exporters. It was China’s second purchase of U.S. soybeans reported this week.

The U.S.D.A. this morning also reported a sale of 113,000 tonnes of soybeans to unknown destinations. On Tuesday the U.S.D.A. reported a sale of 120,000 tonnes of soybeans to China. All three sales were for delivery in the 2013-14 marketing year that begins Sept. 1, 2013.

Despite today’s large export sale, soybean futures prices in Chicago were trading at one-week lows because of improved rainfall prospects in Argentina and southern Brazil. The 2013 harvest of what is forecast to be a record large crop was under way in northern areas of Brazil. Corn and wheat futures also were trading lower at 8:40 a.m. Central Time.

Export sales activity of 100,000 tonnes or more of a single commodity to one destination (20,000 tonnes for soybean oil) must be reported to the U.S.D.A. by 3:00 p.m. Eastern Time the next business day. Smaller quantities of grains and soy complex products are reported in the U.S.D.A.’s weekly export sales report normally released on Thursday. That report is delayed to Friday this week because of Monday’s holiday.

In the export sales report for the week ended Jan. 10, the U.S.D.A. said China bought 845,600 tonnes of U.S. soybeans for delivery in 2012-13 and an additional 180,000 tonnes for delivery in the 2013-14. Total export sales that week were a marketing-year high of 1,608,800 tonnes for 2012-13.

China, the largest importer of U.S. soybeans, cancelled earlier purchases of at least 840,000 tonnes, and possibly more designated as “unknown destinations,” of U.S. soybeans in December, according to the U.S.D.A.