Corn was also weakened by the brisk rate of US sowings while wheat fell again, under pressure from ample world inventories.

The Chicago Board Of Trade's most-active July soybean contract was down 0.4 percent at $9.67-1/2 a bushel at 1004 GMT. Soybeans touched a high of $9.72-1/2 on Monday, ending up 1 percent, partly as a weaker dollar raised hopes of new US export sales.

July corn fell 0.3 percent to $3.64-1/2 a bushel, having gained 0.5 percent on Monday. July wheat dropped 0.2 percent to $4.18-1/4 a bushel, after falling 0.4 percent on Monday.

The US Department of Agriculture said US soybean planting for the upcoming harvest was 6 percent complete, well ahead of forecasts of around 2 percent.

The USDA estimated corn planting at 17 percent complete, also exceeding expectations of about 15 percent.

"US farmers have made a faster start than some had expected to both soybean and corn plantings and the pace of sowing can be very rapid when farmers decide to get going," said Matt Ammermann, commodity risk manager at INTL FCStone. "But I think it is too early to get euphoric after 6 percent of soybeans have been planted and over 80 percent of corn still remains to be done."

"Some rain is expected in US planting regions and a lot of risk to plantings remains in the next couple of weeks. But the markets today do not seem to be focusing on the risk."

Kaname Gokon from brokerage Okato Shoji added: "Farmers are expected to continue planting at a strong pace. US Midwest conditions will remain in focus even as we have record South American supplies hitting the market."

Ample supplies in key export countries continue to depress wheat, which has fallen steadily since early March.

"Wheat remains depressed by the old story of big global supplies which has been dragging the markets down recently. US prices have not been reacting to the weaker dollar," Ammermann said.

"Weather in some important wheat producing areas is far from perfect so risk remains in wheat."

Meanwhile, there could be additional downward pressure looming for corn with improved weather for the country's winter corn crop.