U.S. soybean futures surged more than two per cent on Tuesday as dry weather in key production areas of the U.S. farm belt eroded crop conditions and threatened yield prospects.
But corn futures fell more than one per cent on weak cash markets as the harvest crept northward from the southern U.S., where some farmers produced record-large yields, to the heart of the Corn Belt.
The U.S. Department of Agriculture's ratings of both crops were expected to drop following recent dry weather and severe heat, but the soybean crop was much more vulnerable to the stressful weather as the corn crop was closer to maturity.
Analysts expect the percentage of the U.S. soybean crop rated good to excellent to fall four percentage points in a weekly USDA report due later in the day, while corn conditions were expected to drop three points.
Modest rainfall over the weekend missed some of the driest areas of the Midwest, and the weather was expected to be dry this week, forecasters said.
"Rains over the weekend were pretty good in the Missouri Valley but not so much elsewhere and that's what's driving soybeans higher today," said Sterling Smith, futures specialist for Citigroup.
Commodity Weather Group meteorologist Joel Widenor said the weekend rains were better than expected along the Iowa-Nebraska border but missed many of the driest Midwest soybean areas.
"About 40 per cent of the soybean belt remains severely dry and will continue to see yield losses mount as growth finishes up," he said.
November soybeans on the Chicago Board of Trade rose 29-1/4 cents, or 2.2 per cent, to $13.86-3/4 per bushel after coming to within a penny of last week's 11-month high of $14.09-1/2 a bushel (all figures US$).
Corn followed soybeans higher early but buying interest faded under pressure from a rising harvest and limited export demand amid competition from lower-cost suppliers such as Ukraine and Brazil.
December corn futures fell 6-3/4 cents, or 1.4 per cent, to $4.75-1/4 per bushel, their steepest drop in a week. Selling accelerated as prices fell below the 50-day moving average around $4.88-1/2 a bushel.
"Corn is weakening on some pretty big drops in basis in the cash market, especially in the central part of the country," said Mike Zuzolo, president of Global Commodity Analytics.
Cash corn basis bids around the Midwest have eroded in recent days. The bid at a closely-watched Decatur, Illinois, processor was the lowest in two months.
Wheat futures fell for a fifth consecutive session on spillover pressure from sinking corn and on weak export demand.
Actively traded December wheat fell 6-3/4 cents, or one per cent, to a 1-1/2-week low of $6.47-1/4 a bushel.
Commodity funds sold an estimated net 9,000 corn contracts and 2,000 wheat contracts but bought a net 9,000 soybean contracts, trade sources said.
-- Karl Plume reports for Reuters from Chicago. Additional reporting for Reuters by Sam Nelson.