U.S. grain and soybean futures fell Tuesday, pressured by a seasonal increase in corn and soy supplies as farmers harvest crops in the Midwest.

Profit-taking and technical factors also weighed on futures, as traders acknowledged that grain and soy processors will have to chew through the U.S. harvest before supplies tighten again.

Regardless of how tight supplies are projected to be next year, the pipeline of U.S. grain supplies will be full by the end of September, said Tregg Cronin, an analyst with brokerage Country Hedging in St. Paul, Minn.

The U.S. Department of Agriculture reported Monday that 26% of the nation's corn crop and 10% of the nation's soybeans have been harvested in the week ended Sept. 16, up 11 and 6 percentage points, respectively, from the previous weeklong period. That puts the corn harvest 17 percentage points ahead of the normal pace.

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The grain and soybean markets were pressured by speculative funds shedding risk exposure Tuesday.

"The funds were loaded to their eyeballs with long positions in grain commodities, so it's not a surprise to see them take some profits," Mr. Cronin said.

Soybean futures followed Monday's 4% drop with a 1.7% decline. Soybeans for November delivery slumped 29 cents to close at $16.40 a bushel at the Chicago Board of Trade, their lowest level in a month.

Soybean futures jumped to record intraday and closing highs on Sept. 4, as a searing drought battered crops in the Farm Belt.

Now, however, traders are weighing anecdotal reports of better-than-expected yields as soybean harvests expand north into the Dakotas and Minnesota, said Hugh Whalen, a commodity-risk consultant with agriculture-advisory firm MID-CO Commodities Inc. in Bloomington, Ill.

Soy futures also are under pressure from favorable weather outlooks in South America, where farmers are getting ready to plant soybeans and other crops in the coming weeks.