Soybeans slumped 4% on Monday to their lowest for a month – and wheat set a fresh 2009 low - as fine weather and reports of bumper yields tempted sellers on a day when Chinese stock falls shattered financial market confidence.
Investors rushed to safe havens after Shanghai shares slumped 5.8%, their biggest fall in percentage terms this year, on fears over Beijing efforts to rein in bank lending.
Main European share indices closed at least 1.5% down, a fall matched and more by New York shares in early afternoon trading.
The dollar, meanwhile, gained 0.6% against foreign currencies, making exports such as crops more expensive, while oil tumbled 2.7% to $65.67 a barrel for New York September crude.
Agricultural commodities also proved unable to sidestep the bullet, with first reports from a Midwest crop tour organsued by ProFarmer showing big potential crops, and weather forecasts not currently presenting any threat.
"Crop scouts in Ohio are reporting big yield potential for both corn and beans despite some dry areas in the northern part of the state," Vic Lespinasse, marketwatcher at GrainAnalyst.com, said
"They report yield potential for both corn and beans is better now than a year ago at this time."
Soybean prices had to suffer the extra blow of disappointing US weekly export inspections data of 5.6m tonnes.
Traders have been questioning assumptions of a tight soybean market since data at the end of last week showed weak US crushings and unexpectedly large deliveries on Chicago's expiring August contract, taken as a sign that bean sellers were failing to find trade homes for their stocks.
September soybeans slumped to $9.77 a bushel - the lowest since mid-July - before recovering some ground to stand at 9.87 ¼ a bushel at 18:00 GMT, down 3.7% on the day.
The better-traded November contract stood 3.0% lower at $9.52 ½ a bushel, off a day low of $9.44 a bushel.
Corn was a bit better off, helped by a firmer weekly export inspections figure of 40.9m tonnes.
September corn was 2.7% lower at $3.10 ¾ a bushel, with the better-traded December contract down 2.9% at $3.18 ¼ a bushel.
Meanwhile, September wheat set a fresh year-low of $4.65 ¼ a bushel for a near-term contract before reviving somewhat to $4.70 a bushel, down 2.4% on the day.
December wheat fell below $5 a bushel for the first time in the contract's two-year history, standing down 2.2% at $4.98 a bushel.
European contracts fared modestly better, helped by the slide of sterling and the euro against the dollar. London milling wheat for November lost 1.0% to £96.75 a tonne, while its Paris milling equivalent shed 1.2% to E128.75 a tonne.
Softs got caught in the maelstrom too as funds drew in their horns.
New York cocoa for December slid 3.6% to $2,740 a tonne while its London equivalent - again shielded somewhat by currency moves - closed down 2.5% at £1,767 a tonne.
December arabica coffee beans were 2.7% down at 128.75 cents a pound in New York, with November robusta beans ending down 2.0% at $1,385 a tonne in London.
Sugar, however, staged some rebound, helped by continuing tight fundamentals for the crop.
New York raw sugar for October revived from 21.35 cents a pound to 21.74 cents a pound, down 1.1% on the day.
Its London white sugar counterpart ended down just $1 at $552.0 a tonne, after standing down $17.5 a tonne at one stage.