Soybeans slumped neaarly 8%, and wheat closed within an ace of a two-year low, as external markets, China worries and weather prospects gave bears a handful of negative dynamics to sink their teeth into.
Tuesday was a poor day in many markets. London shares closed down 1.8%, New York stocks were down roughly the same at 19:00 GMT. Oil was 2.5% lower at $68.20 a barrel for New York's October crude contract.
Traders gave a range of explanations for the sell-off, ranging from concerns over China's economic recovery to, in London, some poor UK economic data.
Whatever, the jitters proved significant enough to send the dollar jumping, reducing the allure on export markets of commodities denominated in the currency.
The greenback leaped 1% against the euro.
And investors seemed to have piled up reasons not to invest in food commodities, even without external markets coming into play.
Weather, as ever, was one. Meteorlogix forecast "generally favourable" conditions for filling out Midwest corn cobs and soybeans.
"The main reason for the big losses today in all the grains, especially corn and beans, is the forecast for continued very favourable weather the rest of this week and next week as well with no threat of any freezing temperatures," Vic Lespinasse, the GrainAnalyst.com analyst, said.
Others included the good crop conditions highlighted in Monday's weekly US official crop progress report, with three-quarters of spring wheat rated "good" or "excellent".
And many investors fretted over reports that China is to allow state-owned enterprises to walk away from loss-making commodity derivatives trades.
Caijing magazine reported that the Assets Supervision and Administration Commission, a regulator, told six foreign banks that state-backed investors reserved the right to default on contracts.
Meanwhile, Kuala Lumpur palm oil offered little support to western markets, closing down 2.8% at 2,303 ringgit a tonne amid disappointment at falling Malaysian export data.
In Chicago, soybeans were, once again, the worst affected, as might befit a crop which is heavily dependent on Chinese buying.
The old crop September contract ended down 7.8% at $10.12 ½ a bushel, taking its losses since Friday's high to 13%.
New crop November beans ended down 2.5% at $9.53 a bushel.
Corn for September lost 4.3% to $3.12 ¼ a bushel, with the December lot off 3.2% at $3.18 ¾ a bushel.
Wheat was marginally better off, slipping 2.5% to $4.56 ¾ a bushel for September and 2.3% to $4.85 ¾ a bushel for December.
Not that this may have been much consolation for farmers when it appeared to bring the September contract the lowest close for a near-term lot since April 2007. (Two sets of Reuters data were not agreed on this at the time of writing.)
Indeed, growers' reluctance to sell at low levels was cited as saving wheat from falling further than £2 to £98.00 a tonne in London.
"Limited selling pressure stopped the market going lower this afternoon," Hugh Schryver at Glencore said.
Paris milling wheat for November closed unchanged at E127.00 a tonne.