Chicago's near-term soybean contract slumped more than 6% in its dying hours hurt by technical factors, while the gloom over prospects kept other crops under pressure.
September soybeans stood 60.75 cents lower at $9.20 ¾ a bushel at 16:30 GMT, the lowest for a near-term contract since the end of March, and taking to 26% its decline over the last month.
The slump was not was totally reflective of the broader market. September contracts, which will disappear off trading boards later on Monday, have entered an expiry period which often sees huge volatility on low volumes as investors try to close out last positions.
Just 164 lots had been traded, compared with nearly 55,000 for the November contract.
Nonetheless, the broader market was hardly in a cheery mood on either side of the Atlantic, despite some potentially upbeat news.
US weekly export inspections were good for wheat and corn, and in line for soybeans.
Soybeans also had strong processing data to point to. The National Oilseed Processors' Association pegged the August crush at 112.6m bushels, nearly 2m bushels ahead of market forecasts, indicating robust use.
Soy oil stocks were 100m pounds lower than analyst had expected, at 2.62bn pounds.
And wheat had a forecast of rising imports by Indonesia. OK, the rise forecast by Indonesia's wheat flour producers was only of 5%, and to 4.5m tonnes, relatively small beer in global terms.
Still, in Paris, November wheat closed down E0.75 at E119.25 a tonne, after touching E119.00 earlier to match a three-year low. A rebound in the euro against the dollar did not help.
London wheat for November ended £0.50 lower at £91.75 a tonne.
In Chicago, amid some technical jiggery pokery as nearest-but-one contracts prepared to become near-term lots, December wheat slumped 2.8% to 4.54 ¼ a bushel.
December corn lost 3.75 cents to $3.16 a bushel, while November soybeans eased 3.25 cents to $8.99 ¾ a bushel.
Weather remained a depressant, with the prospect of further benign conditions raising expectations of huge corn and soybean crops.
Some traders also cited a mounting trade dispute with China, which started with tyres and may be spread to vehicles and poultry. The country is the biggest importer of US beans.