Chicago crops succumbed to a late-acting "turnaround Tuesday" effect, as a slide in oil encouraged investors to take the opportunity to take profits.
New York crude for September slumped nearly 4% to stand at $71.612 a barrel at 17:30 GMT, a tumble traders blamed on technical factors after it failed to hold on to a 10-month high of $75.00 a barrel set earlier.
US shares also fell below early highs.
Chicago, meanwhile, suffered a repeat of the phenomenon which often, according to traders anyway, sees a strong price trend set on Monday reverse the next day.
This time, the adage would decree a sharp drop in prices, with soybeans looking particularly vulnerable following a 6% surge in the last session, fuelled by hopes for Chinese demand.
However, actual Chinese demand gave soybeans some support, with news of a further order for US beans, this time for 110,000 tonnes, taking purchases in a little over a week above 1m tonnes.
Rumours that Brazil – America's biggest soybean export rival – may be reduced to importing beans, having run dry of supplies, also helped support the market.
As did talk of a farmers' strike in Argentina, again a big soybean exporter when drought is not ravaging the country.
That help old-crop September beans, at least, make further ground, adding 8 cents to $10.99 a bushel. New crop November soybeans shed 11 cents to $9.96 ½ a bushel.
If that sounded disappointing, spare a thought for corn investors, who stood 9.5 cents, or 2.9%, to the bad if long of the September contract. That meant losing all Monday's gains, and more.
The December lot was doing even worse, down 10.5 cents, or 3.1%, at $3.25 a bushel.
Corn has weaker fundamentals stacked against it, besides the disappointment of falling oil, a major factor for a crop of which a big slice is used to make biofuels.
Furthermore, latest US crop data showed a marked improvement in the condition of the US crop, holding out the potential for bumper yields.
Wheat did better, shedding 1.75 cents to $4.780 a bushel for the September contract and 2.25 cents to $4.97 for December.
Technical factors were credited for some of its resilience.
The crop has held steadfastly above last week's 2009 low, tempting some short investors to ponder whether the crop's downside potential is as strong as it might appear from the fundamentals of a bumper global supply.
Indeed, London wheat crossed back above £100 a tonne, in what appeared an indication that farmers were unwilling to sell for less.