Corn and wheat futures were close to unchanged, with the short-covering that boosted prices on Monday keeping the pressure off the market despite bearish reports from the field.
The soybean harvest flooded country elevators and processors with supplies which pressured bids on the cash market and cast a negative tone on future prices.
"Soybeans caught downward pressure in overnight trading as the pace of the US harvest nearly caught up to the five-year average," Farm Futures analyst Bryce Knorr said in a note to clients.
The US soybean harvest was 70 percent complete, ahead of analysts' forecast of 64 percent, the US Agriculture Department said in its weekly crop progress and conditions report. That leaves farmers only 3 percentage points behind their five-year average.
At 10:21 a.m. CDT (1521 GMT), Chicago Board of Trade November soybean futures were down 4-3/4 cents at $9.76 a bushel. Soy futures held support at Monday's low of $9.75 a bushel.
The corn harvest, at 38 percent complete, remained 21 percentage points behind the typical pace. Analysts had been expecting the corn harvest to be 44 percent complete.
But the USDA assessed 66 percent of the US corn crop as being in good to excellent condition, which compared with analysts' forecast of 65 percent.
CBOT December corn was down 1/4 cent at $3.51 a bushel.
The USDA said 75 percent of US winter wheat was planted, ahead of analysts' forecast of 73 percent.
"US wheat planting is also going well while large wheat export supplies from the Black Sea region continue to be a burden," said Graydon Chong, senior commodity analyst with Rabobank. "Some background support is coming from fears of a large reduction in the Australian crop after poor weather."
CBOT December soft red winter wheat was unchanged at $4.36-3/4 a bushel.