Wheat and corn were also on track for weekly losses.
The Chicago Board Of Trade's most-active soybean contract fell 0.3 percent to $9.39 a bushel by 1016 GMT, setting it up for a weekly loss of 0.7 percent in what would be the longest weekly losing run since September 2014.
"We are caught is a bearish trend, US soybean planting outlook looks good and South American production is so big," said Kaname Gokon at brokerage Okato Shoji in Tokyo.
"We don't see any recovery in prices unless there is a big weather issue in the United States."
Dealers were also keeping an eye on meetings between US President Donald Trump and Chinese counterpart Xi Jinping with China the biggest buyer of US soybeans.
"During his election campaign, and then again after taking office, Trump had repeatedly criticised China harshly for its high trade surplus with the US This is likely to be one of the issues discussed by the two heads of state," Commerzbank said in a market note.
Soybeans found some support earlier this week after rain threatened to disrupt the harvest in key supplier Argentina.
But soybean harvesting in the country has accelerated in the past week despite heavy rain and floods in the western portion of the country's growing area, the Buenos Aires Grains Exchange said on Thursday.
Brazil is shipping record volumes of soybeans to the international market, providing stiff competition to exporters of US products.
This week, consultancy Celeres raised its estimate of Brazil's soybean crop to 113.8 million tonnes from 109.65 million last month. Broker INTL FCStone lifted its projection for the Brazilian crop to 111.6 million tonnes from 109.07 million previously.
CBOT's most active wheat contract was down 0.2 percent at $4.22-1/2 a bushel, heading for a weekly loss of 0.9 percent.
CBOT corn was unchanged at $3.60-3/4 a bushel and was on course for a weekly decline of 1.0 percent.
"Over the past weeks, the market has witnessed increased fund selling in all three US markets, as rainfall brings much-needed moisture to the drought-stricken winter wheat areas," said David Sheppard, managing director of UK merchant Gleadell.
May milling wheat in Paris was down 0.50 euros or 0.3 percent at 163.25 euros a tonne.