* Corn prices weighed by easing U.S. weather concerns
* Wheat firms, but set to finish August down 9 pct
* Soybeans set for first monthly slide since March (Adds quotes, updates prices)
LONDON, Aug 31 (Reuters) - U.S. corn prices were holding around a nine-month low on Thursday and heading for their biggest monthly slide in nearly three years on abundant global supplies.
Wheat edged higher but was set to finish the month down nearly 9 percent, while soybeans were poised to post their first monthly drop since March.
The most active corn futures on the Chicago Board Of Trade was down 0.1 percent at $3.45 a bushel by 1017 GMT, equalling the prior session's nine-month low.
"The corn market, free of widespread weather scares, continues to see a seasonal decline," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia.
While weather had supported prices for much of the year, easing concerns means the U.S. 2017 harvest will be large, if not as big as the record 2016 crop. The global grain pipeline is still absorbing massive South American harvests.
As a result, corn is down more than 10 percent this month, the biggest monthly drop since September 2014.
Chicago wheat prices were slightly higher but remained on course to post a monthly decline as ample global supplies continue to depress the market.
The most active wheat futures were up 0.3 percent at $4.31 a bushel, having closed little changed on Wednesday.
December wheat futures on Euronext rose 0.75 euros or 0.5 percent to 160.50 euros a tonne.
Commerzbank said in a market note that quality concerns in Germany following persistent rains this summer could lead the EU's second largest producer to import high quality supplies from France.
"Growing demand for wheat from France is likely to lend support to the wheat price in Paris," Commerzbank said.
The most active Chicago soybean futures were unchanged at $9.33-1/4 a bushel on Thursday and were on track for a monthly loss of about 8 percent as favourable weather forecasts in the U.S. weigh on prices.
Dealers said a potential pick-up in demand from top buyer China would provide support for prices.
"Having spent most of Q2 2017 in negative territory, Chinese crush margins have been strengthening recently, and have turned positive once again," ING said in a market note.
"The strengthening in the margin should be a positive sign for soybean demand, where prices have been under significant pressure recently," ING said. (Additional reporting by Colin Packham; Editing by Subhranshu Sahu)