BEIJING, Oct 20 (Reuters) - China's soybean imports from Brazil fell 18% in September from a year earlier, customs data showed on Wednesday, as poor crush margins limited demand.

The world's top buyer of soybeans brought in 5.936 million tonnes of the oilseed from Brazil last month, down from 7.25 million in the corresponding period a year earlier, data from the General Administration of Customs showed.

Crushers stepped up purchases last year from top supplier Brazil as a fast recovering pig herd pushed up demand. But their buying has slowed in recent months, as falling hog prices hit margins.

China's hog margins remain in negative territory, despite a pick-up in the past week.

Soybean brought in by Chinese crushers is processed into feed ingredients for the livestock sector, and for use as cooking oil.

Imports from the United States stood at 169,439 tonnes, down from 1.17 million tonnes a year earlier, after hurricane Ida hit shipments by forcing the closure of some grains terminals on the U.S. Gulf Coast.

The supply woes prompted Chinese buyers to turn to costly beans from Brazil.

The impact could last for weeks during the peak U.S. export season, pushing Chinese buyers to buy more Brazilian beans, traders said, although U.S. cargoes are expected to pick up from November.

October's total soybean arrivals are expected to stay lower than last year, fanning concern over supply of soymeal in China, just as an unprecedented power crunch forced some crushing plants to shut or cut operations.

Inventories of soybean and soymeal in China have both fallen in the past week, says agriculture consultancy Myagric.com.

Crushers in Rizhao in the eastern province of Shandong now make 61 yuan ($9.54) for each tonne of soybeans processed, up from negative 650 yuan in June. ($1=6.3912 Chinese yuan renminbi) (Reporting by Hallie Gu and Shivani Singh; Editing by Himani Sarkar and Clarence Fernandez)