CHICAGO, Sept 30 (Reuters) - U.S. soybean and corn supplies were smaller than expected as China stepped up its purchases of U.S. supplies during the summer, the government said on Wednesday.

Soybean stocks as of Sept. 1 stood at 523 million bushels, according to the U.S. Agriculture Department's quarterly stocks report. Corn stocks were 1.995 billion bushels and wheat stocks were 2.159 billion bushels.

Analysts had been expecting corn stocks of 2.250 billion bushels, wheat stocks of 2.240 billion bushels and soybean stocks of 576 million bushels, according to the average of estimates in a Reuters poll. (Reporting by Mark Weinraub)