NEW YORK — While retaining strong credit ratings, the credit outlook of Archer Daniels Midland Co. has been revised to negative by Standard & Poor’s Ratings Services.

S.&P. on Aug. 2 affirmed the A long-term rating of ADM and the A-1 short-term corporate credit rating. S.&P. said a company with an A rating is one with “strong capacity to meet financial commitments but somewhat susceptible to adverse economic conditions and changes in circumstances.”

Outside conditions were the catalyst of the outlook change, S.&P. said.

“We believe ADM’s earnings and credit measures may weaken over the next year, given the current drought across the majority of the U.S. farm belt,” said Chris Johnson, an analyst with the ratings agency. ”The drought could also result in lower volumes for the company’s ethanol facilities. And the company’s grain handling volume may suffer, too.”

S.&P. said it would consider revising its outlook from negative to stable if ADM’s financial performance improved to a point the ratio of debt to EBITDA would be sustained at 2.5 times, and funds from operations to total debt would be greater than 30%.

“We would also consider revising the outlook to stable if the company demonstrates a more conservative financial policy, including curtailed share repurchase activity during weak earnings and/or cash flow cycles, and working capital needs are financed with short-term facilities in order to sustain these credit measures given our R.M.I. (readily marketable grain inventories) adjustment.