America's ability to feed itself is increasingly jeopardized by wasteful energy use, poor soil-tillage practices and a growing concentration of agricultural economic power, the authors of a new study on food supplies reported yesterday.

"The U.S. food system, like a rubber band, is being stretched to the extreme," according to a two-year study of farming and marketing practices conducted by the Cornucopia Project, sponsored by Rodale Press of Emmaus, Pa.

"Our food system maintains the illusion of unlimited success. It still feeds us. But the foundation of the system is being eaten away--slowly, quietly, yet surely," the Cornucopia report said. "The biggest danger is that the situation may become critical before it even seems serious."

Medard Gabel, director of the two-year project, said the picture of the food production future is "distressing, almost horrible." But, he said, changes in farming practices, federal and state actions to encourage conservation and regional food sufficiency, and encouragement of local production for local consumption could help reverse the trend.

These were some of the Cornucopia findings:

American farmers are in serious economic difficulty, with debt now more than $160 billion; last year, net income plummeted 29 percent while fuel, feed and seed costs were rising sharply. The trend has continued through 1981.

As large farms grow larger, consolidation also is growing in the food industry. One percent of the farmland owners control 30 percent of the farmland, 49 firms account for 68 percent of all food processing, and 44 companies control three-fourths of all wholesale and retail revenues.

Regional food self-sufficiency, once a reality, virtually has vanished with the intensified economic concentration. Most of the Northeast, for example, imports more than 70 percent of its food from other states, which intensifies the area's dependency and discourages local farming development.

Cornucopia noted, for example, that the New York area last year bought about 24,000 tons of broccoli, moved mostly from the West Coast at a cost of $6 million. That broccoli, as well as many other vegetables, could have been grown locally. For every $2 spent to grow food in the United States, another $1 is spent to move it.

Major trucking or rail strikes, an oil embargo or sabotage (much of the East's food is shipped over four Mississippi River bridges) could create severe supply problems, the report said. Americans spent more than $16 billion--much of it for energy--just to move their food around last year.

Continuous planting of row crops such as corn and soybeans, along with intensive chemical farming, have put new stresses on land and water resources. Erosion, the loss of prime agricultural land to development and overuse of water for irrigation severely threaten future productivity. Intensive planting for government-promoted export sales (more than $40 billion this year) puts additional strains on the land.

Robert Rodale, head of the health and food publishing and research organization, warned that the shaky state of the farm economy, with commodity prices low, operating expenses steadily increasing and farmers' debts soaring, poses a major and immediate threat to American food security.

The report said that "the only thing standing between many farmers and bankruptcy is the fact that their land is inflating in value, and every year it provides increased collateral for loans. If the inflation bubble bursts, both farmers and rural banks will be threatened."

Cornucopia recommended that farmers diversify their crops to save energy and avoid wild economic swings, reduce their dependence on chemical fertilizers and pesticides and develop new marketing channels. It suggested an end to federal subsidies that promote overuse of land and accumulation of debt.