Within a few weeks, President Carter will order all federal agencies to operate their motor vehicles on gasohol when it is available at "reasonable prices," administration sources disclosed this week.

Carter's executive order could open up an immense new market for gasohol, a blend of 90 percent gasoline and 10 percent alcohol.

With a fleet of more than 415,000 sedans, station wagons, vans and trucks, the federal government is the nation's largest purchaser of motor fuel. The Pentagon, as the fuel purchasing agent for the armed services and 24 federal agencies, spends $210 million a year on unleaded gasoline, said a spokesman for the Defense Fuel Supply Center. In addition, federal employes spend an estimated $250 million on gasoline credit card purchases under auspices of the General Services Administration.

The intent of the order is to increase gasohol demand and nurture the growth of an infant industry -- distilling ethyl alcohol from fermented corn, other grains, sugar beets and other forms of "biomass."

It would be a boon to midwestern farmers who have lobbied Washington for greater federal support for gasohol. It would also amount to a subsidy for alcohol producers and alcohol blenders and distributors because the government would have to pay 6 to 7 cents a gallon more for gasohol than for unleaded gasoline. The chief beneficiaries would be Archer-Daniels-Midland Co. of Decatur, Ill., which produces 80 percent of U.S. alcohol, and Texaco, the leading gasohol marketer.

Carter was expected to sign it "very soon," one White House source said. It would lend force to the policy of promoting federal use of gasohol that the president adopted last January. So far, that policy has been to little avail, with only a few hundred federal vehicles being tested on gasohol.

On Jan. 11, Carter set out to build a national alcohol fuel capacity of 500 million gallons a year by the end of 1981. His intent was to reduce U.S. dependence on foreign oil.