Idaho potato king J. R. Simplot and two of his diversified companies were charged by the Justice Department yesterday with failing to report more than $1 million on federal income tax returns.

Attorney General Griffin Bell said an eight-count criminal information was filed in U.S. District Court in Boise, Idaho, against Simplot the J. R. Simplot Co. and Simplot Industries, Inc.

The court documents indicate that Simplot has agreed to enter a plea of "no contest" to the charges of filing a false personal income tax return for 1970 and assisting the Simplot company in filing false returns for 1970, 1971 and 1972. Simplot Industries was charged with filing a false return for 1970.

"Simplot failed to report more than $100,000 in income, the Simplot Company failed to report more than $450,000 and Simplot Industries failed to report $500,000," the government alleged. No estimate of the amount of taxes and penalties owing on the income was available.

Simplot's attorney, Jess Hawley of Boise, said neither he nor his client would comment on the case or on the statements issued by Bell in Washington and by the Acting U.S.

The court documents indicate that if Simplot enters a no contest plea, the Justice Department will ask the court to dismiss the indictment.

Simplot, who presides over diverse industrial holdings including a chain of potato processing plants, fertilizer plants, sawmills, cattle feedlots and mining companies, was a principle in last year's default on Maine potato futures contracts traded on the New york Mercantile Exchange.

He reportedly divested himself of his sizeable positions during the last contract, which his partner in Simtag Farms, P. J. Taggares of Othello, Wash., did not.

Many in the potato industry had charged that Simplot had attempted to force a default in order to halt futures trading of potatoes. They alleged that Siimplot and Taggares control such substantial potato growing and processing holdings that it was not to their advantage to allow others to "hedge" their commercial potato ventures on an exchange.