Former agriculture secretary Earl L. Butz pleaded guilty in federal court here today to evading payment of $74,957 in 1978 income taxes, a sum owed mainly on unreported fees for speaking engagements.

During a 20-minute hearing before U.S. District Court Judge Jessie E. Eschbach, a subdued Butz acknowledged that he had "consciously violated the law," and in the process had "let down my friends, my family, my former students and my country -- all of whom I love."

Butz, 71, faces a maximum penalty of five years in prison and a $10,000 fine. Sentencing is expected within a month. U.S. Attorney David T. Ready said it was unlikely that the government would make a recommendation on sentencing.

Ready said a charge of evading a smaller, undisclosed amount of federal taxes in 1977 was dropped after But agreed to plead guilty to the "more significant" evasion in 1978.

Ready said he understood that Butz already has filed amended returns and paid the taxes owed for the two years. He noted that tax evasion also carries a mandatory fraud penalty of 50 percent of the amount owed plus interest.

The criminal court to which Butz pleaded guilty today stated that the former Cabinet officer's 1978 return reported income of $97,814 and taxes owed of $39,621. The actual sums should have been $245,928 taxable income and $113,678 in taxes owed, the government complaint charged.

Ready declined to disclose the amount of underpayment in 1977 on grounds that this became confidential information once the government dropped the charge for that year.

Butz told Judge Eschbach that he had waited till the last minute to file his 1978 tax return and then had trouble "getting the information together."

Butz said he "wasn't in a strong cash flow position" because of some debts, but added, "I could have borrowed the money. I didn't."

Instead, he told Eschbach, he put down an "arbitrary sum that was understated."

Ready said following today's hearing that almost all the money that was unreported came from speaking engagements or from Butz' corporate directorships.

Butz, flanked by two attorneys, spoke in a voice that was at times almost inaudible. He described the last 18 months as "a nightmare for me and Mrs. Butz."

"I recognize what I've done is wrong. I feel regretful. There is no justification for what has happened. I'm prepared to accept the punishment. It was an aberration of conduct."

Butz was secretary of agriculture under both Presidents Nixon and Ford, at a time when grain sales to the Soviet Union, rising food prices and tight global food supplies made the job unusually sensitive.

Butz emerged as an outspoken supporter of large efficient farmers and agricultural businesses, which enjoyed unprecedented prosperity during his tenure, thanks largely to strong foreign demand for American food.

However, Butz also stirred up controversy, and seemed to thrive upon it. His unabashed support for agribusiness won his friends, but also enemies in rural America who felt he was unconcerned about the problems of small and medium-sized farmers, and foes among consumer groups who felt that Butz cared little about the impact of rising food prices on the urban centers of the country.

Another controversy arose when Butz was quoted as telling an off-color joke about Pope Paul VI in 1974. In 1976, President Ford accepted his resignation after it was disclosed that Butz had made a slur against blacks.

In private life, Butz bounced back, maintaining an active speaking schedule, hosting a syndicated radio show and serving as a director of a number of companies.

He is the sixth Nixon Cabinet member to be charged with a felony. Two of these, Treasury Secretary John B. Connally and Commerce Secretary Maurice A. Stans, were acquitted. The others were Vice President Spiro T. Agnew and attorneys general John N. Mitchell and Richard G. Kleindienst.

Butz and his attorneys have been attempting to expedite the procedure under which Butz will be sentenced. In the short proceeding yesterday, Butz was read his rights by Eschbach and agreed to waive his right to have the charges aired before a grand jury. He assured the judge he was voluntarily pleading guilty. Eschbach made clear that he had only just seen the information filed against Butz and has given "no indication what the sentence will be."