IN HIS TIME as agriculture secretary, Richard Lyng will take no action more important than the one he took this week -- on civil rights. You might think of that as peripheral to the Agriculture Department's purposes and role. Too many people have thought that in the past. But Mr. Lyng sent a memo to all assistant secretaries and agency heads warning them that the department's reputation on race is central -- that however well it does its other jobs, it will be judged first by its record on discrimination, and from now on, so will they.

The memo is extraordinary, a model. The large but highly decentralized Agriculture Department -- 100,000 employees, many in state and county offices across the country, run as much by local custom as by directives from on high -- has been accused with depressing frequency in recent years of racial discrimination, both in personnel practices and in the distribution of services. Its response has been ragged enough to produce a second tier of charges of attempted intimidation and retaliation against black accusers. Rep. Don Edwards, chairman of a House Judicary subcommittee, one of the few congressional panels to show any interest, had called on Mr. Lyng to seize hold of these issues. That is what the new secretary has done.

"I will not tolerate discrimination in any form," he said in the memo, "and I expect you to make equality of opportunity and respect for civil rights an integral part of all decisions and processes affecting your work force and programs." "All managers and supervisors" are to be educated as to their obligations: "I want civil rights clearly reflected in the performance decisions you make regarding their future employment with this department."

He warned the deputies that, if they fail on civil rights, they will "place the credibility and integrity of this department at risk. Errors . . . reflect upon all of us." He said they will place their own careers at risk as well. "Here is a word of caution," he concludes. "Do not take this matter lightly. I expect you to assume personal responsibility . . . to correct any program or . . . practice that results in inequitable treatment. Failure to do this will be viewed as a grievous weakness in management which, in my view, no other accomplishments can offset."

True, you don't turn around a department just by writing a memo. But surely that's how you start. Who can remember an affirmation of this kind from any Cabinet officer? Hats off to Mr. Lyng.