Washington's major indoor sport, the quadrennial guessing game about who'll be in and who'll be out after a presidential election, has been going full tilt for weeks in the halls of the Agriculture Department.

The No. 2 man, Deputy Secretary Richard E. Lyng, has announced that he will retire if President Reagan is reelected -- a decision that farm lobbyists almost unanimously lament because of their regard for Lyng's managerial talents.

Talk among Republicans, who assume that Reagan will remain, is that Lyng and Secretary John R. Block are promoting William G. Lesher, now the assistant secretary for economics, to succeed Lyng as deputy secretary.Lesher, for his part, for months has denied that he was looking for a job outside of government.

A bigger question is Block's future.

Speculation that he would leave was fueled by reports earlier this year that he had sold his local home. The souring of some of his farming investments back in Illinois and his hospitalization because of a recurrence of bleeding ulcers stirred the rumor pot even more.

But Block is well-liked on Capitol Hill even though Republicans and Democrats alike have been acutely critical of his handling of farm programs, which reached their highest cost in history under this administration. GOP sources say there has been talk at the White House level of moving Block to another spot, perhaps in trade promotion, in a second Reagan term. That version has Lyng staying on for a while to run the USDA while a secretary is found.

Incumbent Block, however, gives no hints of that. He has campaigned vigorously for the Reagan-Bush team, despite some warnings from farm-state pols that the secretary was a liability to their reelection. And lately, he has been insisting to Republican friends that he intends to be on the job in 1985 hammering out a new farm bill.

Insiders also say that Everett G. Rank, administrator of the Agricultural Stabilization and Conservation Service, which oversees farm-support programs, is on the way out. Rank has embarrassed the administration several times -- he had to reimburse Uncle Sam for misuse of a government car, and his farm obtained about $1 million worth of free surplus cotton from the payment-in-kind (PIK) program that he helped design and manage.

The department's inspector general cleared Rank of conflict of interest on the PIK program last year. But recent new charges of inside-dealing on PIK, which Rank has brushed aside as "political," have brought him under investigation by the IG again. BLACK FARMERS . . .

Nearly three years after the U.S. Commission on Civil Rights took the USDA to task for not putting enough emphasis on helping black farmers solve their problems, the Farmers Home Administration has named an administrator to deal with the situation.

The report warned that without special support from the federal government, through funding and training, the black farmer in America soon would become a vanishing species. At current rates, according to some forecasts, by the year 2000 black farmers will own very little of the land they farm.

In the spring of 1983, after prodding by commission Chairman Clarence M. Pendleton Jr., Block announced that he was creating a task force to look at the problem and come up with answers. The panel issued an interim report with some recommendations last fall.

Since then, however, the department has come under intense fire, from civil rights groups and members of Congress, over allegations of mismanagement of the USDA's equal-opportunity and civil-rights enforcement programs. Since 1981, FmHA figures show, the number of farm and housing loans to minorities has gone down steadily.

Last week, 12 days before the national election, FmHA administrator Charles W. Shuman appointed Rozier W. Crew, an FmHA employe since 1977, to oversee the agency's activities with traditional black agricultural schools and to coordinate projects offering training or management aid to black and minority farmers. Shuman also said the FmHA has contracted with North Carolina A&T University to stage a pilot program to train 150 minority farmers. HEDGING BETS? . . .

Political insight of the week, for whatever it might mean: About half the cars in the executive parking lot in front of the USDA's main building on the Mall bear Reagan-Bush bumper stickers. The other half don't.