Well, there is a secretary of agriculture. But after that, it is less clear about who's doing what at the Agriculture Department these days.

The Reagan administration's second term at the USDA is turning into a case of veterans leaving en masse and others trading jobs in the fashion of musical chairs. The high-level gaps are becoming more critical as Secretary John R. Block gears up to do battle with Congress over farm policy.

For example, the White House still formally has not nominated a No. 2 man for the USDA, although sources say Arizona farmer John R. Norton III is expected to succeed the departed Richard E. Lyng.

Nor is there a nominee for assistant secretary for economics to replace William G. Lesher, who went into the consulting business with Lyng.

The leading candidate is said to be Robert L. Thompson, an economist for the Council of Economic Advisers.

And to top it off, Randy Russell, deputy assistant secretary for economics who is running the show since Lesher left, is expected to become Block's executive assistant.

That will leave the department with an all-rookie team in its effort to persuade Congress to change farm programs radically. In the fight over a 1981 farm bill, Lyng, Lesher and Russell played major roles in the administration's lobbying efforts.

Block's current executive assistant, Raymond D. Lett, is expected to get the job of assistant secretary for marketing and inspection services, replacing C.W. McMillan, who resigned.

And farmer Peter C. Myers, who got high marks as head of the Soil Conservation Service, is still in line to become assistant secretary for natural resources and environment. The slot came open with the resignation of former timber executive John B. Crowell.

Another important job at USDA opened up last week with Mary C. Jarratt's announcement that she will leave in late spring as assistant secretary for food and consumer services.

In another switch, John Ford moved from deputy assistant secretary for marketing and inspection to deputy assistant secretary for congressional relations. The person who used to hold that job, Thomas O. Kay, moved to international affairs. IN THE HOT SEAT . . .

Maybe the most controversial job is that of chief of the Farmers Home Administration, which is under fire for its handling of farm and rural housing loans. The job went up for grabs when Charles Shuman left at the end of last month.

Insiders say the front-runner is John C. Musgrave, state FmHA director for West Virginia. If he gets the job, one of his duties would be to defend President Reagan's efforts to zero-out the $3.4 billion rural housing programs.

In fiscal 1984, FmHA in West Virginia spent only $41 million of the $62 million it had been allocated for rural housing programs. Mississippi, in contrast, was allocated $84 million but managed to spend $337 million.

Said Robert Rapoza of the Rural Housing Coalition, which lobbies for the housing funds: "Given what is left of the agency's budget, it's not a very attractive job. The question we have is whether Musgrave is being brought in to house people or wipe out the program."