The secretary of the Army and the Army's top general disagreed publicly yesterday over the need to reintroduce a limited draft for the purpose of filling up the nation's depleted reserve forces.

Gen. Bernard W. Rogers, the Army chief of staff, had supported resumption of the draft in Senate Armed Services Committee testimony, and three hours later, Army Secretary Clifford L. Alexander Jr. said it wasn't needed.

"To enact a draft, I believe, would be unnecessary, unfair and counter-productive to the best interests of the Army. The official position of the Army is abundantly clear. Again, there is no necessity for the resumption of the draft," Alexander said in a hastily drafted statement rebutting Rogers.

The disagreement was an indication of the sharp controversy that has erupted over possible resumption of the draft.

During testimony before the Senate Armed Services Committee, all four of the service chiefs, including Rogers, supported the reintroduction of Selective Service registration.

However, Rogers diverged from the positions of his Air Force, Navy and Marine Corps associates in calling both for a limited draft and for registration of women.

Shortly before the joint chiefs testified on Capitol Hill, a group calling itself Students for a Libertarian Society held a news conference here to announce an all-out campaign against draft renewal.

SLS chairman Tom G. Palmer, claiming chapters and organizers at 150 colleges and high schools nationwide, said the issue is "freedom versus slavery," and attacked a bill in Congress that would set up compulsory civilian service as an alternative to military service.

In Hitlerhs Germany, he said, "If you couldn't serve the state with a rifle, you served it with a shovel."

David Landau, representing the American Civil Liberties Union, said, "It will be an ACLU priority to stop the draft in the next few months."

The draft plan outlined yesterday by Rogers would leave service in the active military on a voluntary basis, but would set up a numbered lottery to create a pool of 400,000 eligible for induction into the individual ready reserve.

Under the all-volunteer armed forces system, the manpower in the ready reserve has fallen to 255,000, far below the 820,000 the Army thinks would be necessary in wartime to fill the ranks of regular units engaged in battle.

To increase these numbers, Rogers called for Congress to enact a draft this session.

Under his plan, 75,000 to 100,000 men would be inducted each year for six months of active duty and training, after which they would be placed in the individual ready reserve pool.

Rogers said he did not favor a draft for women.

He said his plan would cost $590 million a year and save an estimated $160 million annually for the Army by reducing the costs of recruiting and varous bonuses and incentives now offered to persuade veterans of regular Army service to join the reserves.

Army officials said all the services would benefit because the existence of a draft would probably result in more enlistments in the Air Force and Navy.

However, at yesterday's Senate Armed Services Committee hearing only Rogers called for Congress to enact a draft program in this session. Marine Commandant Louis H. Wilson said he believed that a resumption of the draft would be necessary in the 1980s. Chief of Naval Operations Thomas B. Hayward and Air Force Chief of Staff Lew Allen Jr. said they were not in favor of draft resumption at this time, though Hayward said he thought eventual drafting was "likely." None of the other services has the Army's problems in maintaining a huge reserve pool.

However, Wilson told the committee that, "The trend is definitely downward as far as recruiting is concerned."

The services have had some trouble meeting their active-duty manpower needs in the present voluntary system. Since October, the Army has fallen 3,300 short of its recruiting target, and the other services also are behind their enlistment timetables.