The Senate last night approved $486 million in immediate incentives to help stem the exodus of skilled personnel from the military. The Senate earlier sidelined a proposed 3.4 percent across-the-board pay increase for the armed forces.

Voting 46 to 41, the senators opted for a package of selective compensation increases, including higher housing and food allowances, more pay for flight and sea duty and a reenlistment bonus that had been proposed by the Armed Services Committee as an alternative to the across-the-board pay increase.

Then it approved the package as an amendment to a routine House-passed military manpower bill by a vote of 87 to 1. The dissenting vote was cast by Sen. Henry Bellmon (R-Okla.), ranking minority member of the Budget Committee and a consistent opponent of budget-busting proposals.

The measure, estimated to cost $3.2 billion over the next five years -- including $486 million this fiscal year -- now goes back to the House, which could approve the addition or send the bill to a joint House-Senate conference.

Although the draft was not a direct issue in the debate, Sen. William L. Armstrong (R-Colo.) called the pay increase proposal, which he cosponsored with Sen. Spark M. Matsunaga (D-Hawaii), "absolutely vital to preservation of the all-volunteer force."

The implication of draft va. volunteer Army was heightened by the fact that Sen. Sam Nunn (D-Ga.) a leading advocate of draft registration and critic of the all-volunteer Army, cosponsored the committee-backed alternative to the pay increase.

But Nunn denied suggestions that opponents of a pay increase were seeking to pave the way for the return of military conscription.

Nunn, who joined Sen. John Warner (R-Va.) in pushing the selective compensation increases, argued that this approach would be more effective than a uniform pay increase in retaining skilled people, which military leaders have said is their biggest problem within the armed forces.

Armstrong, insisting that his was the better approach, said he will try again -- next year, if not this year -- to get a straight up-or-down vote on his pay increase proposal, which he predicted will be approved.

That did not happen yesterday, just as it did not happen last year, largely because the measure's opponents gained the parliamentary upper hand. Armstrong tried an end run yesterday, seeking a to get a vote first on his proposal.

But Senate Majority Leader Robert C. Byrd (D-W. Va.) argued that Armstrong's gambit violated an earlier procedural agreement and Byrd prevailed, forcing Armstrong to fight his battle over a motion to table the committee-backed substitute. It was on this issue that the 46-to-41 vote occurred, with all Washington area senators voting for the substitute.

The 3.4 percent increase proposed by Armstrong would have been on top of the recently granted 7 percent cost-of-living pay raise for all government employes. It would have brought the pay increase for the military -- but not the Civil Service -- to the full 10.4 percent recommended by President Carter's pay advisers. This had been cut by Carter to 7 percent for cost reasons. It would have cost up to $636 million this fiscal year.

While Armstrong said Congress had an obligation to provide the full cost-of-living increase for the military, his opponents contended it would be unfair to other government workers and might prompt demands from them for comparable increases, thus adding to budget deficits.

Describing low pay as a principal cause of the military's loss of trained personnel, Armstrong said it has contributed to recruiting shortfalls for all the services, a decline in educational levels among enlistees, and reenlistment problems.

Inflation has eaten into military pay to the point where it now is 7 percent less in real dollars than it was in 1972 before the draft ended, Armstrong and his supporters argued. Armstrong said that a unionized janitor earns about as much today as a chief petty officer.

Nunn and Warner, however, countered the Defense Department brass among backers for their proposal, which includes a variable housing allowance pegged to local residential costs, increased mileage reimbursement for changes of assignment, higher sea and flight pay, increased subsistence payments, and reenlistment bonuses for those with 10 to 15 years of service.

The changes, if finally approved, would be retroactive to Jan. 1 of this year, as Armstrong's pay increase would have been.