President Reagan's decision to continue draft registration was welcomed by the Pentagon. But it didn't lay to rest persistent questions about the quality and quantity of armed forces personnel. Will manpower -- generally seen as the key to US defense -- be adequate as the President seeks to ''rearm America?''
Pentagon officials find recent trends promising. All services have met their recruiting goals. Judging by intelligence and aptitude tests, the military is attracting more capable men and women, as well as more high school graduates. This is especially important since those who have finished high school are twice as apt to complete their tour of duty, which reduces training costs.
But there are troubling signs too. The number of minorities in the military remains at about twice the level found in the general population. Judging by the percentage of those with high school diplomas, white recruits in recent years have been less representative of the general population than have minorities.
Reserve forces are running nearly 250,000 short of their needs, according to the Pentagon, affecting the ability to mobilize rapidly should the need arise. The active services continue to lose tens of thousands of highly skilled personnel. Without these men and women, it is asked, how can the Navy increase its fleet by one-third or the Air Force beef up its strategic (nuclear) capabilities as the administration desires?
''We can't compete with private industry,'' a Pentagon spokesman acknowledges.
Officials concede that recent recruiting and retention trends are influenced by the recession and high rate of unemployment, especially among minorities and young people.
''In the next few years, things will turn around and prospects in the civilian sector will begin to outweigh the economic advantages of a military career,'' Assistant Secretary of Defense Lawrence J. Korb warned a Senate Armed Services subcommittee.
The administration wants to increase the 2 million-plus active-duty force by more than 200,000. But it will face a dwindling pool of young people from which to do so. Between now and 1990, the number of 18-year-olds will drop by 19 percent.
For the moment, the administration's answer to quantity and quality problems is more pay and better benefits. Military personnel have received pay raises totaling more than 30 percent in the past 15 months. This has helped recruiting and retention, but the longer-range outlook remains less clear.
''The general picture is more pessimistic than the 1981 figures indicate,'' says Northwestern University sociologist Charles Moskos, who has written numerous books and articles on military manpower. ''I think two to three years down the road we'll be hanging by our fingers with the All-Volunteer Force.''
The same perception has led many lawmakers to favor a return to conscription, which ended in 1973. All the NATO allies, points out Senate draft proponent Ernest F. Hollings (D) of South Carolina, have conscription except the US, Canada, and Britain.
Draft opponents suspect the Reagan decision to retain registration may be a first step toward reinstituting the draft.
''Registration is part and parcel of the draft,'' warned Warren Hoover, executive director of the National Interreligious Service Board for Conscientious Objectors. ''Even though the President says he is opposed to the draft, the decision gives encouragement to those in Congress who are pushing for the peacetime conscription of our youth.''
But, says Dr. Moskos, ''a bungled draft would leave us in even worse straits than the undesirable status quo.'' Moskos recommends what he calls ''a supply-side model of military manpower.''
Since the All-Volunteer Force was created, government college loans have increased fivefold and the GI Bill has been eliminated. Moskos would gradually begin tying student loans to a short term of civilian social service work, giving interim priority to those who enlist in military reserve units.
Eventually, he suggests, the GI Bill for service veterans should be reinstated and all education assistance tied to national service.
Moskos also says the military should move to a ''two track'' personnel and compensation system: ''Citizen soldiers'' would enlist for two years' active duty in low-skill combat and support jobs. They would receive less pay but be eligible for GI Bill benefits. ''Career soldiers,'' he suggests, would enlist for at least four years and receive significantly better pay, benefits, and reenlistment bonuses.