The Army has embarked on a $20 million crash program to fill its ranks with volunteers.

The effort comes at a time when a growing number of lawmakers and some military commanders are contending that the all-volunteer concept has failed and that draft calls must resume.

Army Secretary Clifford L. Alexander, who opposes bringing back the draft, confirmed yesterday that he recently ordered an all-out effort to recruit high-quality young men and women.

Alexander and the Army's highest ranking officer, Gen. Bernard W. Rogers, are split on the draft issue. Rogers recently told Congress that he favors drafting people into the reserve force.

"I certainly do" oppose resuming conscription, Alexander repeated yesterday. "I think the evidence is awfully strong" that the all-volunteer Army is working "because of the high-quality people we are getting and what they are able to do" once they get in uniform.

He added that "I can't explain" why the Army's top civilian, himself, and its top soldier, rogers, have come to different conclusions on the need for at least limited conscription.

The first part of the $20 million crash program to attract more volunteers into the Army got underway in March, Army leaders said. Some other moves must await approval by Defense Secretary Harold Brown and congressional committees that oversee switching money from one military account to another.

About half the $20 million, under current plans, will go for more intensive advertising to entice young people into the Army. The extra $10 million would be on top of the $35 million the Army already has earmarked this fiscal year for recruiting ads.

Robert L. Nelson, assistant secretary of the Army for manpower, said in a telephone interview that his specialists had predicted the Army would end up 10,000 short of its goal for male volunteers this fiscal year.

He said an easy solution would have been to lower the Army's entrance requirements, which put heavy emphasis on having a high school diploma. But, said Nelson, this would have made the Army's manpower problems worse later.

Compared to the Army's current dropout rate of about 30 percent among recruits on their first tours of duty, Nelson said, the historic dropout rate for first-termers with only a 9th-grade education is about 50 percent.

"If you just throw in the towel by lowering your criteria" for acceptance into the Army, said Nelson, "You take the pressure off recruiters to go out and find high school graduates."

Although Nelson did not mention it, coming up short of recruits or lowering entrance standards would strengthen the argument that the all-volunteer concept has failed and conscription must return.

Nelson said the Army needs about 125,000 male volunteers in fiscal 1979, which ends Sept. 30. The most pessimistic predictions, he added, were that it would fall 21,000 short, while the most optimistic forecast, was a shortage of 7,000.

"It's tough on all the services this year," he said of the increasing difficulty of filling the ranks with highly qualified volunteers.

Besides spending more on recruiting ads, Nelson said, Army leaders have doubled the enrollment at schools for Army recruiters; decided to temporarily reassign former recruiters to areas where they did well, and requested a large force of full-time recruiters and aides-recent volunteers who go back to their old neighborhoods to talk up the Army.

While several of those executive actions already have been taken, Nelson said, others, like the request to reprogram money to finance them, are still being worked on.

"We would have been criticized," said Nelson, "if we didn't try these kings of things" before resorting to such a potentially self-defeating step as "backing off on standards."

In a related move to attract more volunteers, the Army recently lowered its entrance requirements for women, which had been higher than those for men.

One of Alexander's allies in the battle to keep the all-volunteer Army has been Rep. Les Aspin (D-Wis.), a frequent critic of the military.

Aspin said recently that "the all-volunteer military is actually meeting more of its monthly goals for recruits than the draft force of the 1960s."

From 1960 through 1969, he said, the draft failed to deliver the requested number of recruits 41 percent of the time, while the all-volunteer system has come up short "only 39 percent of the time" from its inception in 1973 through June 1978. CAPTION: Picture, CLIFFORD L. ALEXANDER . . . "high-quality people" are joining