The Army has drafted a plan to solve its recruitment problems by shortening the minimum enlistment time from three years to two, and offering a better GI Bill for those who sign up.

The plan, just approved by Army leaders and now on the way to Defense Secretary Harold Brown and president Carter, would start in January in response to a congressional directive to test a new market.

The idea is to attract young men willing to sign up for two years, but no longer, in exchange for college money.

The plan is being advanced at a time when the Army is having a hard time filling the ranks of its combat units - artillery, armor and infantry - and is signing up a disproportionate percentage of blacks.

Robert L. Nelson, assistant secretary of the Army for manpower and reserve affairs, said yesterday that the new two-year enlistment plan is designed to get more young men into the combat arms and the under-strength reserves. The racial mix, he said, was not a consideration.

A young man who completed the two years' active duty, Nelson said, would remain in reserve status for four years but would not have to participate in drills.

"This gets us into a new market" where the Army can offer "a mini-GI Bill" to young men who feel a break of three or four years between high school and college is too long, Nelson said.

He termed the program "a test to identify the degree people are attracted to this option."

Gen. Bernard A. Rogers, Army chief of staff, said in a separate interview that lowering the minimum enlistment to two years would fit into the desire of commanders to shorten the tours of American GIs in Europe.

Both Gen. Alexander M. Haig, North Atlantic Treaty Organization commander, and Gen. George Blanchard, commander of U.S. Army forces in Europe, want an 18-month tour for combat troops in Germany rather than the current three and four years, Rogers said.

Duty in Germany, even along the NATO front, become tedious after a while, making it hard to keep troops motivated, Army officials said.

They added that Germany is so expansive for GIs, especially those who live there with wives and children, that three-to-four-year tours often turn out to be devastating economically.

The Army's two-year enlistment plan would enable young men - but not women - signing up for the combat arms to put aside up to $7,400 for college. The Army would add $2,000 to the $3,600 the Veterans Administration will already contribute to GIs who allot $75 of their monthly pay to future education.

Army leaders must in approval of their plan from a White House pledged to reduce any unnecessary government spending in the president's fight against inflation. Under the plan the Army has drafted, between 11,000 and 12,000 volunteers would be signed up for two years in 1979.

Although attracting more whites into the enlisted ranks was not a stated objective of this recruitment initiative directed by the House and Senate Armed Service committees in the closing days of the last Congress, Army officials said a changed racial mix could be one result of recruiting more college-bound men.

The Army, contradicting the predictions which preceded the switch to the all-volunteer military in 1973, is becoming progressively blacker. Back in 1970, the presidential commission headed by former defense secretary Thomas Gates predicted that the racial composition of the armed forces "will not fundamentally change by ending conscription."

The Army in September signed up the highest percentage of black males, 40.7 percent, since going all-volunteer. Preliminary figures indicate the October ratio of black and white male volunteers will be about the same, 36.7 percent black and 59 percent white.

Army officials said they are not disturbed that a disproportionate share of Army duty is being borne by blacks, asserting that blacks see Army service as a way up. Military recruiters target a national manpower pool of young men of which blacks comprise 13 percent.