America's community colleges have "seriously diluted" their academic programs to keep enrollments and budgets growing, and now find it harder to get government support because of widespread uncertainty over what they accomplish, according to a recent Brookings Institution study.

Only 10 percent of the colleges' 4.3 million students graduate from two-year programs, the study reports, and far fewer transfer successfully to universities.

"The two-year colleges really don't have a soul of their own," said David W. Breneman, a senior fellow at Brookings, who headed the three-year study financed by the Ford and Carnegie foundations.

"In their haste and fervor for putting courses into prisons and storefronts, for adult education and remedial education . . . they have lost sight of the academic focus. With some exceptions they're trying to do everything for everybody, and there's a growing concern at the state level that they've created a kind of monster that just keeps growing."

"I'm afraid the community colleges have more and more come down to the people rather than bring the people up to college level," said Susan C. Nelson, a senior economist for the President's Council of Economic Advisers, co-author of the study.

Roger Yarrington, vice president for research of the American Association of Community and Junior Colleges, disputed this evaluation. "A lot of these generalizations are pretty hard to talk about when you're dealing with millions of students at about 1,000 institutions," Yarrington said.

"The major mission of community colleges now is continuing adult education. More than half of the students are part-time. More than half are employed. . . . To judge them on the basis of degrees completed and transfers to four-year colleges has become an irrelevancy."

In their 222-page report, the largest independent study ever made of community colleges, Breneman and Nelson conclude that:

The average cost to taxpayers of educating a student at two-year colleges is no lower than the cost for freshman and sophomores at four-year schools. The small classes and technical courses taught by professors at community colleges, they said, balance out the large lectures and sections taught by graduate students at many universities.

As expected, the two-year colleges enroll a much larger proportion of low-income students than do four-year colleges. But, according to Census Bureau data for 1978, 43 percent of the students at community colleges who get support from their parents come from families earning over $20,000. This undercuts the rationale for low tuition, Breneman and Nelson say. Instead, they recommend that tuition in most courses be raised to about one-third of teaching costs with financial aid for students who can't afford it.

State aid to community colleges should be tied to the cost of particular programs, with more going to health than general studies, for example, rather than being set at a flat amount per head.

Non-credit courses, such as belly dancing, poodle grooming and Christmas caroling, as well as more intellectual subjects, have proliferated. In general, these do not merit state support, they say, and should be financed by tuition or local government funds in the same way as recreation programs.

Remedial programs, set at high school or even junior high level, have greatly expanded as more poorly prepared students enroll. These ought to be free because of their social and economic value, they said, as long as many students do not acquire basic skills in high school.

In 1960, there were 315 public two-year colleges enrolling 392,000 students or 11 percent of those in higher education. By 1979, the report said, the number had grown to 926 schools with more than 4 million students, making up 35 percent of college enrollment. The community colleges spent $6.3 billion in 1979, most of it from state, local and federal sources, according to the report.