The health of science and engineering in the United States is threatened by a shrinking percentage of young Americans going into the fields and by tightening federal budgets that impede efforts to reverse the trend, the director of the National Science Foundation says.

Speaking last week before a joint meeting in Arlington of the American Physical Society and the American Association of Physics Teachers, Erich Bloch said the two trends boded ill for efforts to rebuild American competitiveness in the world marketplace. The NSF is the chief source of federal funding for nonmedical and nonmilitary research.

Making the prospect darker, Bloch said, is the fact that the number of college-age people in the United States peaked in 1983 and is expected to decline for at least several decades. If the country is to maintain the current size of its scientific and engineering communities, the proportion of young people entering the fields must be increased.

"Fewer young people increases the importance of attracting women and minorities to science and engineering," Bloch said. Despite recent increases in the number of women in graduate programs, the share appears to have leveled out at less than 15 percent of all technical degrees. Blacks and Hispanics account for about 20 percent of the population but take less than 2 percent of the doctorates in, for example, physics.

"The large numbers of minorities and women in the country provide an opportunity -- one we cannot afford to miss," Bloch told the physicists and physics teachers. "Our motive should be pragmatism as much as altruism -- the need to develop all our human resources."

An irony of the current situation is that the participation of Americans in science and engineering has been declining in the face of growing demand. The demand has been met almost entirely by foreign students, who now earn more than one-fifth of the science doctorates, one-third of the mathematics doctorates and more than half of the engineering doctorates awarded each year by U.S. universities.

Although many stay in the country, contributing to the American economy and scientific prowess, Bloch warned that this situation could easily change. "It is bad policy to depend on a resource we cannot control," he said.

Although the number of scientists and engineers has been growing, the proportion of the labor force working in research and development is about the same as it was 20 years ago, NSF figures show. In the same period, the chief U.S. overseas competitors -- Japan, West Germany, France and the United Kingdom -- have increased that proportion rapidly.

In 1965, for example, each of the four had only about one-third as many scientists and engineers in proportion to their population as did the United States. By 1984, Japan had nearly matched the United States and the others had grown to roughly two-thirds the U.S. level.

Bloch said the NSF's ability to encourage more young Americans to enter science and engineering has been weakened by federal budget-cutting. He said the NSF appropriation for 1988 increased by less than 6 percent over 1987, slightly more than enough to keep even with inflation. The share going for research, however, was increased by less than 4 percent, a cut in constant dollars.

"The outcome of the budget process this year was very disappointing, especially in view of the high hopes generated by the administration's commitment {in President Reagan's 1987 State of the Union speech} to doubling the NSF budget" in five years, Bloch said.

Still, Bloch said, the NSF hopes to maintain the high priority of programs to encourage more young people to enter the field.

Bloch said expansion of such programs, directed at the college level, will be slower than planned. There will also be an effort to begin new programs for secondary schools.

Bloch said the budget-cutting showed that the public and Congress do not agree "that the investment in science and engineering research and education is central to the nation's economic future. And that investment must take precedence over more immediate political concerns."