IF A VISITOR asked to see the sources of American productivity, you might usefully take the Beltway to Fairfax County and turn west on Route 236. There your visitor would find an unusually clear illustration of a process that is going on, undramatically, throughout the country. You would pass the Annandale campus of Northern Virginia Community College, then Woodson High School and, if you turned left in Fairfax, George Mason University.

As the national debate over taxes and budgets goes bellowing along, you are going to hear much more about the need for tax breaks to encourage investment and raise economic productivity. There's something to that argument. Investment is essential. But it's not the largest source of gains in productivity. For the largest source, look to Route 236.

Woodson High School, with 2,300 students, runs a vigorous program in vocational training--in electronics, construction crafts, business and clerical skills--and most of those youngsters will be not only employable but employed when they graduate. But four out of five of Woodson's graduates will go on to further education.

Although some may go two miles to the east to the community college, most of the students there are older. Typically, they are people who have jobs and are going back to school to qualify for better ones. There are now 13,000 students enrolled at that campus, where classes are in session 12 months a year, seven days a week, from 6:30 a.m. until nearly 11 p.m. The data-processing courses fill up as fast as the college can expand them, teaching people by the hundreds to use this generation's most powerful new tool.

When George Mason University moved to its Fairfax campus 18 years ago, it had 365 students. Currently it has 14,000. Originally a branch of the University of Virginia, it has inherited some of its parent's strength in the arts and sciences. But enrollments are heaviest at George Mason in business administration, followed by nursing and education.

It's worth noting that, although some federal money goes into these institutions, most of the support comes from the state or, in the case of Woodson High, the county. The debate over public finance and economic growth rarely goes beyond the federal budget. But the ability of state and local governments to maintain strong school and university systems is crucial.

It's all very well for Congress to enact federal tax incentives for business investment. But a great deal depends on the ability of business to exploit those incentives effectively. Productivity depends not only on new equipment, but on the skill of the people running it. Those are the concerns--although not the only concerns--of that formidable array of classrooms along Route 236. While private investment in plant and equipment is important, it's not nearly as important as the public investment in the people who will manage it.