The richest man in America is determined to make America even richer.

Sam Walton, the chairman of Wal-Mart Stores Inc., plans to step up his Buy-American campaign next week, calling a major news conference to ask other retailers publicly to join his efforts to prove that American manufacturers can be as competitive as their increasingly popular foreign counterparts.

Walton, who last year was named the richest man in America by Forbes magazine, has been engaged for the last year in virtually a single-handed effort to reduce the growing trade deficit and increase jobs here at home.

Since last March, Wal-Mart, one of the nation's fastest-growing and most successful discount chains, has set out to prove that American manufacturers can make quality goods that will match or beat the prices of imported goods.

Among other things, Wal-Mart offered favorable terms -- including longer lead times and more advantageous financial agreements -- to domestic manufacturers to make it possible for them to compete with foreign firms whose labor costs are far less than companies in the United States.

In addition to helping reduce the deficit, the campaign also serves Wal-Mart, which is based in the rural areas in the Southeast and Midwest. Many of the manufacturers that have signed up with Wal-Mart are based in the Wal-Mart marketing area. Without the Wal-Mart contracts, some of these companies may have gone out of business, reducing business as well for the discount chain, which is second only to K mart Corp. in size.

Wal-Mart says it has been able to save some $200 million from being spent overseas by finding American manufacturers to do the same job. That savings, in turn, has translated into creating or retaining some 4,500 jobs, Wal-Mart contends.

But with the U.S. merchandise deficit totaling $148.5 billion for 1985, Walton realizes he cannot solve the problem alone. As a result, he now is trying to enlist other retailers, including the giants such as Sears, Roebuck & Co. and K mart to join his battle.

So far, only one has agreed: Venture Stores, a discount chain owned by May Department Stores Co.

Walton hopes to change all that next week, however, when he calls manufacturers and government officials together in Little Rock to explore ways to form a stronger partnership between American manufacturers and retailers.