Foreign ministers of the nine Common Market countries adopted a code of conduct today that calls on European businesses operating in South Africa to be even-handed in their treatment of non-white employees.

While not binding, the six-point code urges European companies to alleviate wage discrepancies between races and abolish segregation on the job and aims to encourage blacks to join trade unions.

The ministers shied away, however, from endorsing stiffer guidelines that would have included a provision barring Common Market companies from new investment in South Africa.

Concious of the charge that black African states would view the code as "another pious resolution," British Foreign Minister David Owen asserted that the European plan is "far more comprehensive" than a code drawn up recently by American companies to govern their activities in South Africa.

Owen said that Common Market countries agreed to taper off development aid to governments that flout human rights, citing Uganda as one example.

The EEC code of conduct stipulates that "The same pay scales should be applied to the same work," and that companies should insure adequate pay for black workers. "A minimum wage should exceed by at least 50 per cent" the level of subsistence for a worker and his family, it says.

Black African employees "should be free to form or to join a trade union" and EEC companies should inform the blacks that "consultations and collective bargaining" with freely elected unions" are part of company policy."

The code suggests that EEC firms should furnish adequate housing and leisure facilities for their employees and "do everything possible to abolish any practice of segregation."

It calls on EEC companies with subsidiaries in South Africa to produce annual reports detailing their efforts to subscribe to the code. The EEC governments will then review the companies' behavior.

The ministers will submit the code to the 24-nation Organization for Economic Cooperation and Development to gain full support from other Western nations.

Several ministers said a Common Market consensus on a code of conduct is vital to prevent companies from being aided or penalized by differing national views.Only Britain and the Netherlands had previously prescribed guidelines for their companies with branches in South Africa, and British businessman complained that they had lost out to European rivals whose governments did not condemn wage exploitation and aparthied.

British companies account for about 50 per cent of all foreign investment in South Africa, but West Germany, France and the Netherlands also have sizable interests. The United States holds about 20 per cent of the foreign investment.