ON THE HEELS of a visit by South Africa's foreign minister, the Reagan administration has formally announced a policy of "constructive engagement" with the state that the last administration held at arms's length for its attachment to legalized racism, or apartheid. The new policy sounds good. There is in fact "a limit on the U.S. capacity to use negative pressure" to promote racial change in South Africa, as a State Department official said, and this would be so even if there were great political support for "negative pressure," which there is not. There are stirrings of positive change among the ruling white minority, and it does make sense for this country to see what it can do to strengthen the reformers' hand. One could defend this approach on a theoretical level even if this administration did not place a higher premium than did the last on bringing a pantingly eager South Africa into an anti-communist security system.

The relevant question, however, is how the policy will work out on a practical level. The test case is plainly Namibia, which constituted a large part of the business transacted during the visit of Foreign Minister Roelof Botha last week. Will "constructive engagement" help bring South Africa's longtime colony of Namibia to independence promptly on a basis other nations can respect.

The current indications are at best murky. Even before Mr. Botha had arrived, President Reagan had offered Pretoria an important concession -- support for pre-election constitutional guarantees to protect white minority rights. These seemed worth adding to the mix, but the administration apparently neither asked nor received a South African concession for them. While Mr. Botha was here, moreover, he repudiated his government's former agreement to a military force of the United Nations -- the U.N. is hopelessly biased, in the South African view -- to police Namibia's transition to independence. That would leave South Africa's army as the policeman, clearly an impossible situation. The State Department replied that a U.N. military force is still a "central element" in a solution. But do the South Africans feel that, by standing firm and talking up the possibility of a Soviet penetration on the backs of Namibia's SWAPO opposition, they can get the Reagan administration to go along?Not only did the president give Mr. Botha the tremendous boon of a White House meeting, but the White House described it as "friendly."

A fundamental point is often lost sight of in discussions of Namibia. Yes, the United Nations has been extremely one-sided in the long effort to pry South Africa's 60-year grip from the pre-World War I German colony. This has given South Africa the opportunity to play, profitably, to a Western sense of procedural fairness. The fact remains that South Africa's continuing colonial dominance in Namibia is illegal in international law and antithetical to the spirit of the age. "Constructive engagement" can be considered effective to the extent that it facilitates the early and genuine independence of Namibia. For the United States not to understand this is to make a hash of African policy and, not so incidentally, to hand Moscow a southern African gift on a platter.