President Carter now reaps the reward of his African policies. He is the first American president to visit that continent, and he is getting the big hello in Nigeria, a country that only two years ago refused to receive Secretary of State Henry Kissinger.

But the pride paid for the African visit is not low. The U.S. is now over-committed to Africa in a way manifest in two trouble spots - Rhodesia and The Horn.

The cornerstone of African policy for the Carter administration has been support for blacks who are seeking independence from minority white rule. Associated with that goal has been an emphasis on human rights as a weapon against repressive regimes.

The obvious purpose was to align Washington with the black majorities that are eventually going to carry the day in Africa. The black connection, it was supposed, would block the Soviet tactic of playing on racial feelings to advance communist penetration of Africa. As a bonus, the Carter administration would profile itself favorably against previous Republican regimes, which did little to promote Africa "democracy."

A main test of the new policy has been Rhodesia. The Carter administration joined with Britain in moving for a quick and complete end to the minority white regime under Ian Smith. To that end the United States aligned itself against Smith with the Rhodesian liberation movements - both the moderates inside the country and the radicals on the outside - and with the neigboring black African states.

Smith used the foreign pressure to win a smashing electoral mandate from the white voters. He then put together with the moderate blacks inside Rhodesia a plan for slow and partial transition to majority rule.

Washington regards the Smith plan as a step forward. But the plan has been rejected as unacceptable by the radical Rhodesians and the neighboring black states. So the United States is hung up - half leaning toward a deal that moves toward democratic rule, half standing aloof for fear of affronting the radicals. Thus despite all of Carter's efforts, the United States remains suspect among blacks in southern Africa, and the Russians can easily outbid the Americans by all-out support for the radicals.

As to The Horn, the Carter administration began by cutting off, on human-rights grounds, all aid to the military junta, or dergue, that succeeded the regime of Haile Selassie. Under pressure, the dergue both increased local terror and began to turn to the Soviet Union for help against a secessionist movement in Ogaden province.

The Russians at first responded gingerly. There were loath to compromise their position in Somalia (where they had trained the army and held a major military base), which supported the secession movement in Ogaden.

But President Carter was apparently convinced he could push the Russians out of Somalia. He twice made public statements implying the United States was ready to replace the Russians as Big Daddy in Somalia. Last July, the United States approved (in principle) sending military supplies to Somalia. Thus encouraged, the Somalis undertook a fullscale invasion of the Ogaden.

The Russians thereupon moved weapons, officers and several thousand Cuban troops to Ethiopia. About a month ago the Russian-Cuban-Ethiopian forces broke the Somalis in Ogaden and forced a complete retreat.

The Somali leader, President Said Barre, with his army crushed, is now in desperate straits. He has called on the United States for help, but, in the crunch, Washington has been reluctant. Apart from sending some aid, the main U.S. action has been to warn the Russians that an invasion of Somalia from Ethiopia would prejudice Big Two relations, including arms talks.

By any account, the Russians are the big winners. They stand tall in Ethiopia and may yet take over Somalia. While that would not be the end of the world (and Moscow might gain more of a headache than an advantage), at least there is a lesson to be learned. In Africa, there are at work multifold and unfathombale rivalries - among nations, tribes, leaders and would-be leaders. With no great material interest at stake, the United States is far better off in a posture of general benevolence than mixing deep into conflicts sure to end in bad smashups.