THERE are roughly 20 million blacks in South Africa in a total population of some 30 million. Of the rest, 5 million are white and another 5 million are Indians and ''Coloreds.''
The 5 million whites own or control virtually all of the major natural resources of the country - the gold and diamond mines and the good farmland. Indians and Coloreds are active in service industries and retail trade. Blacks have traditionally provided cheap labor.
On Sept. 3, the whites attempted to win the Indian and Colored communities to their side politically by extending to them representation in a separate Parliament.
There is no political representation for blacks.
Defenders of the government say the groundwork has been laid for eventually bringing blacks into the system. But let us recognize this condition for what it is. The 5 million-member white minority is holding the 20 million-member black majority in a condition of mass servitude.
Even if the whites were in a majority, this would be a politically unstable condition. The blacks outnumber the whites by 4 to 1. Determined denial of political representation to the black majority has turned a normally unstable situation into a newly explosive one. There were two major riots during September in which some 200 blacks were killed. November brought a major black work stoppage.
December brought widespread demonstrations in the United States against the white South African government's treatment of its black majority and increased the political pressure on President Reagan. He responded on Tuesday of this week , saying that ''racial discrimination'' is ''an affront to the human conscience.'' We will be hearing more, not less, about race relations in South Africa and reactions in Washington.
History teaches that there is no end to the political and civil strain and tension when people of one race keep a substantial number of people of another race in a condition of political and economic inferiority.
White superiority and black servitude were supposedly ended in the United States by the 13th Amendment to the Constitution, enacted in 1865. It continued extralegally until the civil rights legislation passed during the Kennedy and Johnson administrations. Traces of black servitude still remain. Blacks are rapidly gaining political representation in government, but equality of educational and economic opportunity are still to be won and consolidated.
Northern Ireland is living proof of how long lasting are the effects of mass servitude. It is now over 300 years since England confiscated the property of Roman Catholics in Ulster and sent Protestants over from England and Scotland to colonize and rule the province. Civil and legal rights were restored to the Catholics late in the last century, but to this day in Ulster most of the land, the factories, the banks, and the big businesses are owned by Protestants. For the most part, Catholics continue to provide the cheap labor of Ulster, just as blacks do in South Africa and Arabs do in Israel.
There is one similarity in Ulster, Israel, and South Africa. In each case one ethnic group dominates the economy while the subordinate group provides the labor. There are differences in the stages of evolution. Ulster is a place where the old ''Protestant ascendancy'' is officially and politically banished but survives simply because most property is still in Protestant hands.
Israel is the newest example of mass inequality. Jews dominate, control the government, and enjoy superior rights of land tenure and access to water. Arabs do the menial tasks for inferior wages and are ''last hired and first fired.'' They have legal rights, but practice is steadily transferring land ownership from Arabs to Jews, especially in the Israeli-occupied West Bank.
South Africa is at the earliest stage, where the subordinated majority have virtually no rights. They are not even considered citizens. This condition will not endure, but the curing of it may, as Ulster proves, be a long time in coming.