Here amid shimmering acres of leafy green stalks in South Africa's rustic corn belt, American construction know-how has linked up with South African ingenuity to build this country's most important insurance policy against a future world oil embargo.

"Sasol Two" will be the world's largest plant producing commercial oil from coal when it is completed sometime next year at a cost of $2.8 billion.

The massive complex is designed to guarantee South Africa at least a minimum supply of gasoline and other strategic fuels from its abundant coalfields if international hostility to its racial discrimination ever result in ban on the flow of imported oil.

The strategic importance of the Sasol project to South Africa, which has no known domestic oil reserves, was underscored last month when, in reaction to Iran's decision to discontinue supplying South Africa with oil, government officials announced that construction of "Sasol Three," at an estimated cost of $3.8 billion, would begin as soon as Sasol Two is completed.

The Fluor Corp., based in Irvine, Calif., the builder of stations on the Alaska pipeline project, holds the management contract for Sasol Two. Officials of the giant construction company, which has also helped build two oil refineries in South Africa, refuse to say how much the contract is worth.

Predictably, Fluor was chosen by South Africa to construct Sasol Three, woth a completion date of 1982. It will be located at Secunda next to Sasol Two.

Under the hot sun of South Africa's highvelt , the Afrikaans name for this corn-growing plateau, construction of Sasol Two is at its peak. Among the 20,000 workers are 300 Amercians, mostly engineers. With their families, they live at the project's prefabricated town of Secunda, a two-hour drive from Johannesburg.

Necessity has been the midwife of these expensive projects. Without a drop of oil of its own, and threatened by repeated calls for an oil embargo by Third World countries in addition to increasing pressure from its Western allies to modify its political system of apartheid, South Africa has had to regard the cutoff of imported oil as a real possiblity.

South Africa began planning for the day when an international oil boycott might materialize as far back as the 1950s. The state-run South Africa coal, oil and gas corporation (known as Sasol) built a tiny pilot oil-from-coal plant-Sasol One-at Sasolburg in 1955. In 1974 it decided to construct the much larger Sasol Two.

"It was a very solid approach not often seen in governments around the world, and it took a certain amount of courage," said one Western expert on oil.

South Africa also begin to build oil reserves in 1966, pouring the precious commodity into abandoned coal mines east of Johannesburg and Pretoria on the country's industrial plateau, and in the Western Cape. Informed observers estimate that the reserves could meet South Africa's needs at present consumption for about 18 months to two years, possibly longer with strict rationing.

Sasol's executives admit that around 40 percent of he entire cost of the Sasol project, more than $6.6 billion, will be spent abroad. Thus, strategic though it may be, the project is still greatly dependent on Western technology, loans and equipment, making it vulnerable to anti-apartheid moves in the West to curb private investment and business ventures in South Africa.

The major problem for the South African government now, however, is how to finance Sasol Three. For the first time, the government has invited private financing for almost a third of the project's cost. The new plant will also be partly financed by recent increases in gasoline prices that brought the cost to nearly $1.85 a gallon.

Although the tremendous cost of the Sasol preject is undoubtedly straining South Africa's economy, government officials say that the hugh investment is a wise one in the face of inevitably rising oil prices.

Sasol directors are telling the public that the three Sasol plants will eventually be saving the country more than $1.3 billion a year in foreign oil bills, which last year amounted to an estimated $1.6 billion. South Africa expects to spend $2.3 billion on imported crude this year.

Since information about South Africa's oil situation is highly classified, it is impossible to know exactly how much of the country's oil-based fuel needs will be met by the three Sasol plants. Government officials have claimed, however, that the first two Sasol plants would meet 30 to 40 percent of the country's gasoline and diesel fuel needs at present consumption. Economic Affairs Minister Christiaan Heunis said recently that the addition of Sasol Three would push that figure to 47 percent.

The minister, however, did not speak of expanded needs. If South Africa is to avoid mass unemployment among its 18 billion black majority, a matter of great concern to the government, then the economic growth rate must expand far beyond the modest 2 to 3 percent it now sustains. That will mean increased fuel consumption.

Some examiners of South Africa's projected fuel needs, notably British economists Martin Bailey and Bernard Rivers, in a report for the U.N. Center Against Apartheid, concluded that Sasol One and Two are likely to provide only 13 percent of South Africa's 1980 projected gasoline and diesel fuel needs. The addition of Sasol Three, by this estimate would be likely to raise that portion only to between 26 and 30 percent.

Whatever the economics of the Sasol project, it is seen as a tribute to South Africans' ingenuity and initiative in the practical application of a technique that "is as old as coal itself," according to Sasol's public relations officer, Clarence Keyter. CAPTION: Picture, As South Africa builds a second coal-conversion plant, a third is projected. By Nico Vandenberg for The Washington Post