Johannesburg — Add the corn kernel to the growing list of things South Africa would rather not discuss.
Food exports to the rest of Africa - precisely where they go and how much - have joined energy and defense among areas the South African government is disclosing less and less information about to the public.
It is not that there is bad news. A short drive from Johannesburg shows the prosperity of South African agriculture. There, combines are sweeping across the cornfields - this country's principal crop land - and unloading their harvest into an already overflowing national bin of maize.
Reluctance to spell out where corn and other exports are destined is linked to Pretoria's ''growing awareness,'' as one top government source put it, of the diplomatic leverage food gives this country over black African states on the receiving end. The gains South Africa makes by refusing to disclose this data is unclear, but the government evidently believes secrecy is worth the price.
Maize, South Africa's major grain export, is a staple of the African diet. Maize imports to sub-Saharan Africa are growing at nearly 6 percent annually.
Most of South Africa's maize export, which came near 5 million metric tons in the 1981-82 year, goes to Japan. Second place belongs to a ''destinations unknown'' category, and it is 30 percent of this country's corn exports. Sales to African states are lumped under that heading to avoid disclosing the names of the importers.
Despite drought this year and a smaller crop, South Africa's maize exports are expected to approach the record performance just turned in for the 1981-82 season. South Africa is among the world's six leading grain exporters, thanks to maize shipments.
The food surplus here is potentially important. A recent World Bank report points to declining food production and soaring imports in sub-Saharan Africa and says it is ''no exaggeration to talk of crisis'' in the region.
How much food is South Africa putting on Africa's plate? No one here feels it is a huge amount, but for the receiving countries it could be significant. Whatever it is, the government prefers to offer intriguing tidbits rather than fully satisfying answers.
It was reported in Parliament this year that trade in all goods between South Africa and the rest of the continent in 1981 was double the 1978 total. And the trade included 47 of the 51 countries that make up the Organization of African Unity. That body regularly criticized South Africa for its policy of apartheid (forced racial segregation) and supports an end to South Africa's white minority rule.
''There is more movement of food (from South Africa to other parts of Africa) than is generally suspected,'' says Prof. Jan Groenewald of the University of Pretoria. He figures 90 percent of the maize export contained in the ''unknown'' category is corn that goes to other African states.
Making deductions from the known needs of African states and the available supply from other exporters, Professor Groenewald reckons Botswana, Lesotho, Zambia, Zaire, Mozambique, and Kenya have bought South African maize in recent years. He ''suspects'' Tanzania, Ivory Coast, and Senegal have imported from South Africa as well.
A maize industry official concedes: ''We sell maize to people who have taken a political posture against us. If we publicly say who we are selling to, it puts them in a rather embarrassing position while we're keeping their populations alive.''
However, South Africa's motives are not principally humanitarian. A high government source says the strategic value of food is increasingly recognized in Pretoria. The prevailing view, he says, is not to use food ''aggressively as a weapon'' to extract short-term political concessions from African states.
''The real potential is more long term. If you have an ongoing program like this, it tends to undermine efforts to fragment the subcontinent,'' he says, referring to efforts of southern African states to reduce their economic dependence on South Africa.
Whether South Africa in the future will be able to play a larger role in helping feed the rest of Africa depends on a combination of economic and political factors. For the foreseeable future, experts rule out any dramatic increase in trade - given the anti-apartheid stance of black African states. Moreover, the African states lack the money to buy more grain on the commercial market.
Over the long term, there are questions about how long South Africa's large maize surplus will last. Maize farmers currently account for about half its total number of farmers. They help make up the conservative bedrock of the ruling National Party, and thus have strong political influence.
That influence, in the view of some informed analysts, is the main reason the maize-farming sector has expanded so far beyond the needs of the domestic population (roughly half the crop is exported).
The government's Maize Marketing Board establishes a local maize price, which consistently has ensured that maize farmers make a nice profit. The current local selling price is considered the highest of all the major corn-exporting countries in the world.
Given the current abundance of maize on the world market, South Africa is exporting its corn at a price considerably below what local farmers are guaranteed. The government, in effect, covers the losses incurred.
GRAIN EXPORTS 1981-82 (million metric tons)
Coarse grain Total rain (including maize) United States 61.8 110.2 Canada 6.8 23.8 European Community 4.8 18.3 Argentina 13.9 18.2 Australia 3.2 14.2 South Africa 4.9 4.9 Thailand 3.1 3.1 Source: Grain and Feed Division of the US Foreign Agricultural Service