WHEN THE PRIME MINISTER of Jamaica, Edward Seaga, calls at the White House this week, he will confront Mr. Reagan with the question of foreign aid. Jamaica provides an excellent example of the case for aid -- and yet, at the same time, of the political limitations on the things that any one country can do.

Jamaica, with all the resources for a strong economy, has fallen into great distress. The island was run, until last fall, by a militantly left-wing government that flirted with Cuba and badly frightened private investors -- many of whom departed, taking their money with them. Agriculture was hurt by mismanaged land reform, and the island became heavily dependent on imported food.

The election last October turned into a revolt against the left and a triumph for Mr. Seaga -- a Carribbean supply-sider who stands approximately as far to the right as Mr. Reagan himself. But without legal foreign exchange, the island's commerce depends on black marketeering and the traffic in marijuana.

When Mr. Seaga is at the White House, he will probably want to talk about better acces to American markets for Jamaican exports. But above all, he will want to talk about aid to buy the food, the fertilizer and the raw materials to get the legitimate economy in operation again. The Carter adminstration was strongly sympthetic, and the day before it left office it signed the agreements that will bring American aid this year to $60 million. There's help coming from other countries as well -- from Britain and, in smaller amounts, from Canada and several of the Europeans.

Mr. Reagan will note that American aid goes further when it's pooled with other countries' contributions. But he will also see that there are very narrow limites to the ability of the United States to impose conditions on the ways in which this aid is to be used. Even friendly and well-intentioned governments like Mr. Seaga's come under fierce domestic pressure to use the money for current speding rather than for structural reform. That is why successive presidents have found the World Bank and the International Monetary Fund extremely useful. Both pool contributions from many countries. And both, with their international staffs, can set enforceable conditions for aid without threatening the infringement of national sovereignty or national pride.

Jamaica was getting help from the IMF under the previous government for a time, until the government lost control of spending and began violating the conditions on a massive scale. Now the new government has reopened discussions with the IMF, but it will take time to renegotiate the loans.

Jamaica's economic experiment will depend, in the end, on what the Jamaicans do for themselves. But as the Reagan administration will note, a fair trial of Mr. Seaga's policy requires, as it begins, quite a lot of foreign aid.