THE PROSPECT of a trade war in farm products is now pretty good. As in other kinds of war, the costs would be high and the outcome uncertain. In trade wars, the costs are, mercifully, only money but the effects on political and military alliances are not necessarily trivial. As with the other kinds of wars, there is every good reason to avoid this one -- and it is dismaying that there's so little inclination among the economic diplomats to find a way around it.

On most of the other topics, the recent international trade talks in Geneva a week ago ended fairly well, at least in the sense that they could have ended much more badly. The chief effect was to demonstrate how far governments' interest in wider trade, as a force for economic growth, has diminished. But of all the various quarrels gathered together there, it was remarkable that tempers seemed to rise most rapidly and positions become most rigid when the subjects were corn, oranges, cheese and so forth.

Why such a depth of passion over agriculture? The conventional explanation is that in the politics of all the rich countries, the farmers are well organized to defend themselves. That's not wrong, but there's more to it. Most countries have had enough experience with food shortages, embargoes and war that they are determined to preserve food production at a certain reassuring level, regardless of economic cost. Beyond that, farm life everywhere stands for certain traditional social values, and it's not only the conservative politicians who respond.

That builds into countries a tendency to over- production, and it has been aggravated by extraordinarily big harvests worldwide for the past two years. For American farmers, the very high exchange rate of the dollar has made it harder than ever to sell abroad. Both the United States and Europe subsidize their farmers heavily, but their systems are fundamentally different and leave each side convinced of the deep unfairness of the other.

Since there's no large, simple solution, the world is going to have to be content with a lot of small, messy ones. For the United States and Europe, the impending trade war in farm products means a competition in subsidizing exports. That's an expensive game, with no winners.