WITH THEIR latest American grain deal, the Russians, as they might put it, are coming to the source. The United States, the world's largest producer, is very anxious to sell. The more interesting question is why the Russians, who expect a good crop of their own this year at least, were willing to accommodate the United States with a new five- year agreement that raises the minimum the Soviets are committed to purchase by 50 percent over the level of the last agreement.

No doubt the American restriction on grain sales to the Soviets over the past three years has been a matter of some concern. The Soviet Union, with its highly irregular agriculture, likes to have an open line to the most prolific and reliable of exporters. But generally speaking, it doesn't make much difference where a buyer chooses to buy. An embargo of one country by another, in a world in which dozens of countries buy and sell, doesn't usually affect supplies and prices very much. The Soviets have reasons that go well beyond food supply.

These grain purchases are being presented in Moscow as a gesture toward Mr. Reagan, one intended to have a positive impact on superpower political relations. They may also have a certain bearing on U.S. policy toward Western Europe. It has seemed outrageous to Europe that the Reagan administration should have tried to force it to call off the construction of the Siberian gas pipeline, and cut back its own trade with the Soviets, while Mr. Reagan kept trying to sell them more wheat. Along with the grain, Moscow is buying insurance against a renewed American campaign against European industrial exports eastward.

All of that, naturally, has little to do with the purpose of this negotiation on the American side. Farmers associate the falling exports and prices of the past couple of years with the restrictions on Soviet sales. They wanted a new agreement with the Russians, and Mr. Reagan has now delivered it.

Perhaps the real point of the whole episode, the semi-embargo three years ago and the renewed agreement now, is that the political structure of the United States is not well adapted to running embargoes. When the Soviets invaded Afghanistan, President Carter prohibited any grain sales beyond the minimum mandated by the earlier agreement--a step conveying outrage yet carrying no military risk. The farmers' vehement reaction was predictable. Having exploited that reaction and now having reinstituted the agreement on a larger scale than ever, Mr. Reagan will find it difficult to use trade as a political weapon in the future.