SINCE PRESIDENT CARTER embargoed those Russian grain sales as an act of national policy, he reasons that the farmers ought not to bear the full cost alone. That's fair enough.The federal government will now proceed to stabilize grain prices for the farmers through the rest of the year, mainly by buying to build up reserves. It will be expensive but -- entirely apart from Russian transgressions -- building stocks is not a bad thing to do. Worldwide supplies have repeatedly been stretched too tight over the past decade, with a bad harvest anywhere promptly causing higher food prices everywhere.

Since farmers are no less willing to support national policy than anyone else, the Midwestern reaction may well turn out to be less hostile than Mr. Carter's various rivals seem to expect. But the farmers are going to remain uneasy about the consequences of this embargo for the longer term. They had been counting on the Soviet Union as a very big customer, if an erratic one, for the rest of the century.They are also aware that large reserves of grain tend to dampen future price increases -- a reason why governments like them better than farmers do.

The other part of the administration's plan -- the gasohol strategy -- is an attempt to answer those anxieties about farm prices in the mid-1980s.It will be perhaps two years before the gasohol project begins to have much effect on grain markets. There is very little excess capacity today in the distilling industry, and expanding production first means building new plants.

The gasohol strategy is, on the whole, a rather questionable idea. It is popular in the Corn Belt for precisely the same reason that the country as a whole needs to be cautious with it. The Midwest sees it, accurately, as a device to sustain and raise corn prices -- which is to say, the prices of bacon, hamburger and eggs. At the relatively modest volumes now under discussion, gasohol production will probably not affect food costs significantly. But it is difficult to foresee exactly the point at which it will begin to have an impact at the grocery store. As for gasohol's contribution to the gasoline shortage, that's hardly any simpler to calculate. Growing the corn, and distilling it, can easily take more energy than the alcohol provides. It depends on the circumstances under which the corn is grown and distilled. Gasohol is worth experimentation on a limited scale, but it's far from a universal remedy.

As a political gesture, this embargo of grain sales was necessary. To refrain from it would have left an unwholesome impression that Americans were unwilling to respond to Soviet misbehavior in any manner that made any material difference to anyone. But it's important not to overestimate the importance of this embargo, either to the Soviets or to the American grain producers. It will not have its principal impact on Russian diets for a while. As for American grain exports, the future remains a promising one of rising worldwide demand in which the Soviet Union has been an important element -- but hardly a crucial one.