THE SOVIETS' tremendous purchases of American grain are evidence of their determination, despite their own poor harvest, to keep improving their diet. That grain will be fed to livestock and poultry for meat, eggs and milk to raise the Russians' intake of animal protein. That's why grain requirements keep rising rapidly even in countries with slow population growth. The Soviets' increased wealth, incidentally, is reflected in their ability to pursue higher nutritional standards in bad years as well as good. In a reversal of the traditional position, it's currently the United States that is under pressure to earn foreign exchange.

In the United States, with its ideal agricultural climates and its reliable rainfall, another huge harvest is being gathered. It's far more grain than Americans will need for the year's food supply, even with provision for normal exports. The question for the Carter administration was whether to increase exports or, instead, to use the surplus to build reserves.

American farmers oppose large reserves on grounds that these tend to hold down prices during shortages. That's quite true.Stabilizing prices and supplies is their purpose. They also hold prices up in times of surplus, but the farmers rarely focus on that side of the case. They don't like large reserves and get very insistent on that point in election years. An election year is coming, and the Carter administration has prudently decided that it's a year for big exports and not for big reserves.

The administration is doubtless right in saying that this sale to the Russians won't add to inflation in the months ahead. But without the additional exports, grain prices might have fallen. The producers' price index, published yesterday, showed another dismaying increase in September. Should the administration have refused the Russians' offer in a deliberate effort to drive down prices? No; it's hard on the farmers and disruptive to American trade. But the country's failure -- and the world's -- to build adequate reserves over the years is certainly a continuing cause of inflation.

The hugely increased exports to Russia over the coming year will please American farmers. They will help the U.S. balance of trade. They will improve the fare on Russian dinner tables. And they will mean that, for another year, the world has chosen to consume its harvest and gamble on more good weather next summer.