WHEAT PRICES are climbing rapidly, an ominous sign for everybody's grocery budget. The Carter administration's economic strategists - not to mention all the people who go through the supermarket checkout lines - had been counting on relatively stable farm prices to help pull down the inflation rate over the next year. But things don't seem to be pointing in that direction.

Grain and soybean prices started to rise sharply several weeks ago. The effects will show up first in bread, dairy products, poultry and eggs. The effect on pork and beef prices will come later, after the end of the year.

The first and largest reason for this worldwide surge in grain prices is the familiar one. The Russians have had a bad crop and are buying, once again, on a vast scale. Much of their crop land is subarctic semi-desert, subjected to extremes of weather with which farmers on the fortunate North American continent almost never have to contend. The Russian shortfall has been compounded by mediocre harvests elsewhere, notably in Europe, and even an abundant American crop is not enough to offset all of these strains.

The central fact of the world grain markets is steadily rising demand, as people in every region raise their standards of living. That process can be seen not only in the rich countries, but in most of the poor ones as well. China, for example, is currently back in the market to buy grain.

This pressure of demand against the world's grain crop is not merely cyclical. It rises and falls with swings in the weather, but over the past decade it has been visibly increasing. The current climb in grain prices demonstrates, once again, the close parallel between the behavior of wheat and oil. In both cases, economic growth and better living standards have put great pressure on supplies and contributed powerfully to the inflation from which most countries are suffering. In both cases, the United States, long accustomed to treating its economic policy as a purely domestic matter, has been very slow to reconcile itself to the profound effects of international demand on prices at home.

There is only one way to try to stabilize grain prices, and that is to try to build reserves. But reserves are costly and, while everybody supports the principle, nobody wants to pay the bill. Here in the United States, the government has encouraged growers, successfully, to hold reserves on their farms. But the lesson of the past three weeks is that those reserves are not large enough. It's a useful lesson but, for the present, there's nothing to be done about it. A bad crop year is not the time to try to accumulate bigger reserves. Even in the best years, building reserves tends to raise prices and is inflationary. That's the dilemma of reserves, and neither the United States nor anyone else has found a good answer to it.