The president's decision to keep the Soviet grain embargo in place for a while gives him, the bureaucracy and other politicians time to come to grips with the unnoted fact that events have handed the United States a major strategic opportunity in dealing with Soviet power.

For the issue is not simply whether President Reagan will respect his campaign pledge to lift the embargo President Carter imposed after Moscow invaded Afghanistan, or whether this is not the right time to lift the embargo. It's whether the new administration will realize that a potent new combination of circumstances exists, ready for American exploitation.

These circumstances are:

1) World grain patterns are changing. Such is the increase in domestic and non-Soviet foreign demand, and such are the rising costs of growing more food at home, that no longer do American farmers require the Soviet market to soak up surpluses and keep domestic prices high. The new pattern is analyzed in a comprehensive report that Agriculture Secretary John Block inherited from his predecessor. Block, in an interview, showed himself definitely unsympathetic to this part of the report. I gathered, however, he has not yet fully digested the material in it.

2) The miseries of Soviet agriculture linger. This is an old story. Nor is there the slightest glimmer that the Kremlin will do the one thing--unleash market forces--that could ease its farm problem. That means depending indefinitely on American farmers to make up Soviet shortfalls, as the Kremlin has for 20 years.

3) The Soviet-American grain agreement of 1976, obligating the Soviet Union to buy eight million tons a year and giving the president the authority to decide whether to sell more, expires in the fall. This gives the new administration the impetus and occasion to make grain policy for years to come.

In brief, we have got the Soviet Union caught between our diminishing need for their market and their large, continuing need for ours, and it's our deal.

Under that 1976 agreement, the Soviets could and did buy eight million tons in 1980. The Carter embargo kept them from buying 17 million additional tons. Most Soviet grain imports are used to feed cattle, so the embargo did not cause hunger by affecting the basic bread ration. But notwithstanding Soviet spot purchases elsewhere, the embargo did affect the supply of meat in the first year (1980) and it will effect meat even more from this point on as slaughtered herds are not replenished.

We must keep in mind that the standard of living in the Soviet Union is still so abysmally low that, except for the elite, the quality of diet is the most important thing. Meat is the principal item with which the Soviet leadership has undertaken to improve the diet.

The removal of meat from the dinner table is, then a blow at the diet, a blow at the standard of living and a blow at the Soviet leadership. It's not stretching to say that every time a Soviet family sits down to dinner, the evidence of the Kremlin's failure is on the table. Call it the Afghanistan red plate special: cabbage or potato, and no meat, washed down with vodka.

Public opinion as such may not exist in the Soviet Union. The Soviet people, compared, say, with Poles, are cows, little given to protest. Certainly the Soviet internal apparatus of repression is stronger. But there is reason to believe that events in Poland over the last decade have sensitized Soviet leaders to the impact that changes in the supply and price of food can have on domestic stability. They cannot be happy looking forward to further erosion of public morale, if only for its effect on productivity in the work place.

There is another angle. If a limit is kept on Soviet purchases, the onus of explaining why there is no meat will lie with those in the Soviet leadership who were keenest to deal with Afghan unrest by tanks. It is the stuff of a good and continuing internal Kremlin argument. Now, with a succession struggle impending, is a particularly good time to nourish that argument. But if the American grain market is again opened wide, the message the United States would be inserting into that argument would be: anything goes.

A decision to make grain an integral and explicit part of American foreign policy sets up certain tasks. Policymakers must determine how to distribute exports among commerical, political and humanitarian/developmental uses. Diplomats will have to head off Soviet end runs in the few other grain-exporting nations.Propogandists must remind foreigners that Soviet adventurism was the cause of it all. The agricultural sector must accept a requirement to institutionalize a new government role in the grain trade. Difficult tasks all, but possible to do.

Will Reagan seize the moment? Surely he will, if he can shed the conviction that government has no business interfering in the market and if he can, instead, think strategically.