Why Reagan plans to ship grain to the Soviet Union

West European reaction to reports that President Reagan has decided to authorize a one-year extension of a US grain trade agreement with the Soviet Union seems to be: We told you so.

As key West European governments see it, the Reagan administration is willing to deal with the Soviets when the United States finds it profitable. At the same time, the administration is trying to get the Europeans to cut their gas pipeline trade with the Soviets.

The result, in the European view, is unfairness, and worse yet,cc11 pincoherence.

Not so, say administration officials. They argue that there is a big difference between taking hard currency from the Soviets for grain - hard currency Moscow could use for military expenditures - and selling the Soviets pipeline equipment that will eventually help them earn a great deal of hard currency. Besides, US officials say, grain is easily available to the Soviets from a variety of sources, not just from the US. They'll get it anyway. Pipeline technology is not so easily available.

Administration officials also acknowledge that the continuing grain trade with the Soviets has what is described as a ''domestic dimension.'' The administration does not want to cut off the trade entirely, because it would hurt American farmers. And this does happen to be an election year.

But in setting down sanctions last year against the Soviet Union over the repression in Poland, the administration vowed that it would postpone negotiations on a long-term grain agreement with the Soviets unless the situation in Poland improved. The US and its allies agreed on a need for the Polish authorities to lift martial law, release prisoners, and renew their dialogue with the Solidarity trade union movement.

Last week, Polish authorities announced the release of 1,227 persons detained under martial law and declared that martial law itself would be suspended by the end of the year if social peace is maintained. As officials here see it, the announcement by Poland's military leader, Gen. Wojciech Jaruzelski, amounted mostly to an expression of hope for the future rather than a significant change. Thus, the administration was not in a position to negotiate a new long-term agreement on grain with the Soviets, such as the five-year US-Soviet agreement which is due to expire Sept. 30. Nor was it in a position to soften its opposition to the European gas pipeline deal with the Soviets.

In the realm of grain, the result has been something of a compromise that is not satisfactory either to a number of American farm groups or to the West Europeans, who view the impending grain deal as a sign of American hypocrisy and incoherence. They also tend to see incoherence in the way the Americans have justified their opposition to the projected Soviet-European gas pipeline. At one point, US emphasis was on a need to punish the Soviets for encouraging repression in Poland, Europeans say. Then it shifted to an emphasis on combatting European dependence on Soviet natural gas, then to an emphasis on curtailing the hard currency earnings the Soviets would gain from Poland. Then back to Poland, etc.

In the European view, it is highly unlikely - and unrealistic to believe - that Polish authorities will do everything Reagan expects them to do, especially if they and their Soviet backers are put in the position of appearing to bend under American pressure. If Reagan had strong objections to the pipeline deal he could have raised them at the recent Versailles summit meeting. As it was, reports from Paris indicate that when France's President, Francois Mitterrand, said that French companies were obligated to fulfill their contracts with the Soviets, Reagan raised no objection. The French, for their part, do not consider trade with the Soviets to be at a rate of significant proportions (it comes to something like 1.8 percent of their total exports).

Both the Americans and Europeans, meanwhile, seem to be interested in entering a cooling-off period while both sides sort things out. Officials here think that there has been some progress - a first small step - in that the French have raised interest rates for export loans to the Soviets.