A giant combine is working to mow down the American embargo on grain sales to the Soviet Union. The pressure of this country's farmers for lifting the ban has now been reinforced by a plea for agricultural trade from the Soviet leadership.

But international conditions militate against any expansion of economic dealings with Moscow. So the right tactic -- a tactic now under consideration in the State Department -- is to roll the embargo into a new statute that would govern trade with Russia as an explicit part of American foreign policy.

A fundamental imbalance impairs all American economic dealings with Russia as presently conducted. The Soviet economy is dominated by the state. What the Russians buy and sell feeds into a national strategy designed to maximize Soviet power -- often at the expense of the United States.

The American economy is relatively free. Companies bid against each other and against foreign competitors for the right to sell to the Russians. The sellers have no clear notion of national interests. Insofar as national interest does come into play it tends to be applied in an erratic and arbitrary way.

Thus the 1972 Trade Accord with Russia was -- largely because of pressure from American Jews -- made subject, by the so-called Jackson-Vanik Amendment, to Soviet practice on immigration. In 1975, to avoid a repetition of the secret buying that sent domestic grain prices skyrocketing, the United States put Soviet grain purchases on a five-year basis with an assured minimum of six million tons. In 1980, after the Soviet invasion of Afghanistan, further grain sales to Russia were suddenly embargoed.

Inevitably, American farmers reacted strongly against the embargo. It denied them, without any due process, markets that had previously been assured. It asked them to make much more sacrifice than any other economic group in the country. It was not all that effective, as the Russians bought grain from other sources. With all these arguments going for them, the farmers were able to elicit from candidate Ronald Reagan a pledge that he would end the embargo if elected.

The Russians undoubtedly had to pay higher prices for grain bought to make up for the shortfall in American purchases. But they were clearly able to live with the pressure. They did not get out of Afghanistan nor significantly modify their political behavior anywhere else. Some Russians claimed that denial of grain only worked against peace and stability in East-West relations. In that vein, Prime Minister Nikolai Tikhonov, in a speech to the 26th Party Congress in Moscow last week, called for improved trade relations with the United States, and specifically mentioned renewal of the longterm grain agreement that expires this year.

The Reagan administration cannot accommodate that appeal. It is just launching a new foreign policy based on putting some starch into the flaccid approach to Russia endorsed by Jimmy Carter. It is moving to beef up defenses here, and in allied countries. It seeks joint action to stop radicals' expansion in Central America, Africa and Asia. It is rightly concerned about a possible Soviet move against Poland.

That basic policy would be undermined if the embargo were lifted at this point; similarly, if the long-term grain agreement were renewed. So the appropriate American move is to subordinate both of these controversial items, and also Jackson-Vanik, to a new approach.

The approach now under consideration in the economic offices of the State Department in effect nationalizes trade with Russia. A new law would provide for general trade agreements with the Soviet Union to be negotiated every two or three years.

In negotiating those agreements, American officials would take into account of the full range of Soviet activities in the world, including foreign policy and human rights. The Russians would be on notice that their general behavior would enter into each successive trade agreement. American sellers, instead of being jerked around by chops and changes, would be subject to a regular routine. This country would be in better position to negotiate cooperative accords with allies who generally organize their own foreign trade in a much more centrally coordinated way than the United States.

Holes can easily be poked in that idea. Russian political behavior is almost surely not going to be appreciably changed by economic pressure. American negotiators will undoubtedly come under all kinds of conflicting pressures as they try to arrange each successive trade package. Allied competitors will seek -- and probably obtain -- competitive advantages.

But at least the basic myth would be shattered. Nobody could seriously argue that trade with the Soviet Union was a function of private initiative. Everybody would understand that it is a part of foreign policy.