THIS YEAR could well turn out to be an exceedingly bad one for the world food supply. (Just to keep things from getting too easy, agricultural years coincide with neither the calendar year nor the fiscal year -- they run from July to June, so that "this year" ends in June 1981.) The latest evidence comes from an unusually self-critical speech delivered by President Brezhnev to the party's Central Committee early this week. Without being specific, Brezhnev hinted that the Soviet grain harvest would fall far below its goal of 235 million tons. How far below is not yet known, but the consensus among experts is that the harvest will be bad but not disastrous.

As it happens, the Soviet Union will not be alone in its bad luck. The U.S. harvest will be way down from last year's. Canada and Australia have also been hit by the drought that damaged U.S. crops. And Argentina is reported to have had such a bad harvest that it has cancelled all its export contracts for the year.

This small group of four countries exhausts the list of major grain-exporting nations. Americans, accustomed to an abundant and reliable domestic food supply, find it hard to remember the extent to which they enjoy a unique situation. Just before World War II every continent except Western Europe exported grain, but today only North America is a net exporter. The United States alone produces 60 percent of the world's supply of exported grain, dominating the world grain market far more heavily than Saudi Arabia dominates world oil trade.

This control of the market brings with it enormous power. Brezhnev's speech underscores the point. Despite the assertions to the contrary by everyone from Sen. Kennedy to Gov. Reagan to anxious farm groups in this campaign year, the U.S. grain embargo has had an effect. The Soviet are feeling the pinch and, fearful of a replay of the Polish food riots, the leadership has felt it necessary to try to explain to the Russian people why meat supplies will be short and how high a priority the government attaches to improving the management of its agricultural sector.

The effects of the embargo are proof that a cutoff of U.S. grain cannot be made up without drawing down world emergency reserves to dangerously low levels. In bad years, such as this one seems to be, even that option may be closed. It should really have been obvious all along. When the Soviet Union entered the international grain market in 1972, it did not willingly choose to be dependent on its principal adversary; it simply discovered that no other combination of suppliers could fill its needs. In this connection, Mr. Carter -- and John Anderson, who has alone among other candidates supported the embargo -- was right.