After three years of good crops and plentiful food supplies, the world appears to be reentering a period of erratic grain production, higher prices and somewhat reduced stocks, in part because of worsening crop conditions in the Soviet Union.

This is the common theme of global projections issued in recent weeks by the Department of Agriculture, the Food and Agriculture Organization of the United Nations, the International Wheat Council and private crop anslysts.

On June 4, the FAO forecast a 1979 world harvest of wheat and feedgrains of 1.13 billion metric tons, about 5 percent below 1978's record 1.18 billion tons. The Department of Agriculture's May figures project a similar decline. The agency foresees a hearvest ranging from a low of 1.09 billion tons to 1.16 billion tons.

All the estimates suggest that the present large stocks of unsold grain, most of it is the United States, will decline as these reserve begin to be used to cover the current requirements of flour millers and farmers.

Grain stocks in exporting countries now total more than 200 million tons, about the level that existed in 1971, just before unexpected Soviet buying and poor crops combined to produce skyrocketing prices and food shortages. Those stocks may provide a somewhat smaller buffer now, however, because of increases in population worldwide.

The major trouble spot is the Soviet Union, which seems headed for a harvest substantially below its record 1978 crop of 238 million tons. Last Friday, new departmental estimates, using a probable range rather than exact figures because Russian crops still are at an early stage of development, predicted the final harvest total would be between 170 million and 210 million tons.

The middle point, 190 million tons, was 12 percent below the one forecast earlier. The government announcement was credited by commodity analysts in Chicago with sending U.S. wheat, corn and soybean futures to new highs this week.

The spot price of wheat in Kansas City Monday reached $4.05 a bushel, more than one dollar above the price on the same day one year ago.

The sudden unfavorable turn of events in Russia, the department reported, was due to hot, dry weather accompanied by wind and low humidity in May in the Volga, North Caucasus, East Ukraine and Southern Urals. These conditions occurred during the critical flowering stage of fall-planted wheat and spring-planted barley.

Despite this initial reaction, government and private analysts said it is too early to predict the impact, if any, that a poor Russian harvest would have on U.S. food prices.

The Soviet Union already has purchased an estimated 15 million tons of grain from the 1978 U.S. crop - the maximum allowed under terms of a 1975 Soviet-American agreement on grain purchases, Current grain prices probably already reflect these large U.S. commitments, analysts say.

U.S. officials say Russian grain purchasers have not asked specifically for lifting the 15-million-ton ceiling on purchases. However, they report Russian officials appear "bullish" about continuing to import large quantities in the months ahead.

In addition to the questionable outlook for crops in Russia, severe winter storms have affected prospects for a good grain harvest in France - western Europe's largest grain producer and fourth largest wheat exporter in the world.

Several other factors also point to strains in the months ahead on the U.S. food supply.

Transportation problems in Australia and Canada have slowed the flow of grain from those two countries to foreign markets. This has caused some importers to turn to the United States, which already supplies more than half of all the grain moving into world markets.

Foreign demand for grain imports has continued to rise steadily throughout the decade.

The International Wheat Council in London reported recently that wheat imports by the 13 memmbers of the Organization of Petroleum Exporting Countries has risen from 2.7 million tons (or 5 percent of world trade) in 1970 to 9.7 million tons (or 14 percent of world trade) in 1978. Only three OPEC countries - Algeria, Iran and Iraq - produce wheat on a large scale, and none is self-sufficient in wheat, except in unusually favorable years.

Earlier this year, efforts by the major grain importers and exporters to establish an international system of grain reserves to stabilize prices collapsed. This means that if crops fail again, there is no system to prevent a rapid depletion of present stocks.