Government reports issued this week have confirmed earlier expectations of big grain crops in the Soviet Union and the U.S., increasing the pressure on the Carter administration to idle substantial amounts of productive U.S. cropland starting this fall because of the mounting surpluses.

The Agriculture Department yesterday estimated the 1977 U.S. corn crop at just over 150 million metric tons, down six million tons from the July forecast but still just below last year's record output.

It said wheat production would be a near record 55.5 million tons. U.S. soybeans, which supply the essential protein for chickens, hogs and beef cattle raised in this country. Europe and Japan, were expected to be at an alltime record.

The department's forecast came one day after it predicted that Russia would harvest 220 million tons of grain in 1977, the third largest crop in that country's history.

The Soviet harvest is a key factor in considerations here on whether to take grain acreage out of production, since Soviet grain output varies widely from year to year.

Administration farm policy makers have to decide whether substantial amounts of farmland can lie fallow without jeopardizing the safety margin in the world food supply. The U.S. still had 30 million tons of unsold, surplus wheat left over his spring when the 1977 harvest began.

Howard Hjort, the Agriculture Department's chief economist said yesterday that there was a "60-40" chance that wheat acreage would be idled in 1978. He said that the chances were "50-50" that the government would also take steps to cut back corn planting.

However, he noted at a press conference that the decision required consultation with other branches of the U.S. government and with other grain producing countries.

If land is idled, there will be a smaller grain harvest next year and customers would be likely to bid higher prices. Hjort said it would take "a substantial set aside (idling of land) to have any impact on food prices."

But Hjort also cautioned that the "line between too little and too much is a pretty thin one," because population growth and improved diets abroad required a steadily increasing amount of food each year.

The last time the United States idled productive grain acreage was in 1973. The decision was sharply criticized later as a factor in the grain shortages and food price inflation of 1974.

A decision on whether to retire wheat land must be taken in a few days, because about two-thirds of the wheat is planted in September and October and farmers already have begun to prepare their fields for seeding.

The National Association of Wheat Growers favors a 30 per cent reduction in wheat acreage, as the best way to increase sagging wheat prices.

Wheat farmers in the Midwest report financial and credit difficulties. The Agriculture Department predicted earlier that U.S. farmers would earn $18.5 billion in 1977, the lowest for several years. but Hjort said yesterday that payment and support provisions of a farm bill that is expected to go to the President in September should bring this up to about $20 billion.