By Kevin Klose and Dan Morgan

The Soviet Union's 1977 grain harvest has fallen 19 million metric tons short of its target, and private dealers reportedly have sold the Soviets 6 million to 7 million tons of American grain the last few days.

The shortfall in the Soviet harvest - one of the key economic barometers in that country - was announced by Soviet President Leonid I. Brezhnev yesterday at a Kremlin rally marking the 60th anniversary of the Bolshevik Revolution.

Western experts had been predicting bumper crops, but Brezhnev announced that the harvest would produce only a disappointing 194 millions, which he attributed to weather conditions that were "far from being the best."

U.S. officials said this is bad news for the Soviet Union and could be the first good news in some time for American grain farmers, who have seen the prices they get for their crop plummet to five-year lows this summer because of slack demand abroad.

Ultimately, the disappointing Soviet crop results could help President Carter by getting the large wheat and corn surplus moving off farms and out of the country, and propping up low prices that agitated Midwest farmers. In some parts of the country, farmers have been talking about united action to withold stored grain from the markets, and they have been blaming the President for not doing enough to support farm incomes.

However, whether the administration is helped will depend on how much grain the Soviets buy. So far, the Soviet trade agency Expertkhleb has ordered 2.3 million tons of grain from this year's U.S. crops.

Under Secretary of Agriculture John White yesterday said it is likely that "6 or 7 million tons" of exported grain reported by companies as bound for "unknown destinations" will go to the Soviet Union.

These sales represent shipments abroad for which there are still no final contracts. White said it is likely that some or all of these shipments will be delivered against contracts now being arranged by foreign subsidiaries of global grain firms.

Secretary of Agriculture Bob Bergland said at a press conference that it was "likely" the Soviet Union would purchase 15 million metric tons of U.S. grain. That would be the largest Soviet purchase in a single year - larger than the 14.5 million tons of 1975 and the 13.7 million tons of 1972.

The United States told the Soviet Union several weeks ago that they could purchase up to 15 million tons because of the unusually large buildup of unsold grain here this year. The 1975 Soviet-American grain agreement required the Russians to get special authorization for buying anything more thna eight million tons.

The Agriculture Department set the total Soviet need for foreign grain at 20 to 25 million tons. According to some reports, India has sold the Soviet Union some wheat this year.

Bergland said that the United States "can meet whatever Russian demands there may be," and that the shipments would have to reach 50 million tons before the exports would have an appreciable impact on food prices, such as occured after the first big Soviet purchase in 1972.

However, Bergland said the new situation in the Soviet Union would be a factor in the Carter administration's decision whether to ask farmers to idel corn acerage next spring. Part of the reasoning behind idling land is that there are insufficient customers abroad to buy as much corn as farmers can produce without restrictions on planting.

A decision on the corn land "set aside" plan will be reached in a few weeks.