President Reagan yesterday announced a program aimed at helping American farmers, which includes payment of farmers in free surplus grain in return for idling part of their acres and an increase in federal credit supports to help bolster sagging foreign purchases of agricultural products.

Addressing the 64th annual convention of the conservative and friendly American Farm Bureau Federation, Reagan outlined a program aimed at reducing farm surpluses that depress farm income, increasing farmers' income and reducing federal crop support outlays.

He also expressed sympathy for the plight of dairy farmers, expanded on his dismay about press leaks in his administration and designated 10 flood-stricken Louisiana parishes (counties) as federal disaster areas.

Net farm income fell for the third consecutive year in 1982, and loan foreclosures and bankruptcy among farmers are increasing. The value of the average American farmer's land dropped last year for the first time in 18 years.

Under Reagan's program:

The U.S. Department of Agriculture this month will begin a payment-in-kind (PIK) program under which farmers will be given surplus federal grain, rice or cotton in return for not planting a portion of their land this year. They would be free to sell or use the commodities as they wish, which USDA officials say they believe will decrease huge surpluses that have helped depress prices.

Agriculture Secretary John R. Block estimated yesterday that the PIK program--Reagan called it a "crop swap"--could remove as many as 23 million additional acres from production this year. Block said that, combined with an existing acreage-reduction program, would have only a slight impact on prices, but would remove the "dark cloud" of surpluses that deflate prices. Related story on Page A17

Reagan also told the 6,000 Farm Bureau members that he is more than doubling the $100 million credit program for foreign buyers of U.S. farm commodities in an effort to increase sagging exports, which fell by $4 billion last year. The low-rate credit program led to an additional $500 million in farm exports in 1982, the president said.

In another move aimed at pleasing his audience, the president anounced that he had signed legislation reauthorizing the Commodity Futures Trading Commission. The Farm Bureau had supported the legislation, but it was resisted by the White House because of a "contract sanctity" provision that would in effect bar farm export embargoes except in "extreme foreign policy situations."

The law guarantees that any farmer's grain export contract will be honored by the government if delivery is scheduled within nine months of an embargo announcement.

Under the PIK program farmers will be given free surplus grain in return for not planting up to half their acreage. They would get 80 percent of their normal production on the idled acreage for corn and feed grains and 95 percent of their production in wheat.

The president also said he is sending the message to other countries that the United States now expects "fair access" to international agricultural markets.

He continued complaints that he began Monday about press coverage of his administration, telling reporters that they sometimes give the public the wrong information.

Reagan said he was surprised that newspapers quoted him yesterday as telling aides he was up to his "keister," slang for buttocks, with leaks in the press.