President Reagan announced yesterday that he will nominate fellow Californian Richard E. Lyng, a longtime Republican ally and policy adviser, to succeed John R. Block as secretary of agriculture.

Lyng, who resigned as deputy secretary a year ago to set up a consulting firm, is expected to win easy confirmation in the Senate, where he is well-regarded.

In an Oval Office meeting to present Lyng to the news media, Reagan said farmers would have a "sound and solid friend" in Lyng as he moves to administer new farm laws that "will help get farming more into the market economy and rectify some of the things that have been wrong."

Administration sources indicated the choice of Lyng -- easygoing and politically low-profile -- reflected a desire to avoid more animosity in the economically troubled midwestern farm states, where many GOP lawmakers expect difficulty in fall elections. Reagan's selection of Lyng heightened speculation that John R. Norton, deputy secretary who has major farming interests in California and Arizona, would be pressured out of the administration and replaced with a more politically palatable midwesterner.

Norton, in Iowa and unavailable for comment, has indicated that he might leave Washington because of economic difficulties on his farms, although he also was interested in succeeding Block. Capitol Hill sources are floating the name of Duane Acker, president of Kansas State University, as a new deputy secretary.

Block, who will leave the administration Feb. 14, increasingly had been criticized by farm-belt Republicans as a political liability as he spearheaded the White House drive to lower federal farm-support spending and make U.S. crops more competitive overseas.

That tension was noted yesterday by Rep. Edward R. Madigan (Ill.), top Republican member of the House Agriculture Committee, who had been critical of Block. Madigan said he hopes that Lyng "will be able to be more effective politically than Block was" because of Lyng's longstanding ties to Reagan.

Block, who reportedly will take a job next month with a food trade association here, "applauded" Reagan's choice of Lyng as "a proven, experienced, highly capable individual."

Lyng, 67, was director of the California agriculture department when Reagan was governor. He was an assistant agriculture secretary in the Nixon administration and an agriculture adviser in Reagan's 1980 presidential campaign. He became deputy secretary, second in command at the Agriculture Department, in 1981.

Most speculation here centered on Lyng as a logical successor to Block, although a quadruple-bypass heart operation in 1982 raised questions about his health. Lyng, however, lobbied vigorously for clients on Capitol Hill and told reporters that his health was good.

Reaction to the president's announcement was predictable. Rep. E (Kika) de la Garza (D-Tex.), chairman of the House Agriculture Committee, praised Lyng but warned that congressional differences with the White House about farm policy will not go away.

De la Garza said Lyng's nomination "will not change the basic direction of this administration's farm policy, and that may mean we are going to have some of the same policy differences with Dick Lyng that we had with Jack Block."

Ellen Haas, director of Public Voice for Food and Health Policy, called Lyng "one of the most politically astute and pragmatic agriculture leaders around." But she said that his life-long association with agribusiness interests means that "we must remember from whence he comes . . . a consumer advocate, he is not."

Lyng headed a family seed business in Modesto before he joined Reagan in California state government. He was president of the American Meat Institute between 1973 and 1979 and held various positions with agribusiness lobbying and policy organizations. Another frequent critic of Reagan farm policy, Sen. John Melcher (D-Mont.), said he thinks that Lyng will put more emphasis on export development than Block. "He's like an old hightop shoe," Melcher said. "There's some substance and support there, some trustworthiness. He's been around the loop."