In an attempt to defuse growing social tensions, Poland today took the unprecedented step for a Communist-ruled country of appointing a prominent Roman Catholic layman, who is an independent member of parliament, as a deputy prime minister.

The appointment of Jerzy Ozdowski, 55, as one of six deputy premiers coincided with continuing labor unrest and a threat by Poland's railway workers to go on strike Monday.In a government reshuffle at the end of a two-day session of the Polish parliament, four Cabinet ministers lost their jobs.

Ozdowski, who belongs to the tiny six-member Zank faction of independent Catholic deputies in parliament, will have special responsibility for family and social affairs. His appointment was seen as reflecting the Communist Party's desire to broaden its popular support, which was severly damaged following last summer's labor unrest.

It is understood that Ozdowski, an agricultural specialist who lectures at Warsaw's Theological Faculty, secured the agreement of Poland's powerful Roman Catholic Church before accepting the post.

In another apparently conciliatory gesture toward the church, the Polish government agreed to a sharp increase in the circulation here of the Polish edition of L'Osservatore Romano, the official Vatican newspaper.

Ozdowski's appointment hardly adds up to any dramatic power sharing between Communists and Catholics in Poland, but it is a move in the direction the church has been seeking. By Communist standards it is a unique gesture and it reflects the extent to which Poland has changed during the past three months.

For roughly two decades until 1976, the Znak deputies represented the sole voice of opposition to government policies in the Polish parliament. Since 1976, the group has been more muted in its criticisms following an internal split caused partly by Communist Party maneuvering.

Some noncommunist members of parliament have privately been urging the establishment of a government of national unity to help Poland overcome its grave economic and social problems. They are understood to have the backing of the Polish primate, Cardinal Stefan Wyszynski, who no doubt pressed their case at a meeting last month with Communist leader Stanislaw Kania.

Ozdowski was granted an audience with Pope John Paul II recently -- and it is likely that the Polish-born pontiff has also been kept informed of developments.

The need for a gesture such as the appointment of Ozdowski was underlined by speeches in the parlimentary session revealing the extent of Poland's current economic problems.

Ministers revealed that -- largely as a result of the poor harvest -- this year's grain crops will be 3.2 million tons short of target, potatoes 18 million tons short, and sugar 6.6 million tons short.

Mieczyslaw Jagielski, the deputy prime minister who negotiated the Gdansk agreements with striking workers, said that production losses as a result of strikes and disturbed supplies during the last four months amounted to $3 billion.

Cabinet members who lost their jobs in today's shake-up included Labor Minister Maria Milczarek, Construction Minister Edward Barszcz, Health Minister Marian Shwinski and Maceij Wirowski, a minister without portfolio.

Ozdowski's appointment was approved unanimously but, in an indication of the parliament's growing independent mood, some deputies voted against or abstained from approving other government nominations. The ministers of agriculture and food industry remained at their posts despite calls for their resignation by some deputies.

An Army general complained that Poland was unable to fulfill its obligations to the Warsaw Pact military alliance because of supply difficulties. Mieczyslaw Moczar, the former interior minister who is emerging as an increasingly influential force in Polish politics, blamed what he described as "astonishingly unreasonable strikes" for Poland's bleak economic position.

Moczar warned that those who attempted to oppose Communist power would "not be treated leniently."

At least five groups of workers are involved in disputes with the government. The latest to threaten strike action are the railway workers, who have accused the Ministry of Transport of failing to fulfill an agreement concluded with them last month.

The railway workers are affiliated to the independent trade coalition Solidarity, which has, however, asked them to work normally pending the outcome of last-minute negotiations.

At a meeting of the national coordinating committee earlier this week, Solidarity leader Lech Walesa appealed for greater order and discipline in the process of wage bargaining. Otherwise, he said, there was a danger that everything that had been gained could be lost.

In a speech to the parliament before his nomination, Ozdowski called for investments in agriculture to be directed toward the small peasant farmer. He said it was necessary to overcome a loss of confidence among them toward the authorities.