At President Carter's urging, the Immigration and Naturalization Service is allowing Mexicans field-workers to enter the United States to help harvest onions in southern Texas, an action the Labor Department refused to approve.

White House press secretary Jody Powell said yesterday that Carter after receiving a telephone call from Rep. Richard C. White (D-Tex.), called Labor Secretary Ray Marshall, who passed the President's wishes on to the Justice Department, which administraters the INS.

The Mexican workers were needed because farmers could not find U.S. worlers to harvest the onions, which were rotting in the fields Presidio, Tex., about a mile from the Mexican border.

Powell said Carter told Marshall to tell the farmers they could not count on similar arrangements next year.

Before Carter became involved in the problem, the Labor Department found its regulations prohibited admitting the workers, even though it was satisfied that no domestic workers were available, Powell said.

A department spokesman said that before it could certify the admission of foreign workers, the farmers were required to provide housing and to be willing to pay "adverse wage rate," in this case $2.83 an hour. That is the wage at which, it was determined, U.S. workers' wage rates would not be adversely affected.

The spokesman said the farmers balked at paying this wage.

Powell said the field-workers were being paid the minimum wage, $2.30 an hour. Because the laborers are returning to Mexico at the end of each day's work, the farmers did not have to meet housing standards for migrant workers, Powell said.

He said regional director of the INS advised against allowing the Mexicans into the country, but that the district director suggested that they be permitted to enter.

"The INS was looking at it for a week before the President called and asked that they expedite the situation in as much as rotting onions don't wait," Powell said.