President Reagan today said that his administration has exposed abuses in Pentagon purchasing "that had been going on for years." In his weekly Saturday radio speech, delivered from his ranch near here, the president said it was Defense Secretary Caspar W. Weinberger's auditors and inspectors who rooted out the abuses.

At the close of his five-minute talk, Reagan said, "There's something I'd like to get off my chest. It deals with all those headlines about the Pentagon paying $100 for a 4-cent diode or $900 for a plastic cap."

"What is missing or buried in all those stories about waste is who provided those figures for all the horror stories," he added. "This administration exposed those abuses, abuses that had been going on for years."

Reagan said that Pentagon auditors and inspectors "ordered the audits in the first place, conducted the investigations and formed a special unit to prosecute defense-related fraud." This unit obtained 650 convictions in 18 months and "we're going to keep on exposing these abuses where we find them," Reagan said.

The president took a different approach toward Pentagon waste a few months ago, when the administration was battling Congress over defense spending. Then, Reagan said that claims of fat in the Pentagon budget were overstated. Today, Reagan said his administration has been "making progress" in combating waste and fraud in government.

As a result of a review of each department and agency, Reagan said, "There were 112,000 fewer people working in non-defense federal agencies" than before. He added, "Fifteen departments, agencies and commissions have been able to reduce their payroll numbers by 20 percent or more."

Reagan said 90 percent of the reductions came not by layoffs but by attrition. "And guess what? The government is actually doing things more efficiently than it was before," he said.

The president also took credit today for reducing the number of government employes who retire on disability, saying there had been "considerable abuse of the system."

While civilian government jobs are "not particularly hazardous," he said, "we found that one out of every four retirements . . . was disability retirement." The administration has reduced this by nearly 40 percent, the president said.

He also charged that the civil service retirement system had "sometimes" provided "preferred treatment" to federal workers because benefits were indexed to inflation twice a year, "an advantage enjoyed by virtually no one in the private sector."

The president noted that the administration had shifted to indexing once a year.

Reagan also defended administration efforts to base certain pay increases for federal workers on merit. He said the administration is still working with Congress to "refine" the merit pay proposal.

In the Democratic response to Reagan's speech, Rep. Matthew G. Martinez (D-Calif.), who represents a heavily Hispanic Los Angeles district, said Hispanic Americans have received "maltratado," or "rotten treatment," from the administration.

"The sad truth is that this administration's policies have been disastrous for Hispanics," Martinez said. He noted that about 30 percent of Hispanics nationwide live below the official poverty line, almost double the national average.