President Reagan, gearing up for major votes this week on tax-revision and farm legislation, urged Congress yesterday to pass bills on both topics but did not say which versions he would be willing to sign.

In his weekly radio address, Reagan did not endorse either the tax plan produced by the Democratic-controlled House Ways and Means Committee or a Republican alternative. But he made his opening pitch in what White House aides say will be a week of strong lobbying for the Democratic bill if, as expected, the GOP version fails.

"While the proposals before the House are far from perfect, they do represent an essential step toward a tax code that is fairer, simpler and encourages greater growth," Reagan said.

The president was conciliatory on the measure to shore up the ailing Farm Credit System, which his administration has repeatedly threatened to veto as a budget-buster. Praising the sponsors of both the House and Senate versions, Reagan asked Congress "to send me a bill I can sign."

Reagan did not say which provisions he finds objectionable or mention that the farm credit bill is $20 billion more than he sought. Instead, he twice blamed the Carter administration for the depressed farm economy.

"The last administration contributed to this by imposing grain embargoes, a failed policy that our administration will never repeat," Reagan said. He said the struggling farm economy reminds him of the Depression and that "government must do its part" because it helped create the problem.

In the Democratic response, Ways and Means Chairman Dan Rostenkowski (D-Ill.) said Reagan has been "an ally" in overhauling the tax code. "I've got every reason to believe he will back his support with some political muscle in the days ahead," Rostenkowski said.

Urging listeners to call their legislator to support the Ways and Means plan, he said, "Most Washington insiders bet that we'd never have the guts to take away special tax preferences from powerful industries like oil and gas . . . . They were wrong. But that isn't going to stop the special interests from trying to kill reform on the House floor."