American business executives have scaled down their plans slightly for new capital investments this year, the government reported yesterday.

The Commerce Department said surveys in January and February showed nonfarm businesses planned to increase spending for new plant and equipment in 1981 by 10.2 percent, or about $325.7 billion.

That would be slightly more than 1 percent above the increase in prices of capital goods attributed to inflation, if prices rise at about the same rate as last year.

Commerce Secretary Malcolm Baldrige called the new survey "somewhat encouraging," but he also said congressional passage of President Reagan's economic plan is vital to spur additional capital spending.

In a prepared statement released by the department, Baldrige noted that the survey showed predicted spending picking up speed in the second half of this year.

"While these plans are somewhat encouraging, a further quantum jump in business investment is needed to attain our goals of national economic renewal in terms of enhanced productivity, energy efficiency and competitiveness in domestic and international markets," he said.

A survey of nonfarm businesses taken near the end of last year had indicated plans to increase capital spending by about 10.8 percent in 1981, the new report said.

Actual spending in 1980 was about $295.6 billion, 9.3 percent higher than in 1979, the report said. Inflation was measured at about 9 percent last year, according to one department method of calculation.

Department officials noted that business spent somewhat more in the fourth quarter of 1980 than surveys had indicated they planned to spend.

Capital spending had risen 0.6 percent in the third quarter of 1980.