American businesses plan to boost spending on plants and equipment just 0.4 percent in 1991, the smallest gain in investment in five years, the government reports.

The Commerce Department said a survey conducted in October and November found businesses planning to spend $524.2 billion for expansion and modernization, the fifth consecutive year of increased business investment.Business investment actually declined 3.1 percent in 1986.

The survey found businesses planned to boost spending 4.1 percent this year, to $522.1 billion. That was down from a 5.1 percent increase projected for 1990 in the previous survey last July and August.

In fact, the quarterly surveys found projected spending slowing throughout the year as the economy grew more sluggish. For instance, the January-March survey found businesses planning to increase capital spending 8.1 percent.

Still, the 1990 investment would top the record $501.6 billion spent on plants and equipment in 1989, when capital spending was boosted 10.4 percent over the previous year.

Most economists now believe the nation is in a recession and any increase in investment spending would be a stimulant to the economy.

In addition, spending for business investment to replace aging plants and equipment and expand facilities helps to boost U.S. productivity, an increasingly important factor in view of the stiff competition the nation faces from overseas.

All of the figures have been adjusted for inflation.

Before adjusting for inflation, the survey projected a 2.4 percent increase for all businesses in 1991, to $546.7 billion.

It said the investment increase for all industries includes a 4.6 percent gain for manufacturing firms and a 5.6 percent advance for companies engaged in mining, transportation and other non-manufacturing businesses.