The nation's retailers face a major shakeout during the 1990s that will leave half of them out of business by the turn of the century, an economist says.

Ira Kalish, senior consultant and economist of Management Horizons, the retail consulting division of the accounting giant Price Waterhouse, said a reduction in retailers would result from changing demographics, and new stricter spending habits following the binge of the 1980s.Quoting Retail 2000, a report issued by the firm earlier this year, he said the retailing industry has too many stores, too much sameness and too much debt.

"By the end of the decade half of the retailers in the United States will be out of business," he said.

He said retailing as a share of gross national product would slip to about 31 percent from the current 34 percent by the year 2000, representing about $150 billion worth of business.

Retailers would have to contend with significant shifts in population, he said. For example, the crucial 18-to-34 age group toward which most retailers aim their merchandise, would be reduced by 8 million by the year 2000.

Spending habits and priorities will change, too, he said. "In the 1980s, people spent more and saved less. The 90s will be the reverse. People will be less acquisitive and more concerned with their children's education and their own retirement."

In addition, the black and Hispanic populations of the United States would rise to 24 percent by the year 2000, demanding another shift of emphassis on the part of retailers.

"There will be no more homogeneous mass market," he said. Success will come to the retailer who focuses on individual customers so that it becomes a large share of each consumer's budget. It will be a "relationship market."

The key to survival, Kalish said, would be an emphasis on the individual consumer, rather than the mass market. Successful retailers would have the ability to focus sharply on their customer in terms of age, income and lifestyle, and lure them with things such as liberal return policies and private label credit cards.

The fragmentation of the customer base would demand "the ability to manage at the micro level and to have a decentralized system of control."

He cited The Limited Inc. of Columbus, Ohio, a women's apparel retailer, as being a successful recent example of this approach, through its portfolio of stores that cater to narrow markets.

Successful retailers will use technology to identify their customers. And they will be able to react quickly to a changing market. "They will be able to move the right products to the stores at the right time," he said.

Retailers will be more successful as part of a "strip center" or multi-use neighborhood complex than as part of a mall, Kalish said. "America will be `demalling."'