WEBSTER CITY, IOWA, OCT. 11 -- The secretary of the Soviet Politburo cut short a presentation by an Iowa State University economist today, saying he did not need to learn about capitalism, and asked instead to see a farm or laboratory.

Viktor Nikonov, who heads a Soviet agriculture delegation touring the United States, said the Soviets were not studying American methods so that they could replace their socialist system with capitalism.

"We're not going to redo your social structure," Nikonov said through an interpreter. "You're not going to redo ours, either."

His comments came when he halted a presentation on computer modeling by Iowa State agricultural economist Stanley Johnson, who heads the Center for Agricultural and Rural Development.

"I'd like to see a laboratory or farm," Nikonov said.

Nikonov got his wish about an hour later, touring the Van Diest fertilizer and chemical operation and farm near Webster City.

He watched Arlo Van Diest combine corn on his 1,300-acre farm and examined the machinery that shreds, disks and plows under cornstalks.

"I think he likes this," said Bob Van Diest, owner of Van Diest Supply Co.

Nikonov, 58, an agronomist, also visited the Garst Seed Co. plant at Slater.

Garst Seed chairman Steve Garst, whose father, Roswell, squired Soviet leader Nikita Krushchev around his Coon Rapids farm nearly 30 years ago, invited the Soviet officials to spend a weekend at his home so he could show them the real Iowa. "Feel free to come back," Garst said.

Garst told Nikonov that he was not gleaning a true picture of the state on a two-day visit to Iowa agricultural manufacturing and research companies.

Garst said he hopes Nikonov's visit opens doors to cooperation and trade like a visit by a Soviet agriculture minister in 1955 that led to Krushchev's visit four years later.

Nikonov invited Johnson to make his presentation at a future conference on humanitarian development of scientific efforts in the Soviet Union. He said Soviet experts would be available to rebut Johnson's assertions.

House Agriculture Committee Chairman E. (Kika) de la Garza (D-Tex.), who invited the Soviet delegation to the United States, said the Soviets were here to see science, technology and biology, not economic reports.

Johnson said he offered the analysis "to show you how we view the world. We have great respect for how others view the world as well."