President Reagan's nominee to manage U.S. forests has asked to amend his financial disclosure statement in the wake of growing controversy over his past involvement with a company found guilty of conspiracy to rig bids on purchases of public timber.

John B. Crowell Jr., Reagan's choice as assistant secretary of agriculture for environment and natural resources, made the unusual request to the Senate after Republicans, fearing an embarrassing confrontation agreed to postpone a final vote on his nomination until next week.

New questions about Crowell's activities as general counsel for the Louisiana-Pacific Corp., the major buyer of federal timber, and his connection with an Alaska subsidiary found guilty of antitrust practices have put an additional cloud over the nomination.

As assistant secretary, Crowell would oversee the U.S. Forest Service, which manages about 190 million acres of public forest land, and would be the chief watchdog over anticompetitive practices among timber firms.

Crowell's original disclosure statement, filed with the Senate Agriculture Committee, gave no indication of a connection with Ketchikan Pulp Co., which a federal judge in Seattle has held guilty of conspiracy. Late Wednesday, a new information surfaced about his role with the firm, Crowell asked to be allowed to amend the document. There were no indications of how he would do so.

Crowell was approved unanimously by the Agriculture Committee six weeks ago. A floor vote has been blocked by Sens. Edward M. Kennedy (D-Mass.) and Patrick J. Leahy (D-Vt.), who are probing his corporate background. GOP leaders had intended to force a floor vote this week, but changed signals at Leahy's request.

Rep. Jim Weaver (D-Ore.), chairman of the House forestry subcommitte, has called on Crowell to withdraw, charging serious conflicts of interest. Failing that, Weaver said, the Senate should reject the nomination.

A key question is the extent of Crowell's involvement with Ketchikan Pulp CO. (KPC), the Louisiana-Pacific subsidiary recently found guilty of price-fixing and conspiring to keep small loggers out of the Tongass National Forest in Alaska, the nation's largest.

The judge held that company executive actively engaged in the conspiracy with officials of the Alaska Lumber and Pulp Co. by rigging bids to force down prices on the public timber and by colluding to squeeze out small firms.

Publicly, Crowell has insisted that his only involvement with KPE was in choosing defense counsel for the trial and hearing periodic reports on trial proceedings.

"Beyond that, I have never been involved in the remotest way with any of the occurrences, negotiations or contracts out of which the plaintiffs' claims allegedly rose," Crowell said in a letter last week to Agriculture Committee Chairman Jesse A. Helms (R-N.C.).

Since he wrote that disclaimer, however, these new elements have come to light:

In a 1975 company memo, Crowell wrote at length about Louisiana-Pacific's dealings with a small logging firm, asserting that it was in KPC's interest to prevent smaller companies from gaining a foothold in the Tongass. The small firm's complaints about KPC figured in the conspiracy case.

A 1973 memo from a KPC official told Crowell about a meeting with officials of Alaska Lumber and Pulp Co. to discuss timber supply in the forest, where the two companies have 50-year leases. Alaska Lumber, a codefendant, also was found guilty in the conspiracy.

Crowell has conceded that he was assistant secretary of KPC, at least as far back as 1973, although he insisted to a Senate investigator that it was solely a "ministerial function."

None of those points was mentioned by Crowell in his letter to Helms, nor did they come up at his confirmation hearing. As assistant secretaty of USDA, a principal duty would be to assure fairness of competition in the bidding on federally owned timber.

In private interviews with Senate staff this week, Crowell reportedly acknowledged that he had been involved in a variety of KPC business functions, including negotiation a consent decree with the Environmental Protection Agency in a pollution case, preparing KPC contracts for lumber sales to Japan, negotiating on chemical supplies for Kpc and consulting on KPC dealings with small firms in the Tongass.

Crowell also reportedly told a Kennedy aide that he was the "in-house savant" for the Portland, Ore.-based Louisiana-Pacific on antitrust matters and that he had been involved in acquisition questions for the company, which has expanded by buying up many smaller operations.

Kennedy has submitted a lengthy list of questions to Crowell, asking for amplification on those points. Leahy interviewed Crowell Tuesday, but would not comment on the meeting except to say that he believed the Agriculture Committee should reconvene to discuss the new issues -- a proposal that Helms has rejected informally.

Environmental organizations have mounted a vigorous campaign against Crowell, contending that his past role as a severe critic of Forest Service policies and timber-selling practices made him unsuited for his new job. His candidacy is strongly supported by large companies that dominate the industry.

Crowell promised the Senate committee that he would avoid conflicts by staying out of specific appeals brought by his old company and by keeping hands off the Forest Service's small business set-aside program, of which he has been critical in the past. The program is designed to protect small firms from predatory competitors by assuring them a share of the public timber sold on bids.

Rep. Weaver has argued that Crowell, despite his promises, cannot avoid conflicts at USDA because of the immensity of Louisiana-Pacific and its widespread involvement in national forests.

Louisiana-Pacific, created as a spin-off of the huge Georgia-Pacific Corp. by order of the Federal Trade Commission in 1972, has in less than a decade become the second-ranking U.S. lumber producer, heavily reliant on timber from the national forests, with sales over $1 billion in 1979.

Louisiana-Pacific began with 20 percent of the larger company's assets and hired Crowell away from Georgia-Pacific to become general counsel.

With an aggressive policy of acquiring smaller firms and buying up federal timber, Louisiana-Pacific by last year had amassed 62 mills around the country and surpassed Georgia-Pacific as a lumber producer. Throughout its growth period, the firm -- often by way of Crowell -- was a vociferous critic of the Forest Service's program to assure small firms a share of public timber.

Along the way, the company has acquired a reputation as a litigious and quarrelsome client of the Forest Service. The company has filed at least five suits against the government in timber disputes and has brought at least 10 appeals before the Agriculture Board of Contract Appeals, which rules on timber contract problems.

Of three pending suits, one is a Louisiana-Pacific challenge to USDA's recomputation of the small business set-aside in the Mendocino (calif.) National Forest. The other two suits involve the firm's challenges to environmental requirements in the Deer lodge National Forest in Montana.