The pungent smell of pot that blankets a popular quadrangle at the University of Colorado-Boulder every April 20 is being replaced by the stench of fish-based fertilizer Friday as administrators try to stamp out one of the nation‘s largest annual campus celebrations of marijuana.

After more than 10,000 people — students and non-students — attended last year’s marijuana rally on Norlin Quadrangle, university officials decided this year to apply the stinky fertilizer to the quad to deter pot-smokers. They’re also closing the campus Friday to all unauthorized visitors and offering a free campus concert by Haitian-born hip-hop star Wyclef Jean timed to coincide with the traditional 4:20 p.m. pot gathering.

The measures pit Colorado’s flagship university, which has tired of its reputation as a top party school, against thousands who have assembled, flash mob-style, each year to demand marijuana‘s legalization or simply to have a good time.

With more than 30,000 students, Colorado was named the nation‘s top party school in 2011 by Playboy magazine. The campus also repeatedly ranks among the top schools for marijuana use, according to a “Reefer Madness” list conducted by The Princeton Review.

“We don’t consider this a protest. We consider this people smoking pot in the sunshine,” said university spokesman Bronson Hilliard. “This is a gathering of people engaging in an illegal activity.”

“I do not see any justification for the university shutting it down,” said student organizer Daniel Ellis Schwartz, who contends the measures infringe on First Amendment rights to protest. Schwartz, a physics major, and other supporters of the 4/20 smoke out plan to move it to a nearby park off-campus. He suggests there also will be some form of off-campus protest against the measures.

“We do have to play a game of chess with the authorities,” Schwartz said.

Many students at the University of Colorado and other campuses across the country have long observed 4/20. The counterculture observation is shared by marijuana users from San Francisco’s Golden Gate Park to New York’s Greenwich Village.

The number 420 has been associated with marijuana use for decades, though its origins are murky. Its use as code for marijuana spread among California pot users in the 1960s and spread nationwide among followers of the Grateful Dead.

Like most counterculture slang, theories abound on its origin. Some say it was once police code in Southern California to denote marijuana use (probably an urban legend). It was a title number for a 2003 California bill about medical marijuana, an irony fully intended.

Others trace it to a group of California teenagers who would meet at 4:20 p.m. to search for weed (a theory as elusive as the outdoor cannabis crop they were seeking). Yet the code stuck for obvious reasons: Authorities and nosy parents didn’t know what it meant.

In Colorado, recent 4/20 observations have blossomed alongside the state‘s medical marijuana industry. Approved by Colorado voters in 2000, medical marijuana boomed after federal authorities signaled in 2009 they would pursue higher-level drug crimes. All marijuana is illegal under federal law, though Colorado voters this November will consider a ballot measure to legalize it for recreational use by adults over 21.

A larger rally is planned for Denver near the state capitol on Friday and Saturday. Police have suggested they’ll be taking a hands-off approach to the gathering, which could draw tens of thousands of people, said chief organizer Miguel Lopez.

Others are rebelling against the gatherings.

In Colorado, several high schools across the state are hosting drug-free events on Friday. The University of Colorado’s student government supports the university’s anti-4/20 actions this year. And other Colorado students created a Facebook campaign urging their colleagues to wear formal clothing to school on Friday to repudiate the party-school reputation.

Campus police officers will be stationed at school entrances, allowing in only those with university IDs or permission. Anyone on campus without proper ID could be ticketed for trespassing, which carries a maximum $750 fine and up to six months in jail, said campus police spokesman Ryan Huff.

Anyone caught smoking on campus will be ticketed, just as they would any other day, Huff said. That includes anyone with a medical marijuana card, which requires that consumption be in private.

Off campus, Boulder police could also issue tickets for people smoking pot, and the Colorado State Patrol will be watching for any motorists under the influence, Huff said.

“This is not about the war on drugs. It isn’t even about marijuana per se,” insisted Hilliard, the university spokesman. “Ten thousand to 12,000 (people) doing anything in the academic heart of the campus would be a problem.