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

After more than 10,000 people - students and nonstudents - attended last year's marijuana rally on Norlin Quadrangle, university officials this year applied the stinky fertilizer to the quad to deter pot-smokers.

They also closed the campus Friday to all unauthorized visitors and were offering a free campus concert by hip-hop star Wyclef Jean timed to coincide with the traditional 4:20 p.m. pot gathering. His contract bars him from making any direct references to marijuana, other drugs, or 4/20.

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," university spokesman Bronson Hilliard said. "This is a gathering of people engaging in an illegal activity."

Student organizer Daniel Ellis Schwartz, and other supporters of the 4/20 smoke-out planned to move it to a nearby park off-campus.