Google is trying to clean up its search results by cracking down on dubious Web sites that contain little content but lots of ads.

The search giant has notified a number of Web publishers in the last few days that they will be dropped from its popular “AdSense” program starting June 1.

The cut-off notices jolted the online ad world, where hundreds of thousands of people make money in exchange for allowing Google to place ads on their Web sites.

Google’s AdSense software has made it easy for advertisers to piggyback on any Web site – no matter how obscure – that attracts an audience.

For example, on a travel Web site, Google’s AdSense will automatically serve up relevant travel-related ads on that site. Web site publishers then get paid each time a visitor clicks on one of the ads supplied by Google.

The easy money has led to a rise in “made-for-AdSense” Web pages that critics say clutter up the Internet and divert online searches. These content-free sites, which often are nothing more than links to other sites and a bunch of Google ads, exist solely to exploit AdSense.

“The economy has built up to game the Google system,” said Darren Chervitz, the director of research for Jacob Asset Management.

Web site publishers do it by pocketing the difference between what they pay Google to drive traffic to their site and the amount they get for running Google ads.

For instance, a publisher can bid on a cheap search term, say, “purple raincoat” so that its site purporting to be about raingear is displayed each time someone searches for the term on Google.

The publisher may pay a nickel to Google each time someone clicks on the link to their Web site, which may be nothing more than a picture of a purple raincoat.

The publisher makes money if, on average, they collect a dime each time a user then clicks on one of the Google-supplied AdSense ads.

Although Google still makes money off of these sites, they don’t like them because they hurt the quality of search results and reduce the “click through” rate for advertisers.

“If advertisers get a bad return on their investment, they will stop spending money,” said Jeremy Schoemaker, an AdSense expert who runs the popular ShoeMoney.com blog.

Search experts said Google routinely cuts off publishers who run afoul of AdSense rules, but they believe this latest round of notices was a more widespread effort to clean up its ad network.

“I don’t really remember where this many people reported it at one time,” said Chris Winfield, president of Internet search marketing firm 10e20.

Google confirmed that it had sent out notices, but characterized it as part of an ongoing review.

“In some cases, violations of our program policies will result in termination from AdSense,” Google said in a statement.

holly.sanders@nypost.com