As of Jan. 1, the city government has banned the use of fertilizer that contains phosphorus - a nutrient that helps plants blossom, but also runs off into the Chesapeake Bay in rainwater and spurs the growth of harmful algae.

The measure was quietly passed by the City Council over the summer as a step to reduce harmful nutrient pollution that runs off the city's streets and lawns.

There are some exceptions - for newly planted grass, if soil tests prove the ground is lacking phosphorus - but most of the 10,000 property owners in Annapolis will have to grow grass without a phosphorus boost.

And it's still OK to use phosphorus-containing fertilizer on trees, shrubs, vegetable gardens and indoors. Compost is OK, too.

"We're not saying you can't use phosphorus, but you have to show you need phosphorus," said Frank Biba, the city government's chief of environmental programs.

Annapolis may be the first government entity in Maryland to ban fertilizer with phosphorus, although jurisdictions in other states have similar laws.

Phosphorus has already been phased out of laundry detergent and soon will be banned from dishwasher soap sold in Maryland.

"One of the things we know is that phosphorus creates algae blooms, which create dead zones," said Alderman Julie Stankivic, a Republican who was the lead author of the measure, which passed the City Council unanimously.

The decaying algae blooms rob the water of life-sustaining oxygen. Fish, oysters and crabs suffer from the oxygen-deprived dead zones that appear in the Chesapeake Bay each summer.

"Essentially, what we're trying to do is limit the amount of phosphorus that does enter the bay from fertilizer that comes from non-agricultural sources," she said.

In addition to the limits on using phosphorus-containing fertilizer on lawns, there are requirements for businesses, too.

Businesses within city limits that sell fertilizer have to post signs alerting customers to the new law. And starting in 2010, they'll have to pull phosphorus-containing fertilizer off the sales floor, though they keep it in the back and sell it by request.

Mr. Biba of the city environment office has been handing out posters to the handful of businesses that sell fertilizer. He said he doesn't expect to have fertilizer cops poking around people's yards and assessing fines - which are $100 a pop. More likely, city inspectors will respond to complaints.

"Enforcement will be interesting," he said.

Businesses that don't follow the fertilizer-selling rules can face a $500 fine, as can lawn companies that improperly use phosphorus-containing fertilizer.

Education will be important, Mr. Biba said, though he said it still will be a few more months before homeowners start thinking about greening up their lawns.

The new law applies to all city-owned property - including the Eisenhower Golf Course in Crownsville - but does not cover state-owned land, Housing Authority properties or the Naval Academy, Mr. Biba said.

Improper use of fertilizer on residential lawns is a big problem for the bay. One federal study estimated that 11 percent of excess nutrients entering the bay in the Washington, D.C., metro area came from residential lawns.

Suburban homeowners tend to use too much fertilizer, according to the federal-state Chesapeake Bay Program. When the grass and plants don't use up all the nutrients, rainwater washes it into streams and ultimately, the bay.

Phosphorus is represented by the middle number in the three-number rating system for fertilizers. City property owners will now need to look for fertilizers that have a zero in the middle.

But as large as 10,000 properties in Annapolis may seem, the city contributes just a small portion of pollution in the Chesapeake Bay. Still, Mr. Biba and Ms. Stankivic put importance on the symbolic nature of the bill.

"What's the impact on the bay with Annapolis doing it? Minimal," Mr. Biba said. "But what if other jurisdictions pick up on it?"

Ms. Stankivic said she thinks Annapolis residents will buy into the idea of curbing phosphorus use through the fertilizer law. She's hoping to get Anne Arundel County or state lawmakers to pass versions of the law, too.

"One of the key things is by adopting this law, we help to educate the public," she said. "And in doing so, we will probably get good adherence, because people who live in Annapolis and Anne Arundel County, they care about the bay and they want to do the right thing, but sometimes they don't know what it is."