Maryland has joined the list of states banning phosphorus in lawn fertilizers used on established lawns. The new law also limits the amount of nitrogen fertilizer that can be used, and restricts the timing of applications to warm-weather months. Other states with restrictions on lawn fertilizers include Florida, New York, New Jersey, Michigan and Wisconsin. (Organic fertilizers may contain small amounts of phosphorus and are exempt from the laws, and the fertilizing of newly seeded lawns is also exempt.)

Several others states are considering similar bans, and a major manufacturer of synthetic chemical fertilizer, Scotts, recently announced it will phase out the use of phosphorus in all its lawn fertilizers by the end of 2012.

Why?

While phosphorus is important for root growth and is considered critical in establishing new lawns, existing lawns require little if any phosphorus. Meanwhile, phosphorus and nitrogen may help lawns grow at certain times, but their over-use leads to a major source of non-point-source water pollution. Unlike water pollution we might imagine spewing from an industrial pipe, non-point sources like lawns are diffuse, but nonetheless significant sources of pollution.

Excess nutrients feed algae in lakes, streams and bays, and when algal blooms die, the process starves the water of oxygen, creating “dead zones” where fish and other marine creatures can’t survive. The Mississippi River, which drains water from dozens of farming states, discharges so many nutrients into the Gulf of Mexico that a dead zone the size of New Jersey forms each year. A similar problem, fed by farms and suburban runoff, occurs in the Chesapeake Bay, the Great Lakes and in countless other bodies of water across the U.S. and world.

Paul Tukey, author of The Organic Lawn Care Manual ($13.50 at amazon.com) recommends using compost tea instead of commercial fertilizers, fertilizing and mowing only when light rain is in the forecast, and — most importantly of all — fertilizing only after first testing your soil to see what nutrients it really needs. See more of Tukey’s lawn fertilizer tips.

Related: 21 Organic Lawn Care Tips from Paul Tukey