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Getting ready to fertilize your lawn? Make sure you aren't doing so illegally.

A ban on lawn fertilizer containing phosphorus goes into effect in Wisconsin on April 1, so it will be illegal except in limited circumstances to put that type of fertilizer on lawns, including golf courses and municipal properties.

The Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) sent out a reminder on Thursday about the new fertilizer law.

Retailers will not be able to display turf fertilizers containing phosphorus, but retailers will be allowed to post signs that say fertilizer with phosphorus is available upon request.

The law is designed to protect Wisconsin waterways from phosphorus runoff, a main contributor to algae problems.

A ban on phosphorus in lawn fertilizer was enacted in Dane County and the city of Madison five years ago.

The state Legislature passed similar legislation last spring, and Gov. Jim Doyle signed it into law in April 2009.

Homeowners will be allowed to apply fertilizer containing phosphorus if the fertilizer is being put on new lawn or if a soil test shows a deficiency in phosphorus. Fertilizer with phosphorus will be allowed to be applied to farmland, pasture and home gardens.

How do you know if your fertilizer is phosphorus-free? The package should have three numbers on it, denoting (in order) the percentage of nitrogen (N), phosphorus (P) and potash (K) inside. If the middle number is 0, the fertilizer is phosphorus-free.

The new law also prohibits all types of turf fertilizer from being applied to frozen ground or to impervious surfaces such as driveways or sidewalks, even if the application is unintentional.

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"If you get fertilizer granules on the driveway or sidewalk, sweep or hose them into the grass or sweep up and put back into the bag or in the trash," said Charlene Khazae, DATCP fertilizer program manager.

If you have lawn fertilizer from last year that has phosphorus in it, don't use it.

"After April 1 that old fertilizer is not legal to use unless you meet the exceptions (new lawn, soil depleted)," Khazae said.

Throw the old fertilizer in the trash or take to a local Clean Sweep program, she said.