Phosphorus is a component of most fertilizers that helps plants to grow. When too much is applied or is applied at the wrong time—such as right before it rains—most of it is washed away and ends up in the local waterways. This type of pollution is called nonpoint source pollution. It causes eutrophication (a reduction of dissolved oxygen in water bodies caused by an increase of minerals and organic nutrients) of rivers and lakes. This reduced level of oxygen in water ends up suffocating fish. Several municipalities have banned the use of phosphorus-containing fertilizers for turf or lawn areas. These laws are designed to protect local water quality in lakes, streams, and ponds. According to several experts, in most cases phosphorus is not needed to maintain a healthy lawn. Retailers near towns that have enacted a ban are required to alert customers about the prohibition of phosphorus in fertilizers for lawn and turf by posting a sign where fertilizers are sold.