Lawn fertilizers are not dangerous if they are applied properly and if the label instructions are followed. In some cases, lawn fertilizer manufacturers may recommend keeping children and pets off of the lawn for a specified period of time. In other cases, the manufacturer may recommend not applying the fertilizer when rain is forecast. The key to safe use of lawn fertilizers is following label instructions.
Dangers to Children
Properly applied, fertilizers that don't contain pesticides are generally not harmful to children. However, many commercially produced lawn fertilizers also contain herbicides. Those herbicides can be harmful to children. Lawn fertilizers often contain ammonium phosphate for phosphorus and ammonium nitrate for nitrogen. Ammonium nitrate, for example, can be toxic in high concentrations. Children have lower body weights and may have a lower tolerance to different forms of chemical fertilizers. Ammonium phosphate can also be harmful if swallowed. Keeping children off of your recently fertilized lawn is the best way to prevent inadvertent ingestion of potentially toxic lawn fertilizers.
Dangers to Pets
Pets can suffer from chemical sensitivities that are very similar to children. However, because toxic levels are often measured in milligrams per pound of body weight, some pets are more susceptible to poisoning because of their lower body weights. Both cats and dogs are known to eat grass. By eating recently fertilized grass, they are also eating any fertilizer that has not been washed from the leaves and watered in to the soil. Be sure to carefully follow all watering instructions for the particular brand of fertilizer.
Potential Soil Problems
Over fertilizing lawns can cause changes in pH that can sometimes result in the death of soil organisms that help to convert soil organics into nutrients necessary for your lawn. By fertilizing at the low to moderate end of the suggested application rate, you can help to minimize later soil problems.
Phosphorous and nitrogen runoff from lawn fertilization can cause systemic environmental problems in other nearby areas. Phosphorus runoff can result in lake and river enrichment. This enrichment creates algae and weed bloom because of the increased nutrients. Algae and weed blooms can remove other water nutrients. In fact, toxic algae blooms can shut down shellfish farms and cultivation in ocean shallows.
Excessive nitrogen can percolate down through the soil and contaminate groundwater. Excessive nitrates and nitrites in well water can lead to reduced thyroid function and an increased need for vitamin A. Nitrates and nitrites can sometimes form nitrosamines, which are a common carcinogen.