In the case of fertilizers, more doesn't mean better. Overuse of synthetic fertilizers is especially dangerous because it can "burn" plants. It can also result in unnatural growth spurts which can in turn attract more pests and diseases.
More importantly, however, is the toxic runoff that results from watering excessively-fertilized plants and lawns. This runoff is called "non-point source pollution" because the pollutant cannot be tracked back to one single source. When large amounts of nitrogen and phosphorus are present in local streams, algae blooms are created that can destroy ecosystems. When algae starts dying, it uses up the oxygen and kills off the aquatic organisms.
Carefully follow application instructions provided on fertilizer containers. Better yet, compost your grass clippings, leaves and yard trimmings, and use the compost as your fertilizer.
