Fertilizer Burn

• Quick-release fertilizers are designed to provide a large amount of nitrogen to the soil. In moderation, this can be a good thing. If you use too much quick-release fertilizer and then overwater or have a heavy rain, your soil can suddenly become saturated with nitrogen. This can kill plant roots. If plants get five to 10 times the amount of nitrogen they need, they will "burn," turning the color of straw and dying from the tips down. Once this happens, plants can not be saved. In the case of lawn grass, water heavily to flush away the excess chemicals.
  
  

Overgrowth

• A more common problem with overfertilization is that your plants will produce a lot of leaf matter very quickly. Overfertilized tomatoes, for example, will grow leaves very quickly and produce less fruit. Overfertilization of lawns can mean the grass grows very quickly, needing to be mowed more often; this can be tough on your mower as well as your grass.
  
  

Weed Encroachment

• Fertilizers don't just affect the plants to which they are applied. In fact, fertilizing the soil around your "wanted" plants can also encourage weeds. This can particularly affect lawn grasses that have been growing very quickly due to overfertilization: because of their fast growth, they are mowed more often, which can kill them off and allow hardier weeds to take hold. Weeds can sometimes be treated with a "weed-and-feed" fertilizer that kills some plants while encouraging others, but these chemicals can be dangerous to use with sensitive plants such as dogwoods.
  
  

Fungi

• Brown patch and Pythium blight are two types of fungus-caused diseases that commonly affect lawns and golf courses. These diseases are often related to overfertilization: they thrive on heavily fertilized, moist and poorly-drained soils. Brown patch and Pythium are most common in warm, humid weather. They spread via mycelium (fungus threads) and can be devastating to plants.
  
  

Recommendations

• Instead of giving plants one big dose of fertilizer, fertilize in small amounts throughout the season. If this is too much work, choose a slow-release nitrogen fertilizer. Follow the manufacturer's instructions carefully, and if you aren't sure, err on the side of caution. It is much easier to add more fertilizer later than to flush it out of the soil when plants start dying.