Pet Feces and Urine

• One of the most obvious causes of unpleasant smells in the backyard is dog feces. Even if you don't have a dog, the odor generated by the dog feces left in your neighbor's yard may be the cause. Cats may be using your yard as a litter box. Another possibility is urine, especially if cats spray in your backyard.
  
  

Stagnant Water

• Stagnant water not only attracts unwanted mosquitoes, it smells unpleasant as well. Sources of stagnant water include puddles of water left from rainfall or water leaks, ponds, spas, forgotten buckets of water and unattended children's wading pools.
  
  

Compost Piles

• If you or one of your neighbors has a compost pile, the pile can stink if it's not properly tended. If it's too wet, too dry or not turned on a regular basis, a compost pile produces unpleasant odors. Instead of removing the compost pile, turn the pile with a pitchfork, and maintain a proper moisture, which should be that of a wrung-out sponge. You might need to add more leaves or fertilizer to the pile for it to maintain its proper composition and allow for decomposition without generating bad smells.
  
  

Septic Tanks

• Check your septic tank, as the smell might be an indication of a larger problem. If the smell is not coming from your septic tank, there could be a problem with your neighbor's septic tank or cesspool.
  
  

Dead Rodents

• Dead and rotting flesh generates a distinct and unpleasant smell, similar to that of fertilizer. If that is the smell you notice, start looking for the remains of a dead rodent or animal in the vicinity of your yard, or in a nearby trashcan.
  
  

Debris

• Garbage and debris generates bad odors. There might be a pile of trash or rotting leaves in your backyard, or kitchen debris sitting in a trashcan. Clean up the yard and remove the trash and debris from your property to get rid of the smell.
  
  

External Sources

• That unpleasant smell may not be coming from your house, or from one of your nearby neighbors. It might be drifting your way from another external factor, such as the city's sewer plant, a dairy farm in the area or another odor-generating business.