Anyone who has a dog knows that dogs poop. They poop a lot. Part of being a responsible dog owner is picking up this hazardous waste material. Unfortunately, huge numbers of people do not feel like stooping and scooping. This is a problem for all sorts of reasons:
- Stepping in dog poop is gross
- Often, children play in the grass where dogs poop
- Dog poop can contain harmful bacteria (which is why we process human waste)
- Dog poop is like a little fertilizer packet that can wreck havoc on your lawn, community, or nearby water bodies like oceans and rivers
I’m going to talk about that last point and try to put it in a frame of reference. I did a little bit of journal searching and some back of the envelope calculations to determine just how much fertilizer a dog puts out. Typically, when we talk fertilizer we think of nitrogen and phosphorus as the two elemental biggies, and I’m going to focus on nitrogen.
To determine how much nitrogen dogs dump onto your lawn each day, we need to know how much a dog actually poops. Fortunately, people know this. A 40 pound (18 kg) dog puts out about 0.07 pounds (30 g) of dry poop material per day  . It looks like more than that because poop contains a lot of water, but once you dry it down, this is what’s left over. We also know that dog poop is about 5.21% N (per unit of dry material) , which means a 40 lb dog squats out about 0.004 lbs of nitrogen onto your lawn per day.
Doesn’t sound like much? Well, Osmocote fertilizer is 15% nitrogen . So 0.004 lbs of nitrogen that your dog puts onto your lawn is equivalent to about 0.03 lbs of Osmocote slow-release fertilizer per day. Now, Osmocote recommends only 0.02 lb pounds of fertilizer per square foot applied every three to seven months. Your 40 pound dog is dumping out more nitrogen per day than Osmocote’s recommended once or twice per year fertilizer rate.
Odds are you’re not the only dog-lover in your city. In fact, roughly 36.5% of households own dogs, often more than one . Using demographic pet data, we can estimate how many dogs are in a given community. For example, the greater Miami metropolitan area has 2,097,626 households . This translates to approximately 1,225,013 dogs in the Miami area.
Not all dogs are equal. In Miami, most are much smaller than 40 pounds . We can assume a particular distribution of dog body weights that is heavily skewed towards small body sizes, like this:
If we assume constant poopage across body sizes, given the information above we know that dogs excrete roughly 0.002 pounds per pound of body mass per day. Multiply this number by each weight class, and we can estimate the amount of poop that a dog puts out in each weight class (i.e. a 5 pound dog puts out 0.002 * 5 = 0.01 pounds of poop per day). Then, multiply that number by the estimated number of dogs in each weight class (above) and we get the total amount of excrement in each weight class (isn’t math awesome?). We can add all of this poop up, which comes out to a staggering 65,039 pounds of poop per day in the city (anyone looking to solve the oil crisis should look into dog poop as an alternative energy source). This equates to 3,382 pounds of nitrogen per day or 22,457 pounds of fertilizer per day. This is equivalent to over 450 bags of Osmocote (50 lbs) dumped across Miami every day. It doesn’t stop on your lawn either. This stuff gets into the groundwater and leaks into nearby rivers, lakes, canals, and yes, even the ocean.
So do us all a favor. Pick your dog’s poop up.