Dogs and Trees

Trees and Pee: The Science That Binds Them
by Adam Romsdahl CPDT-KA

A separation between socially accepted fallacies and the way nature works has somehow evolved in urban environments. You’ve seen the signs “curb your dog,” perhaps even seen someone lecturing an owner for letting their dog relieve themselves on or near a tree. Maybe you yourself have at one time or another scolded someone for the same activity. This is a thesis to describe why there need not be a quarrel between plant lovers and dog lovers in this modern day of science. In fact, nature has created a symbiotic relationship where the former should welcome the latter.

“What fallacy?” It is a fallacy to say that “pee kills trees.” Urine does not kill trees; in fact quite the opposite. Urine is one of the best natural fertilizers out there.[1] It is sterile therefore bacterial infections aren’t an issue; unlike fertilizers made from fecal matter. The content of urine is 95% water, 2.5% urea (which breaks down into ammonia), and 2.5% other constituents including, urea nitrogen, creatinine, nitrogen, uric acid nitrogen, ammonia nitrogen, amino nitrogen, sodium, calcium, chloride, sulphate, phosphate.[2] It also contains potassium and magnesium, both essential nutrients in the production of chlorophyll. Chlorophyll is used by the plant to absorb sunlight, converting it to energy used to break nutrients down into usable sugars.[3] All of these ingredients listed are non-toxic and have no harmful effects on the plant’s systems, including many ingredients that the tree needs to survive.[4]

To have knowledge of the nitrogen cycle it is vital to have a clear understanding of plant nutrition. The nitrogen cycle is part of the biogeochemical cycle, commonly known as the “circle of life”. The nitrogen cycle involves a series of bacteria that help break down the nitrogens, including ammonia, into the nitrates that will be absorbed by the plant’s roots (a.k.a. plant food).[5] The trace amounts of ammonia are consumed by a bacteria known as Nitrosomonas, which have enzymes in them to break the ammonia down into nitrite.[5] The nitrite is then broken down by another bacteria, Nitrobacters, and converted into nitrates.[6] Nitrates are absorbed by plant roots as well as recirculated into the atmosphere.[7]

“Doesn’t the acid in pee burn the tree?” Another false assumption is that the acid in urine can harm the tree. The pH scale goes from 1 to 14, 7 being neutral. Anything below 7 is considered acidic; the lower the number the more acidic something is. Conversely, anything above 7 is alkaline also known as basic. For the best conditions trees need nutrient enriched soil that is just slightly acidic, with a pH being between 6 and 7.[8] Urine in healthy dogs is right between 6.2 and 6.8 typically.[9] The perfect match.

One common problem is that urban soil becomes too high in alkalinity, which is the opposite of what the plant needs. The reason for this is that cement works very much like limestone, which leaches alkalinity into the rainwater tipping the pH scale in the ‘wrong’ direction.[10] Therefore acidity in urban environments needs to be constantly corrected, which urine happens to accomplish.

Not only are trees a natural filter for urine, but the trees are fed in the process. Therefore one might make the argument that the pee belongs on the tree and not in the street. Though urine is sterile, most people prefer not to walk in it or roll their strollers through it. Aside from social decorum there is a looming danger in allowing urine to be flushed into the rivers and waterways. Aquatic environments use the same nitrogen cycle as the soil. A big difference is that if the aquatic nitrogen becomes overly abundant algaes tend to form in dense plumes, through an ecosystemic response called “eutrophication”.[11] This can be dangerous to both plant and animal life in the water. Therefore washing the urine into the waterways not only deprives the urban trees but also creates an imbalance in the surrounding waters. New York has one of the few remaining estuaries in the United States; this is a way to help protect it.

“Ok that makes sense. But then tell me why urine kills grass?” This is a common argument and seemingly a good one. Urine will kill grass, but not for the reasons people think. When a dog goes in one spot over and over without the area being flushed with water, there is a buildup of concentrated nitrogen which can burn the root system.[12] Many canine supplements on the market claim to be able to counter this burn in the grass by neutralizing the pH with ingestible chemicals like methionine. Now these chemicals do in fact neutralize pH as claimed, the problem being that pH is not the reason urine kills grass.[12] The reason these chemicals work is that they simply make the dog thirstier. The dog will drink more water and dilute the nitrogen in the urine. The only way to ‘save’ a lawn is to water it, especially in the spots where the dog goes over and over. The same rule applies to trees, but for a tree to experience ‘nitrogen burn’ you would need to compare the scale of the tree to a shoot of grass. The adjusted amount of liquid would call for hundreds, if not thousands of gallons of urine at once that isn’t washed away.

“Why then do trees die in the city?” Far more complicated than the environmental processes which provide the tree nutrients, such as urine, are the factors that damage them. For instance, salts used on the sidewalks and roads are made up of sodium chloride. In heavy doses like this both of these chemicals damage trees. Chloride is taken up by the tree’s roots.[13] It enters the branches and sap, leading to ‘leaf death’ and preventing new bud development. Sodium uses the same ‘chemical route’ as potassium and magnesium restricting their uptake. This deprives the tree of essential macronutrients that create chlorophyll.[14] Salt in the soil also creates physiological drought to trees by absorbing the water and blocking intake.

Physical damage can also be detrimental to a tree’s health. The damage itself may seem like a mere scratch, but that tiny scratch can lead to infection in an urban tree.[15] Many types of fungi, bacteria and pests take advantage of these kinds of openings in trees. The current method of ‘fertilizing’ the trees is to inject nitrogen into the soil at a much higher concentration than urine. Often if the injection needle hits the roots it can cause physical damage resulting in what’s known as a canker.[15] Cankers look like a crater shaped dead section on the trunk or branches and are signs of either fungal or bacterial infection.

Though placing flowers and plants around a tree can make the city beautiful it also gives the tree competition for resources. Competitive species (e.g. flowers, ivy and decorative plants) are closer to the surface, thereby taking up a majority of the top soil nutrients and more importantly use up the water. Drought is the leading urban stress in trees. They simply are not watered enough. A higher amount of water and nutrients should be provided to urban trees especially because the root systems aren’t allowed to expand the distance that they should. Normally a tree’s root system would go out in each direction as far as the height of the tree.[16] In the city they are confined to tiny boxes, a fraction of what they need.

“Look, that’s all well and good, but it doesn’t change the fact that it’s the law.” This also is a social misconception. Many signs are posted telling people to “curb your dog.” Curb in this context would not refer to a sidewalk curb as you can’t noun a noun. Curb in this case is a verb meaning to control, as in ‘curb your enthusiasm.’ There is no such law that has the words “curb your dog.” What people mean to refer to is either one of two laws. The first commonly referred to as the ‘pooper scooper law.’ is New York Public Health – Title 1 – section 1310 which states that an owner must “remove any feces left by his or her dog on any sidewalk, gutter, street or other public area.” The law does not state where a dog may or may not relieve himself. The second aptly called the ‘leash law’ is New York Public Health – Title 24 – section 161(a) which states “the dog [must be] restrained by a leash or other restraint not more than six feet long.”

“What would you propose we do then?” Trees in both rural and especially urban environments should not have competitive species planted too closely, instead the best coverage is bark mulch. Mulch retains soil moisture, regulates temperature, improves efficiency of beneficial microorganisms and eliminates competitive species such as weeds.[17] Tree beds should be slightly raised to prevent salts and alkalinity from entering the soil. Selection of trees is important to make sure that the trees planted can even survive in an urban environment. NYC parks provides a great selection on their approved species list.[18] Most of all trees NEED to be watered often and plenty if they are going to have a good life in this environment of extreme urban stress. But more to the point of this thesis is to go ahead and let fido relieve himself on that tree, you’re doing more good than harm.

Works Cited
“Urine: The Ultimate ‘organic’ Fertiliser?” The Ecologist. Web. 28 Jan. 2015. .
Putnam, David F. Composition and Concentrative Properties of Human Urine. Washington: National Aeronautics and Space Administration;, 1971. Print.
Marchuk, William N. A Life Science Living Lexicon. Dubuque, IA: W.C. Brown, 1997. Print.
Saxe, George Alexander De Santos. Examination of the Urine; a Manual for Students and Practitioners. Philadelphia and London: W.B. Saunders, 1909. Print.
“Nitrosomonas.” Encyclopedia Britannica Online. Encyclopedia Britannica. Web. 24 Mar. 2016
“Nitrobacter Winogradsky.”Encyclopedia Britannica Online. Encyclopedia Britannica. Web. 24 Mar. 2016.
Coder, Kim D. 2012. Nitrogen & Trees. University of Georgia Warnell School of Forestry & Natural Resources Monograph WSFNR12-1. Pp.72.
Šimek, Miloslav, Linda Jı́šová, and David W. Hopkins. “What Is the So-called Optimum PH for Denitrification in Soil?” Soil Biology and Biochemistry 34.9 (2002): 1227-234. Print.
Johnson, Kristen Y., Jody P. Lulich, and Carl A. Osborne. “Evaluation of the Reproducibility and Accuracy of PH-determining Devices Used to Measure Urine PH in Dogs.”Journal of the American Veterinary Medical Association 230.3 (2007): 364-69. Print.
Significance of Tests and Properties of Concrete and Concrete-making Materials. Philadelphia: ASTM, 1978. Print.
“Eutrophication.” Definition Page. Web. 24 Mar. 2016.
“Dog Urine Damage on Lawns: Causes, Cures, and Prevention.”Dog Urine Damage on Lawns. Web. 24 Mar. 2016.
White, P. “Chloride in Soils and Its Uptake and Movement within the Plant: A Review.” Annals of Botany88.6 (2001): 967-88. Print.
“Understanding The Role Of Magnesium In Plants – How Do Plants Use Magnesium.” Gardening Know How. 15 Apr. 2012. Web. 24 Mar. 2016.
“Trees and Shrubs: Diseases, Insects and Other Problems.”SULIS: Sustainable Urban Landscape Information Series : University of Minnesota Extension. Web. 24 Mar. 2016.
Susan Day, Eric Wiseman. “At the Root of it” International Society of Arboriculture. Web 2010.
“Mulching Trees and Shrubs.”Mulching Trees and Shrubs. Web 2016 2016

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