Urine as a Fertilizer: Benefits, Concerns, and Discoveries

Most people don’t think about their pee a lot. Under normal circumstances, once the toilet is flushed, the urine is forgotten. The liquid may be important, however. Plants require nitrogen and phosphorus in order to grow. Human urine is a good source of these elements and is readily available. This suggests it could be used as a fertilizer for the plants that we cultivate.

Though the idea may sound strange and perhaps even repulsive, some people are already using urine as fertilizer, and some have been doing so for some time. Scientists are becoming increasingly interested in the process. Using a urine fertilizer not only has benefits but also avoids some of the drawbacks of commercial fertilizers. It has some disadvantages of its own, but these problems may be solvable.

A human urine sample (Photo by Mark Hamilton, via Wikipedia, public domain license)

Functions of Nitogen and Phosphorus in Plants

Nitrogen and phosphorus play many vital roles in plants. For example, plants need nitrogen to make the proteins in their body, including the enzymes that control chemical reactions. Proteins consist of chains of amino acids. These molecules contain nitrogen. In addition, nitrogen is an essential component of chlorophyll molecules. Chlorophyll absorbs light, enabling photosynthesis to occur.

Like nitrogen, phosphorus has many roles in a plant. It’s an essential part of ATP (adenosine triphosphate) molecules. ATP provides the energy needed for chemical reactions. Nitrogen and phosphorus play the same roles in our body as in plants, except in the case of chlorophyll, which we don’t produce. A deficiency in either element would hinder plant growth.

Spreading Antibiotic Resistance via Urine

Though the use of urine as a fertilizer is an interesting and potentially important idea, using “raw” urine fresh from the body might not be a good plan. Researchers have discovered that urine can contain bacteria even when a person doesn’t have a urinary tract infection. The old idea that urine is sterile is not necessarily true. Viruses, components of medicinal drugs, and other chemicals specific to the person’s body may also be present in the liquid. There’s a concern that some of the bacteria in the urine may contain genes that confer antibiotic resistance. Genes can pass from one bacterium to another in certain circumstances. Bacterial resistance to the antibiotics that used to kill them is becoming a serious problem.

Fortunately, researchers have found away to reduce the risk of spreading antibiotic resistance via urine. They say that if the liquid is aged “for several months” in a sealed container, 99% of the bacterial genes providing antibiotic resistance are inactivated. They also say that the genes are broken down by various microorganisms in the urine.

Concentration of a Urine Fertilizer

Most of the nitrogen in urine exists in the form of urea. Urea is too concentrated in urine fresh from the body for most plants. It’s often recommended that one part urine is mixed with ten parts of water for fertilizer use and that the liquid is applied to the soil without touching the above-ground parts of the plant. A much higher dilution would likely be needed for potted plants. I have read that a thirty fold to fifty fold dilution is required for houseplants, but I’ve no idea whether this recommendation has been obtained experimentally. Though dilution of urine is probably the best tactic for someone using the liquid in their home garden, in commercial situations with large, open fields, some people have used a urine fertilizer successfully at full concentration.

I suspect that the effects of a urine fertilizer’s concentration depend in part on the type of plant being grown and the landscape. Some plants, such as beets, have a high demand for nitrogen. A large and bare area of soil may be able to accept full-strength urine. The practicality of doing this is a concern, however, depending on the size of the area that needs to be fertilized and whether the urine is in a liquid form or the solid form described below. The scientists who completed the described project say their research was specifically aimed at creating a fertilizer for farmland, though it could apply to other situations as well.

Fertilizer burn (Photo by Fenrisulfur, via Wikipedia, CC BY-SA 3.0 license)

Creating a Solid Fertilizer From Urine

Scientists in Sweden have designed a way to make a solid fertilizer from urine. They have also planned a way to make the process sustainable. They call their plan the alkaline urine dehydration process. The steps in the process are summarized below.

  • First, urine would be collected from specially designed toilets.
  • A substance such as calcium or magnesium hydroxide would then be added to the urine to raise the pH, making the liquid more alkaline or basic. This would prevent an enzyme called urease from converting urea to ammonia, a substance than can create a bad smell.
  • The water in the urine would be evaporated (and then condensed for use in other processes).
  • The solid remains of the urine could be used as a fertilizer.

The researchers point out that while electricity would be needed to evaporate the water from the urine, the process could be done by using solar energy, making it more environmentally friendly.

The majority of fertilizers are either made by converting nitrogen in the air to ammonia, which alone consumes 2% of the world’s energy and relies heavily on fossil fuels, or by mining finite resources, like phosphate rock.

Quote from Prithvi Simha, Bjorn Vinneras, and Jenna Senecal, via The Conversation

The researchers say that their trial urine fertilizer worked as well as traditional fertilizers in their field tests. They also say that the alkalizing agents would be inexpensive. They don’t say how environmentally friendly it would be to produce the agents.

(Converting urine to fertilizer) would require a service chain capable of supplying homes with alkalising agent, collecting the dried urine and processing it into fertiliser for farmers to use. A similar service chain already exists for the recycling of plastics, metals, paper and glass – dried urine could simply be another component.

Quote from Prithvi Simha, Bjorn Vinneras, and Jenna Senecal, via The Conversation

Many of the nutrients that we ingest but don’t need are sent into the urinary bladder and then eliminated from the body when we pee. It does seem like a waste to simply send them down the drain. Making use of urine is an excellent plan. The substance seems to have great potential as a fertilizer. An effective and sustainable process for making soil fertilizer from urine could be a beneficial development for people and the Earth.


Human urine as a fertilizer from Scientific American

Antibiotic resistance and urine fertilizers from the phys.org news service

A solid urine fertilizer for farmland from The Conversation