Urea

Have you ever peed on a tree?

You may have heard garden lore about peeing on lemon trees to improve tree health and fruit flavor, but is it true? And is it safe? [Mostly yes, and yes.] And what is in that bag of urea, advertised as such an excellent source of nitrogen?

Nitrogen deficient young cabbage (Rasbak) CC BY-SA 3.0

As the human population and the demand for food and water continue to increase, new solutions are being sought. Using urea and urine to fertilize edible plants is one of those solutions. Before you get grossed out, you need to know that urine is practically sterile and it is an important part of the nitrogen cycle. As mammals urinate on the ground, nitrogen fertilizes the soil, helping plants grow.

What is urea?

Our bodies use urea to excrete nitrogen in our urine. Urea is colorless, odorless, soluble in water, and non-toxic. Urea and urine both contain a lot of nitrogen, and all plants need nitrogen to grow and thrive. Nitrogen is the fundamental building block for chlorophyll and plant enzymes and proteins, including a plant’s DNA. Without nitrogen, photosynthesis cannot occur.

Over 90% of the world’s manufactured urea is used in agriculture as the most concentrated, affordable nitrogen source available to plants. Pure urea has an NPK value of 46-0-0. For comparison, ammonium sulphate has an NPK of 21-0-0, followed by blood meal at 13-1-1. On average, human urine has as NPK value of 18-2-5. This means urine contains 18% nitrogen (N), 2% phosphorus (P), and 5% potassium (K). The potassium and phosphorus found in urine, human or otherwise, are in forms that are quickly absorbed by plants.

Keep in mind that nitrogen, in all its forms, is highly mobile and easily leached into ground water. This causes out of control algae blooms and water pollution. Regardless of where you get your nitrogen, don’t add more than is needed. [Get a soil test before adding anything!]

How urea feeds plants

When urea comes into contact with the soil, specific bacteria convert it into ammonia (NH3), ammonium ions (NH4+), and bicarbonate ions (HCO3−). Other bacteria use the Calvin Cycle to oxidize the ammonia, converting it into nitrites, in a process called nitrification. This makes the ammonium and nitrites readily available to plants. It also acidifies the soil slightly.

Research published in the 2007 American Chemical Society [J. Agric. Food Chem.200755218657-8663] reported that cabbage grown with human urine as a fertilizer grew larger than those fertilized with industrial fertilizer. Those same plants showed less insect damage that their commercially fed counterparts, though unfertilized cabbages showed the least amount of insect damage. The same study demonstrated that urine performed equally well as commercial fertilizers on cucumber and barley crops, without increasing the risk of disease. In each case, there was no noticeable change in the flavor of the food being grown. [So much for those lemons! ] 

In a similar study, published by Cambridge University Press, larger harvests of amaranth were noted on the crops fed with urine. Other studies have found similar results with beets and tomatoes. In fact, beets grown with urine tend to be 10% larger than those grown with commercial fertilizer.

Top dressing your garden with urea or urine just before it rains or irrigating can add a lot of nitrogen to the soil. If you buy a bag of urea, be sure to keep it tightly closed. Nitrogen evaporates rapidly into the atmosphere and urea absorbs water from the air very quickly. Also, you should know that urea can contain biuret, an impurity that can be phytotoxic, or poisonous to plants.
Thank 
Did you know that the average adult in the Western world pees enough in a year to fill three bathtubs? That’s a lot of plant food!

Basil plants fertilized with urine (left) grow stronger and healthier than those without (right) (SuSanA Secretariat) CC BY 2.0

But there’s a catch.

Too much fo a good thing can be a bad thing

There is so much nitrogen in urea and urine that it can prevent seeds from germinating and burn seedlings, roots, leaves, and your lawn. This is especially true if a plant’s moisture content is low. Urine also contains salt, which can dehydrate or kill plants. You know those dead spots in the lawn where Fido relieved himself? That damage is nitrogen and salt burn.

​If you want to use urine to water and feed your garden, it is a good idea to dilute it first. The recommended dilution is one part urine to 2-8 parts water, depending on who you ask. This nutrient rich mix can then be dispersed using a watering can. If you are taking prescription or recreational drugs, you may want to discuss transference with your doctor or local pharmacist first. Chemicals in our water and food supplies is real.

Otherwise, go unzip yourself at the more mature plants in your backyard, such as that lemon tree, or you can contribute some nitrogen and moisture to the compost pile and conserve some tap water.