The Tree Center

Urea Crystals

Written by Dave Gs • September 23 What Is Urea?

Urea is a form of nitrogen that is made in the kidneys to excrete excess nitrogen, mostly from eating protein, which cannot be stored in the body. It is also manufactured on a large scale for use as a fertilizer, although there are some other uses in explosive making, medicine and even cleaning car exhaust gases.

Almost all the urea manufactured is converted into urea-formaldehyde, which is the most common type of nitrogen fertilizer used in agriculture and gardening.

Urea is very soluble in water, so if applied as a fertilizer by itself only a very small amount can be used and it will quickly be carried away in drainage. The urea cannot be used directly by plants but it is naturally converted into ammonia on contact with water in the soil. This then dissolves in the water and can be absorbed by plants for growth. Nitrogen is needed by plants to make protein and also to make chlorophyll, the green pigment in plants used to trap light and make sugars to grow.

However, if there is too much urea or ammonium in the soil it will draw water out of the roots and cause ‘fertilizer burn’, with the leaves shriveling and dying, often also killing the plant. This means that pure urea must be applied in very small amounts very often to be effective as a fertilizer. This is also what happens when dogs or cats urinate on your lawn.

Methods For Slowly Releasing Urea

So scientists worked on ways to save the cost and labor of these frequent small applications. They developed two methods of slowing the release of urea.

Water-Resistant Coating

The first method adds a water-resistant coating to a granule of urea and is called controlled-release. Examples are Nutricote™ and Osmocote™. These forms have a resin or polymer coating to slow down the release. Release of nitrogen is mostly influenced by temperature and higher temperatures accelerate the release. This is usually a good thing, since during warm weather when plants are growing more will be released, but in cooler weather less will be released, matching the growth of the plants and preventing pollution of drainage water. These granules are often found in fertilizers that last a whole season and are great for pots and planters. Other plant nutrients are added to make a complete fertilizer.

A less-expensive type of coating is SCU (sulfur-coated urea). Here the urea granules are coated with molten sulphur, wax and then clay. The activity of soil microbes and water penetration through cracks in the coating allow the urea to escape into the soil. This material is cheaper to manufacture, so it is widely used in lawn fertilizers. Be careful handling lawn fertilizer, because if the granules are crushed they will release all the urea at once and cause burn which could kill the lawn.


For cheaper fertilizers, especially for agriculture, the urea is turned into a less-soluble form called urea-formaldehyde. This does not contain the formaldehyde used to preserve dead animals. Since this does not immediately dissolve in water, it will stay in the soil as a solid until it is broken down. The molecules of urea-formaldehyde are of different sizes, depending on how exactly they are manufactured, and they have different degrees of solubility in water, affected by temperature.

As well, soil microbes are needed to break down this material so plants can absorb it. Microbes are also more active at higher temperatures, so overall the rate of release of the fertilizer depends on the temperature, since both solubility and the amount of microbe activity is influenced by the soil temperature. Release is very slow below 50 degrees, so this material can be applied to lawns and gardens in the late fall. No nitrogen will be released until the soil warms up in the following spring, and a spring fertilizer application is not needed– a job saved at a busy time of year.

Should I Use Urea On My Plants or Lawn?

So you probably did not know that when you use a fertilizer on your trees, your lawn, your vegetables or your flowers, you are probably using urea in one form or another. Because it is also present in urine, there are those who suggest peeing on your compost heap is a good idea. The choice is yours!

These kinds of fertilizers are essential for large-scale agriculture to feed a constantly growing world population. Chemically speaking what the plant takes up through its roots is identical, no matter where it came from. Organic material is essential for preserving the quality of the soil, but it is an often inefficient way to give plants nitrogen. If chemical fertilizers are used wisely they can be a great benefit to humanity.

Comments 10 comments

  1. Hi there
    We have a mature mulberry tree that is fruiting at present
    After many years the local bats have finally discovered the free meal and are … expressing …. their gratitude – all over the neighbours paving

    Is there a way of deliberately over-using nitrogen fertiliser to stop it fruiting but in a way that is safe for the tree ?


    1. December 26, 2017 by David Goodfellow

      Hi Geoff,
      Mulberry trees planted near paving sure can be a mess, and with the bat poop too, that sounds nasty! Sadly, I don’t think over-feeding a tree that is already a vigorous grower is going to work – probably just turn it into an even bigger fruit/poop producer. A better approach might be to prune very hard each year, so that you get only young, one-year old shoots, and so no fruit. However on a ‘mature’ tree that doesn’t sound like a realistic option. Maybe remove it completely and plant a beautiful flowering tree that won’t make a big fruit mess!

  2. October 29, 2018 by Aaron

    Hi Geoff,

    This is an interesting read.

    I’ve planted approx 30 magnolia Little gems at are approx 1.2m tall now. I’ve been removing the flowers to try to encourage the plants to take off with a heap of new growth.

    What are your thoughts on potentially using ag. Urea to encourage growth? I’m trying to create privacy from neighbours. I obviously don’t want to harm to trees either. If not urea, what can I use?

    1. October 29, 2018 by G Dave

      I don’t think anyone, farmers included, are using straight urea – the agricultural urea is probably urea-formaldehyde. I would not suggest using a straight nitrogen source on your trees – it will probably cause unbalanced growth, with week stems, and be more pest and disease prone. Better would be to use a blended fertilizer for evergreen trees. These have a lot of nitrogen, but with phosphates and potassium too, plus iron and other micronutrients, for balanced growth. If you choose something more expensive you should be able to get a granular form with Osmocote or SCU in it, so you only need to feed once – in early spring.

      Removing the flowers will only have a limited value – enjoy them and remove them after flowering. That way you prevent seed formation, which does divert a significant amount of energy away from growing leaves and stems. Seems a shame to remove the flowers, when youthful flowering is one of the reasons to grow this variety. . .

      Remember that ‘Little Gem’ is a small form in part because it simply doesn’t grow at the same rate as the wild plant. A season’s growth is probably about half the length of that seen on full-sized trees. The advantage is that it will naturally grow dense, and won’t become too big in your lifetime to need elaborate trimming. It is not like smaller forms of trees grow at the same rate as big ones, and then miraculously stop growing when they reach a smaller height. It doesn’t work that way.

  3. July 29, 2020 by Juli Goodhue

    I have a box wood elder that has a carpenter ant infestation. I was told to spray for the ants and use a high nitrogen fertilizer to boost the tree’s health and help fend off infestation and disease. I bought Cessco 46-0-0 fertilizer (little white pellets). Unfortunately it gives no directions for application. Are you able to advise? Thank you! Juli

    1. July 30, 2020 by Dave G

      See my response above to Aaron. I am not familiar with that product, so I can’t advise you on dose, but ‘high nitrogen’ doesn’t mean ‘only nitrogen’. It just means a fertilizer where the first number is the largest. I suggest you use a product for evergreen hedges, but at this time of year not a slow-release formulation – but slow-release would be good next spring. Follow the directions on the product. Too much nitrogen now will stimulate a lot of soft fall growth that will probably die back over the winter. By the way, it’s ‘box elder’ – a kind of maple tree, right?

  4. November 11, 2020 by Christine Kelly

    I have been told that urea mixed with water and painted to a cut in the tree will kill it. How true is this.

    1. November 11, 2020 by Dave G

      I am afraid all these ideas about stump removal, from urea to pennies, are of no value at all. File them under ‘fake news’. The only effective ways to remove stumps without waiting the decade or so for them to rot naturally (or watching them re-sprout!) is to dig them out, or use a stump chipper (arborists and some landscapers have them) to grind it down to a foot below ground level. Actually, for a reasonably fit person, digging out a 8-inch stump with a spade, axe and saw is not that big a deal, and that way, once it gone, it’s gone for good.

  5. March 16, 2021 by Diane Sargeant

    Can Urea help my Avocdo tree and my cherry tree ? they are not bearing

    1. March 17, 2021 by Dave G

      How old are they? Both take a few years to start bearing fruit, and the cherry, unless it is a sour one, needs a pollinator of a different cherry variety. If it flowered, but didn’t fruit, that is probably the problem.