Thu, 23 Jun 2016
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Urea: Friend or Foe?

Once upon a time, ammonium nitrate was readily available to nurseries.  This ‘all nitrogen’ fertiliser was useful for quickly supplying some additional nitrogen, or for inclusion within fertigation solutions as a major source of nitrogen.  A main advantage of ammonium nitrate over other nitrogenous fertilisers is its equal proportions of the ammonium and nitrate forms of soluble nitrogen, as is now explained.

Ammonium and nitrate

Most of the soluble nitrogen in a growing medium is present as either ammonium or nitrate ions (NH4+ or NO3)

When a root takes in an ammonium ion to maintain electrical neutrality, it releases a hydrogen ion (H+) into the growing medium.  In other words, the root releases acidity into the growing medium. The pH of the medium decreases (pH is defined as the negative logarithm of the concentration of hydrogen ions present).

When a root takes up a nitrate ion, it releases a hydroxyl ion (OH) into the growing medium. Hydroxyl ions readily react with hydrogen ions to produce water: OH + H+ → H2O, thus the acidity of the growing medium is decreased.

Therefore, if equal amounts of ammonium and nitrate ions are taken up by plant roots, there should be no change in growing medium pH.

Another source of acidity is when ammonium ions are converted to nitrate in the growing medium itself.  The reaction is: NH4+ + 2O2 + (microbes) → NO3 + H2O + 2H+

Whichever way you look at ammonium, it makes a growing medium more acidic.

Ammonium nitrate in the age of terrorism

It is still possible to obtain a license to buy and use ammonium nitrate – under strict government supervision – although the hassle involved is quite considerable.

There are many other potential sources of fertiliser nitrogen – as shown below – although the one that is now most often used is urea.  A liquid formulation that contains both urea and ammonium nitrate (UAN) is also available.

Nitrogen carrier Pros Cons
Urea Relatively cheap, highly soluble More acidifying than ammonium nitrate; may contain biuret, which is toxic to plants. Some losses of nitrogen as ammonia gas from surface applications.
UAN (urea-ammonium nitrate solution) Highly concentrated solutions in water of ammonium nitrate and urea with 28-32% N.  

Highly corrosive to metals, so plastic equipment must be used. Less acidifying than urea alone. Must not be used in mixtures that also contain calcium nitrate.

Ammonium sulphate Highly soluble  

Highly acidifying; the sulphate may not be needed

Potassium nitrate All the nitrogen is present as nitrate, so its use tends to raise mix pH  

Expensive; the potassium may not be needed

Calcium nitrate All the nitrogen is present as nitrate, so its use tends to raise mix pH  

Absorbs water from the atmosphere; difficult to handle; a dilute source of nitrogen

Calcium ammonium nitrate Contains both ammonium and nitrate  

Not totally soluble, so cannot be used in fertigation solutions unless its lime content is allowed to precipitate out.

Ammonium phosphates  

Expensive; only slightly soluble; only useful where phosphorus is also needed

Magnesium nitrate  

Very expensive.

Sodium nitrate  

The last thing most nurseries need is extra sodium

Ammonium chloride  

The last thing most nurseries need is extra chloride

 

Urea a few facts

Urea has the chemical formula CO(NH2)2. It is produced by reacting liquid ammonia with carbon dioxide, under high pressure. Reaction conditions have to be carefully controlled when horticultural grade urea is being produced so that the production of biuret (two molecules of urea joined together – NH2CONHCONH2) is minimised. Biuret is toxic to plants. In recent years, horticultural grade urea has consistently been found to have a low (and non-toxic) biuret content.

When urea is added to moist growing media, it is rapidly converted to carbon dioxide and ammonia: CO(NH2)2 + H2O + urease → CO2 + 2 NH3

The ammonia dissolves in the water in the growing medium to give ammonium ions (NH4+).

Some precautions in relation to urea use

If urea is left sitting on the surface of the medium, some of the ammonia released from it may be lost into the atmosphere before it can dissolve in water. This loss can be as high as 60% of the added nitrogen, so additions of urea should be quickly watered into the growing medium.

Because all of the nitrogen in urea is ultimately present as ammonium ions, it is more acidifying than the same amount of nitrogen supplied via ammonium nitrate. This acidification may need to be countered via an inclusion of coarse dolomite in the growing medium.

Another potential problem with urea is that the high concentration of ammonium ions produced from it can be toxic to some young plants. In warm to hot weather, these ammonium ions are rapidly converted to nitrate ions. However, in the cold of a southern winter, the microbes that do this conversion have a rest, so a high concentration of ammonium ions can persist for weeks. Seedling growers must monitor ammonium concentrations to prevent ammonium toxicity in their plants.

As long as attention is paid to the foregoing potential problems with urea, there is no reason not to use it as a source of nitrogen for plants in production nurseries.

More information 

For more information, please contact one of our grower services representatives.

Disclaimer

Please note, any advice displayed is of a general nature only, and an intending user of a product should obtain and only rely on professional advice particular to their intended purpose, climate, soil conditions and other factors.