Why is urea subject to volatilization?

Urea in the soil needs to be converted to ammonium (NH4+) and nitrate (NO3-) before plants can absorb it.

Urease enzymes in the soil drive this process. Since it requires humidity too, it is called hydrolysis. During hydrolysis, the soil pH around the urea granules temporarily rises. As a consequence, the balance between dissolved ammonium (NH4+, bound to soil particles) and gaseous ammonia (NH3) is shifted to the latter form, increasing nitrogen losses by volatilization.

By contrast, nitrate fertilizers do not undergo hydrolysis and are therefore much less prone to volatilization. Volatilization generally occurs within two weeks upon application of urea. The losses are strongest when urea is not immediately incorporated into the soil, under hot and windy conditions, on high pH or light sandy soils grassland, or cow tillage.

The ammonia volatilization from the three main sources of nitrogen vary greatly according to the source of nitrogen [1].

Why is volatilization a problem?

Ammonia volatilization creates a twofold problem:

  • Ammonia volatilization is a direct loss of nitrogen, and thus money. Losses from urea represent 6 to 47 % of applied nitrogen while losses for nitrate fertilizers make up less than 4 % of applied nitrogen [1]. Losses for UAN are inbetween these values. The European Environmental agency has retained emission factors of 19.9 % for urea, 10.8 % for UAN and 3.0 % for AN [2].Volatilization losses are largely unpredictable and therefore hard to compensate. They depend on various environmental parameters. The nitrogen form is the most important factor influencing volatilization.
  • Volatilized ammonia represents an environmental burden. Most ammonia emitted to the air derives from agriculture. It travels beyond national borders, causing acidification and eutrophication of land and water upon deposition of ammonium (NH4+). This is why the European Union works on reducing ammonia emissions and has set emission ceilings per country.
  • Ammonia in the air reacts with other substances, forming fine particules. Particulate matter impairs air quality and creates a health hasard, especially for young children, asthmatics and people with respiratory insufficiency

The European Nitrogen Assessment Group [3] has evaluated the social cost of health damage from lost ammonia at 9,5 €/kg NH3 and for ecosystem damage at 2 €/kg NH3. More than 100 kg of NH3 can be lost per ton of urea. Reducing ammonia emissions throughout Europe therefore has a high priority.

Best agricultural practice 

Best agricultural practice strives to avoid or mitigate ammonia losses by different means. One of the most efficient is to use nitrate fertilizers instead of urea or UAN. Split application is another means to reduce volatilization. Incorporation of urea into the ground immediately upon spreading is highly efficient but hard to apply. On the other hand, volatilization risk is increased by the following conditions. If one or more of these conditions are met, spreading of urea or UAN should be reconsidered. 

  • hot and windy weather conditions with no rainfall expected upon spreading 
  • grassland or fields with large amounts of plant residues (eg. low tillage cultivation) that exacerbate ammonia losses 
  • alkaline soils or soils with low buffer capacity (sandy soils) that result in temporary high peak pH values.

Nitrate fertilizers - the golden standard

Urease inhibitors mitigate volatilization from urea-based fertilizers. When the only available nitrogen choice is urea or UAN, urease inhibitors therefore improve environmental and agronomic outcome. Nitrate fertilizers, however, remain the gold standard in terms of application precision, agronomic reliability and environmental compatibility.