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What is urea?

Urea is a white crystalline substance with the chemical formula CO(NH₂)₂. It is highly soluble in water and contains 46.7% nitrogen. Urea is considered an organic compound because it contains carbon. It was the first organic compound ever synthesised by chemists; this was accomplished by accident in 1828 by the German chemist Friedrich Wöhlerin.

How is urea made?

Urea is made by reacting carbon dioxide (CO₂) with anhydrous ammonia (NH₃) at pressures above 14 000 kPa (2000 psi) and temperatures above 180 °C in a two-step process:

CO₂ + 2NH₃ --> NH₂COONH₄ --> CO(NH₂)₂ + H₂O

The water that is produced during the reaction must be removed by dehydration to maximise the yield. The resulting molten mixture is further processed into either prills (solidified pellets) or granules.

Urea also occurs naturally in urine (hence its name). We each excrete 20 to 30 grams of urea a day.

How much nitrogen does urea supply?

As urea contains 46.7% nitrogen, each tonne supplies 467 kg of nitrogen. For comparison, a tonne of ammonium nitrate supplies only 350 kg of nitrogen and a tonne of ammonium sulphate supplies only 212 kg of nitrogen. The high nitrogen content means lower transport and application costs per tonne of nitrogen.

How fast does urea convert to nitrate-nitrogen?

This depends on the environmental conditions. When soil conditions are favourable for microbial activity - in other words, warm and moist with an appropriate pH - the conversion is rapid, and will begin within a few days. In a band application, it may continue for a month or more.

How does urea compare with ammonium nitrate?

Urea contains about one-third more nitrogen, is less corrosive and is less prone to caking. In addition, until nitrification occurs, the nitrogen is less subject to leaching or denitrification than the nitrate portion of the ammonium nitrate.

Is the nitrogen in urea different from nitrogen in "organic fertilisers" in terms of plant nutrition?

No; the form of nitrogen taken up by plants is chemically identical regardless of the source. Nitrate and ammonium that result from urea application are indistinguishable from the nitrate and ammonium that result from manure application. Plants take up inorganic ions, even from organic manures, so the source is irrelevant to the yield and quality of the produce.

What role does urea play in world commerce?

Urea is the major fertiliser traded in international commerce. In the very near future it is expected to account for more than 50% of the nitrogen fertiliser in world trade, and already accounts for more than 65% of the world trade in dry fertilisers.

Why is urea increasing in world trade?

Urea has a number of advantages over other nitrogen fertilisers: it is safe to ship and handle, it is less corrosive to equipment, it has a higher nitrogen content than any other dry nitrogen fertiliser and it can be used on virtually all crops. Urea can be stored and distributed via conventional systems. It can be applied in many different ways, from aircraft to hand spreading. It is also highly water soluble, so it moves readily into the soil.