Originally Posted by
Halloamey
Well, well, looks like you have opened another Pandora's box here hehe. Ok so the general opinion expressed here is avoid urea nitrogen, though I some what agree to this opinion if you fertilize at the roots (foliar fertilization is a very different issue) since you are interested in the chemistry, I will divulge.
The reason cited here as to why urea is not a good source for orchids at the roots is scientifically wrong. It is not due to its unavailability without the break down by micro-organisms but due to the ammonia toxicity. Urea is an organic compound [CO(NH2)2] as against nitrate (NO3-) and ammonia (NH3) which are inorganic compounds. Urease is an enzyme that breaks down urea into carbon dioxide (CO2) and ammonia (NH3), it is a very robust enzyme produced by many microbes and most of the plants in very high concentrations. So breaking down the urea is not a problem, the problem is what happens next. Ammonia is a highly water soluble gas and it quickly dissolves in acidic water to produce the ammonium ion (NH4+). Actually the degradation of Ammonia can be written in the form of a balanced chemical reaction as follows
CO(NH2)2 + H2O = CO2 + 2NH3
NH3 + H2O = NH4(+) + OH(-)
The Ammonia produced is a very strong reducing agent and hence very caustic, it needs to get protonated that is convert from NH3 to NH4+ . This Ammonium hydroxide is a strong base and is what causes the most damage to the plant. The ammonia produced simply looks for water in its vicinity any where from any tissue and reacts with it, 'burning' the tissue. Soil has more water and other buffering cations and anions that limit the formation of ammonium hydroxide. And thus prevent this burning phenomenon.
Fertilizing with Urea is like fine art, you need to know the correct balance that will be assimilated quickly without producing too much ammonia to burn your plants. And if you fertilize by foliar application, then urea is the fastest assimilated source of Nitrogen, since it is already in the organic form. Many growers in Thailand use pig urine (high in Urea) as almost the sole source of fertilizer for their plants, and I don't need to tell you how their plants are.