Discovery of the essentiality of nitrogen is often credited to de Saussure , who in 1804 recognized that nitrogen was a vital constituent of plants, and that nitrogen was obtained mainly from the soil. De Saussure noted that plants absorb nitrates and other mineral matter from solution, but not in the proportions in which they were present in solution, and that plants absorbed substances that were not required for plant growth, even poisonous substances . Other scientists of the time believed that nitrogen in plant nutrition came from the air. The scientists reasoned that if it was possible for plants to obtain carbon from the air, which is a mere 0.03% carbon dioxide (by volume), then it would be easy for plants to obtain nitrogen from the air, which is almost 80% nitrogen gas. Greening was observed in plants that were exposed to low levels of ammonia in air, further suggesting that nitrogen nutrition came from the air. Liebig  wrote in the 1840s, at the time when he killed the humus theory (the concept that plants obtain carbon from humus in soil rather than from the air), that plants require water, carbon dioxide, ammonia, and ash as constituents. Liebig supported the theory that plants obtained nitrogen as ammonium from the air, and his failure to include nitrogen in his “patent manure” was a weakness of the product. Plants will absorb ammonia at low concentrations from the air, but most air contains unsubstantial amounts of ammonia relative to that which is needed for plant nutrition. 


PROPERTIES AND USE OF NITROGEN FERTILIZERS
1 Anhydrous Ammonia (82% N) 
Anhydrous ammonia is the most-used nitrogen-containing fertilizer for direct application to land in the United States . Worldwide, consumption of anhydrous ammonia is ranked fourth or fifth among nitrogen fertilizers . In agriculture, anhydrous gaseous ammonia is compressed into a liquid and is applied under high pressure with a special implement by injection at least 15 cm deep into a moist soil. The ammonia gas reacts with water to form ammonium ions, which can be held to clay or organic matter. If the ammonia is not injected deeply enough or soil is too wet or dry, ammonia can be lost by volatilization. Anhydrous ammonia is usually the cheapest source of nitrogen, but equipment and power requirements of the methods of application are specific and high. 

2 Aqua Ammonia (21% N)
Aqua ammonia is ammonia dissolved in water under low pressure. Aqua ammonia must be incorporated into land to avoid losses of nitrogen by ammonia volatilization; however, it needs not be incorporated as deeply as anhydrous ammonia. 

3 Urea (46% N)
Urea is the most widely used dry nitrogen fertilizer in the world . After application to soils, urea is converted into ammonia, which can be held in the soil or converted into nitrate. Ammonia volatilization following fertilization with urea can be substantial, and if urea is applied to the surface 

4 Ammonium Nitrate (34% N)
Ammonium nitrate is a dry material sold in granular or prilled form. It can be broadcasted or sidedressed to crops and can be left on the surface or incorporated. It does not give an alkaline reaction with soils; hence, it does not volatilize readily. However, incorporation is recommended with calcareous soils. Ammonium nitrate is decreasing in popularity because of storage problems, e.g., with fire and explosion. 

5 Ammonium Sulfate (21% N)
Ammonium sulfate is marketed as a dry crystalline material. It is recommended for use on alkaline soils where it may be desirable to lower soil pH. Nitrification of ammonium is an acidifying process. Ammonium sulfate can be broadcasted or sidedressed. It can left on surfaces or incorporated, although on calcareous soils watering in or incorporating is recommended to avoid ammonia volatilization . 

Nitrogen Deficiency
Symptoms: 
1. Old leaves turn pale green and may eventually yellow and fall from the plant. Although some yellowing of old leaves may be normal, if more than a leaf or two on your plant suddenly begins to yellow it is a good indication that your plants are suffering from a nitrogen deficiency.

2. New leaves are paler green than existing leaves and appear to be smaller than normal. Young leaves should be a rich green and should grow rapidly. If they are tiny and fail to increase in size within a few days, they may need a boost of Nitrogen.

3. Plant growth slows or stalls. This should not be confused with plants that have reached their mature size. If the plant has not reached maturity or fails to produce new leaves and branching ceases, the culprit may be a Nitrogen Deficiency.

4. Undersides of leaves may take on a red or purple hue. Although this depends on the type of plant you are growing, leaves that discolor on the underside may be a sign for you to add Nitrogen to the soil. 

What kinds Nitrogen fertilizer terrabetter company have ?
Urea
Ammonium sulphate
Ammonium chloride
Ammonium nitrate
Mono ammonium phosphate NPK 12-61-0
Diammonium phosphate NPK 21-53-0
Potassium Nitrate NPK 13-0-46
Calcium Nitrate NPK 11-0-0+23CaO
Calcium Ammonium Nitrate NPK 15.5-0-0+26CaO
NPK 12-12-17+2MgO
NPK 13-13-20+2MgO
NPK15-15-15
NPK 19-19-19+ME
NPK 20-20-20+ME