Lets see if the SDMB can put an end to an argument here at work. Are there plants that exist that absorb nitrogen out of the air?
No. Atmospheric nitrogen (N[sub]2[/sub]) cannot be used by plants. It must be in the form of nitrate (NO[sub]3[/sub]) or ammoniam (NH[sub]4[/sub]). These forms are referred to as “fixed nitrogen”. The scarcity of fixed nitrogen is one of the limiting factors for plant growth, as nitrogen is used in all amino acids. Some plants, such as legumes, have symbionic nitrogen-fixing bacteria in the nodules of their roots. The bacteria fix the nitrogen, making it usable by the plant.
Here’s a pdf that talks about nitrogen fixation in legumes.
This site also talks about nitrogen fixation.
Nitrogen is also a component of the nucleotides required to synthesize DNA and RNA.
-Apoptosis
What about orchids and such that don’t live in soil?
No, orchids don’t have nitrogen fixing bacteria. They do have the normal symbiotic bacteria that all plants have. These help by digesting any detrital material witin range of the roots and mineralising the nitrogen, enabling it to be absorbed by the plant.
And also remember carnivorous plants, that extract their nitrogen from insects and small animals.
slight hijack
Where does the nitrogen used for fertilizers come from? is it taken from the atmosphere?
Nitrogen is fixed industrially using the Haber-Bosch process. Atmospheric nitrogen gas and hydrogen gas (I’m pretty sure actual hydrogen gas is required, which must be provided separately, since hydrogen gas is too light to stay in the atmosphere) are reacted to produce ammonia. The reaction must be carried out at high temperatures with a catalyst (otherwise it’s extremely slow), with the product ammonia continuously being removed to keep the reaction moving toward product.
Then, the Ostwald process is used to produce nitric acid from ammonia and oxygen gas. This is a multi-step process, in which ammonia is burned with excess oxygen, the resulting nitrogen monoxide is reacted with more oxygen to produce nitrogen dioxide, and finally the nitrogen dioxide is reacted with water to produce nitric acid.
Put the two together, and you get ammonium nitrate (fertilizer).
Lightning is the major nonbacterial source of fixed nitrogen (nitrates and ammonia). Here’s a brief overview of the nitrogen cycle.
I see jharmon’s already mentioned the Haber-Bosch process for industrial ammonia production. Here’s a more detailed description of the work that led to Haber’s 1918 Nobel prize in chemistry.
Most common fertilizer is UREA - CO(NH2)2. NH3 (Ammonia) is produced by reacting Atmospheric Nitrogen with H2 (hydrogen) produced by steam reforming of Natural Gas or other Petroleum Cuts (e.g. Naphtha). NH3 is reacted with the CO2 produced during steam reforming to give UREA.
Oxydizing the NH3 to Nitric Acid is rather wasteful for fertilizer purposes.
Other Industrial N Sources used for fertilizers are natural Nitrate salts like Chile Saltpetre, etc.
Clover takes nitrogen out of the air and can be found in low-nitrogen soils.
I’ll be back to cite.
cite:
http://www.ext.nodak.edu/extpubs/plantsci/soilfert/sf728w.htm
A few years back, when I decided to go to an almost all organic lawn care program, I quickly learned about clover and what it meant to the soil, and what it meant about the soil.
Clover = poor nitrogen level in soil, and clover will fix nitrogen from the air and make it available to the soil.
The clover itself doesn’t fix nitrogen. It’s the symbiotic bacteria in the legume’s root nodules that convert N[sub]2[/sub] to ammonia.