Here is another highly informative article on plant nutrition from http://www.hydroponicunlimited.com/. I am not affiliated with that site, but I find the forums to be very educational and another great platform for sharing information and shortening the learning curve.
Nitrogen and your plants.
Nitrogen isn't really used by a plant in it's raw form that you see when you hold some nitrogen sulfate in your hand. Nitrogen is absorbed by a plant and then used in many ways by the plant.
When adequate nitrogen is present, a plant grows fast and has lots of green foliage. It allows plants to grow to maturity instead of stunting the plant due to deprivation of nitrogen which causes plants to remain small and develop slowly as a result of the lack of nitrogen required for structural and genetic materials and processes.
When nitrogen is in amounts insufficient for the plant's total health, often the older leaves will be seen to become necrotic and die. This is because the plant is moving the nitrogen from the older, less important tissues into the younger, more important parts of the plant.
With nitrogen deprivation, a distinctive purple coloration of the undersides of leaves can been seen also. Root growth is restricted and flower and fruit development is delayed.
Many of the processes that a plant uses to survive and grow are accomplished by the use of chemical compounds. Each of those compounds are made up of various chemicals.
The compound that plants use to convert sunlight to produce plant sugars from water and carbon dioxide, "photosynthesis" is just one of the compounds that require nitrogen to function.
Many of the proteins used within plants are made up of amino acids that rely on nitrogen. The proteins that make up the structural components of a plant use nitrogen. Nitrogen is also a component in plant enzymes that are necessary to create the biochemical reactions on which life itself is based.
Many energy-transfer compounds like adenosine triphosphate, (ATP), allow cells to conserve and use the energy produced during metabolism.
Nucleic acids also rely on nitrogen. DNA, the genetic material that make it possible for plants to grow and reproduce, uses nitrogen in it's processes.
Bluntly put, without nitrogen, life as we know it would cease to exist.
In the soil itself, nitrogen is available in three different forms:
1. Organic Nitrogen Compounds
2. Ammonium ions
3. Nitrate ions
Of all the nitrogen in the soil that has the potential of being used by plants, 95 to 99 percent of that nitrogen is in organic forms of plant or animal residues, organic matter or in living organisms like microbes such as bacteria. Nitrogen in it's organic forms is not directly available to the plants to use.
Almost all of the nitrogen that is available for a plant to use is in the form of Ammonium ions (NH4) and as Nitrate ions (NO3). These two forms of nitrogen are usually referred to as "mineral nitrogen", and are what is used in hydroponic plant growth via a water/nutrient solution.
Direct link to this article is at http://www.hydroponicunlimited.com/showthread.php?20-Plant-Nutrients-and-how-they-are-used. Thank you hydroponicunlimited for allowing me the use of content for my blog and as always, thanks for providing quality information.