It gets a little deeper than this and I'd have to look at some references in order to give you some numbers, however generally speaking, nitrogen is an element that the plants use as a nutrient. Nitrate is just one of several different forms that nitrogen can be presented in. Nitrate contains a certain percentage of nitrogen, so does ammonium for example. So when one talks about nitrogen, they are not referring to a specific form of nitrogen, just the basic element itself.
Nitrogen is a basic element (N). Nitrate is an inorganic compound NO3. It is composed of one atom of nitrogen (N) and three atoms of oxygen (O). Nitrite (NO2) can be formed from nitrate by a chemical process called reduction.
Nitrogen is a macro nutrient for plants. It is an essential component of amino acids and, therefore, of proteins which include nucleic acids, enzymes, and the green, light-harvesting pigment, chlorophyll.
Thanks guys! That helped, atleast now I have somewhere to start from when I try digging depper into this. When I first startet out with plantet tanks, all the chemistry and technical equipment just pissed me off, I wantet to work with layout's, I didn't want to stress with learning about PO4, KNO3, MGSO4, lux, lumen, RA index, laterit, clay, peat and lordy lord how ever much else there was... But, lately I have been really digging into this, and man does it show on my tank! For one thing, very singel species of plant I have but into my thank has been growing like crazy! Even to such a rate that just from selling out my trimmings, I have made enough money to buy myself a PH controller and UV-filtering device. And the worst part of all this... It's actually fun!
I enjoy reading all your various post, and hope to contribute more in the future!