nitrate 1) "Plants can absorb nitrate or ammonium ions from the soil via their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants which have a mutualistic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. Animals, fungi, and other heterotrophic organisms absorb nitrogen as amino acids, nucleotides and other small organic molecules." Source and further information: http://en.wikipedia.org/wiki/Nitrogen_cycle 2) difference between ammonium ion and ammonia: "In an ammonium ion, the positively charged nitrogen atom forms four covalent bonds, instead of three as in ammonia. This reaction is reversible. The ammonium ion can act as a very weak Brønsted-Lowry acid in the sense that it can protonate a stronger base using any one of its hydrogen ( H ) atoms and convert back to ammonia. This means that the ammonium ion is a conjugate acid of the base ammonia. In a solution, the degree to which ammonia forms the ammonium ion depends on the pH of the solution." Source and further information: http://en.wikipedia.org/wiki/Ammonium

The form of Nitrogen most usable to plants is?