Nitrogen (N)
Occurrence
Around 80% of our atmosphere consists of N. However, the extremely stable form of atomic N (N2) is not available to plants. Both free living and symbiotic microorganisms are capable of fixing atmospheric N2 in the form of NH4+ that can be directly taken up by plants or converted into NO3− by nitrifying bacteria.
The preferred form in which N is taken up depends on soil conditions and plant species [1 and 4•]. In general, plants adapted to low pH and reducing soil conditions tend to take up NH4+. At higher pH and in more aerobic soils, NO3− is the predominant form. Both NO3− and NH4+ are highly mobile in the soil. By contrast, organic N compounds such as amino acids, are far less mobile but there is growing evidence that these can also form important N sources [4• and 5].