If you’ve ever sifted through fertilizers at a garden store, you’re likely familiar with NPK ratios. These elements – nitrogen (N), phosphorus (P), and potassium (K) – are the three biggies, the primary elements that all plants need. The first of these, nitrogen, is a key element in amino acids (the building blocks of proteins, enzymes, and DNA) and central to the photosynthetic macromolecule chlorophyll. When nitrogen levels in the soil are low, chlorophyll production is stunted and plant leaves pale to a sickly yellow. To compensate for low nitrogen levels, a plant will slow stem growth and put more resources into root development, allowing the roots to forage in the soil for limiting nutrients.
The vast majority of nitrogen in the biosphere is in the form of gaseous N2 (dinitrogen). When you take a big deep breath of air, most of what you’re inhaling is this form of nitrogen (about 78% of the atmosphere is dinitrogen). And when you exhale, out comes the nitrogen, unchanged, unused, as unfortunately for most organisms, a triple bond binds the two nitrogen molecules together and is just about impossible to break.
This unusable nitrogen needs to be “fixed” into a biologically useful form like nitrate (NO3) or ammonia (NH4) before organisms can use it to build proteins and other important macromolecules. A small amount of soil nitrogen (<10%) is fixed by lightning. The rest is fixed biologically (called diazotrophy) by a variety of different species of bacteria. While most of these bacteria are free-living (like cyanobacteria, including the blue green algae responsible for that pea soup-like slurry that appears in the summer), some plants have very casual associations with these bacteria, and they’re often found in closer proximity. In these associative nitrogen fixation relationships, small clusters of bacteria live in the soil immediately around the roots of plants. Excess nitrogen products are absorbed by the plant’s roots, while excess photosynthates (carbohydrates) are exuded into the root zone