Why Crops Require Fixed Nitrogen

Nitrogen that is in a stable and combined form is called "fixed nitrogen." Plants cannot use gaseous elemental nitrogen (N), such as exists in the air we breathe, but require fixed nitrogen. In fact nitrogen gas is inactive chemically to the point it is sometimes included for practical reasons among the "inert" gases. Fixed nitrogen is water-soluble and can be absorbed through a plant's root system and sometimes through the leaves.

Nitrogen becomes available to plants when it is fixated. Nitrogen becomes naturally fixated during lightning flashes in rainstorms. Water soluble nitrogen compounds that are formed dissolve in raindrops, which then fall to earth and enter the soil. In addition to available nitrogen produced in this way, fixed nitrogen is supplied in commercial fertilizers as ammonium compounds, nitrates, and urea-depending upon the particular plant needs and the specific chemistry and physics of the soil involved.

Going Green

Despite the type of plant and the form of nitrogen utilized, what are the reasons plants need to be provided with fixed nitrogen? To begin with, not only animals but also plants grow by building cells-cells require chemicals. One of the best known of these chemicals is chlorophyll-the substance that makes plants green. Chlorophyll is vital to plants, which enables growth and converts light energy into a form usable by the plant. Each molecule of chlorophyll contains four atoms of nitrogen. Without nitrogen availability, no chlorophyll could exist.

Proteins and Nitrogen

Do plants contain proteins? Indeed they do. In fact, according to the American Journal of Clinical Nutrition,¹ plants provide per capita, worldwide, approximately 65 percent of the world's supply of protein. Proteins contain amino acid linkages, each of which includes a nitrogen atom. Hence, fixed nitrogen is essential to the formation of plant proteins.

DNA and Nitrogen

Each cell contains DNA. DNA, or deoxyribonucleic acid, the substance that identifies what a living thing will be, whether a fish or a frog, celery stalks or bananas. DNA contains four smaller structures within it called nucleotides. Nucleotides are rich in nitrogen content, hence fixed nitrogen is critical for any and all plants, whether they are green or not. The four nucleotides are thymine, cytosine, guanine, and adenine. How much nitrogen does each of these nucleotides contain? Thymine contains two atoms of nitrogen per molecule, or about 22 percent by weight. Cytosine contains three atoms, or approximately 38 percent by weight. Guanine and adenine each contain five atoms of nitrogen per molecule. Guanine thus contains approximately 46 percent by weight of nitrogen, while adenine contains an amazing 52 percentage points!

Conclusion

Thus, it has become evident that plants need, not just a little fixed nitrogen, but really quite an extensive amount. A nutrient that is needed in relatively large amounts is termed a macronutrient.² In fact, when one buys a commercial fertilizer mixture, three macronutrients are generally found included in the mixture: nitrogen, phosphorous, and potassium. Later we shall see why the other two substances are needed in abundance, as well as nitrogen.

¹ Am J Clin Nutr 1994;59 (Suppl): 1203S-12S "Plant proteins in relation to human protein and amino acid nutrition," by Vernon R Young & Peter L Pellett.
² A nutrient needed in only small quantities is termed a micronutrient.

Additional References:

EarthSky - What's the difference between plant DNA and animal DNA?