All about Nitrogen

Picture of a metal pot containing boiling liquid nitrogen
By David Monniaux – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=374418 Liquid nitrogen boiling at room temperature

Nitrogen (N) in it’s normal state is a colourless and odourless gas. It is highly abundant and makes up a whopping 79% of the air that we breathe. It is relatively unreactive and the atoms happily exist in pairs as N2 molecules as a gas at normal temperatures and pressures. Although nitrogen is a gas at room temperature, when it becomes extremely cold it turns into a liquid.

The temperature that nitrogen turns into a liquid is close to as cold as anything as get. The concept of “heat” is caused by the vibration of molecules or atoms in a substance. At -273.15 degrees Centigrade (known as “absolute zero” or zero kelvin) the atoms or molecules stop vibrating completely, and as such this is the lowest temperature that anything can get. Nitrogen turns into a liquid at -196 degrees centigrade so it is fairly close to this “absolute zero” temperature. Liquid nitrogen has many uses from making gourmet ice cream to the long term storage of sperm.

Although nitrogen is fairly unreactive, it will form bonds. Ammonia (NH3) is an example, as is nitrous oxide (NO3), both of which plants can uptake and use.

Nitrogen is incredibly important for life. It is the basis of amino acid molecules which are the building blocks of proteins, and even DNA. Without nitrogen, life would probably not exist as we know it.

For this reason, plants require nitrogen in relatively large amounts. Although nitrogen is abundant in the air, plants cannot simply breathe it in. Instead, it has to be absorbed through the roots, usually in the form of ammonium (NH3) or as nitrate (NO3). Once it is inside the plant it is used to make amino acids which are used to make proteins, chlorophyll, and other plant matter. In the form of nitrate, it is mobile and reusable within plants. This means that if there is shortage of available nitrogen, the plant can take it from lower, older parts of the plant to use for the growth of newer shoots and leaves.

Some plants (most notably legumes) work with certain microorganisms which “fix” nitrogen from the air. Fixing nitrogen is the process where nitrogen is taken from the air and turned into ammonium. Certain naturally occurring strains of bacteria that do this are present in most soils. The roots of most legume plants have special nodules where “nitrogen-fixing” bacteria are actively nurtured by the plant.

In this symbiotic relationship, the plant sends sugar down to the roots to feed the bacteria. In return the bacteria take nitrogen out of the air and “fix” it in the soil in the form of ammonium (NH3). Plant roots can then absorb and use the nitrogen in the ammonium. The “nitrogen-fixing” bacteria also enrich the soil with organic matter too. Therefore they play an important role in “crop rotation” where they replace the nitrogen in the soil which has been used up by other types of crop.

Nitrogen is a part of most plant matter. However, it is required in the largest amounts during the vegetative stage. Leafy growth is at its fastest during this phase and chlorophyll production is at a high. Chlorophyll molecules require 4 Nitrogen atoms. Once flowering/fruiting begins, the demand for Nitrogen is reduced but it is still needed for making plant matter and proteins.

Plants can relocate Nitrogen within themselves. When there is a shortage of Nitrogen, plants will move it from older, lower leaves, and send it to the top of itself to build new growth. A deficiency begins with yellowing leaves at the bottom of the plant. The yellowing progresses up the plant as the deficiency continues and leaves may begin to fall off. Flowering may be triggered prematurely as the plant tries to reproduce before it is too late:

Picture of a plant with very yellow leaves which has been caused by a deficiency in nitrogen
By Rasbak – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7384318

An overdose of Nitrogen first shows itself as very lush dark-green leaves. The dark greenness moves up the plant. The leaves may form a “claw” with burnt tips. The leaves and stems of the plant may become weak and brittle because the salt content within the plant makes it difficult to transport water and fluids around itself. The crop will often have a grassy taste of chlorophyll and/or salty.