Question

Why do plants need nitrogen? How do plants obtain nitrogen?

Answer 1

Hint: Nitrogen is a critical component of many structural, genetic, and metabolic compounds found in plant cells. It is also a basic component of many important organic compounds, such as amino acids, proteins, nucleic acids, enzymes, and the chlorophyll molecule.

Complete answer:
Nitrogen is the most frequently limiting nutrient for crop growth of all the essential nutrients. Nitrogen is the nutrient that typically produces the highest yield response in crop plants, promoting rapid vegetative growth and providing the plant with a healthy green colour.

Nitrogen is taken up by roots in the inorganic forms of nitrate $NO_{3}^{-}$ and ammonium $NH_{4}^{+} $ ions. Once inside the plant, $NO_{3}$ is converted to $NH_{2}$ and assimilated to form organic compounds.

Nitrogen addition is not recommended for legume crops such as soybean because they produce their own nitrogen supply. Nitrogen-fixing soil organisms (rhizobium) associated with legume roots capture atmospheric nitrogen and provide it to the plant.

Nitrogen is obtained naturally by plants. Fertilizers and animal and plant wastes add nitrogen to the soil. Bacteria in the soil convert nitrogen to ammonium and nitrate, which plants absorb through a process known as nitrogen fixation. Plants require nitrogen to produce amino acids, proteins, and DNA. The nitrogen present in the atmosphere is not of the type that plants can use.

Above-ground tissues of healthy plants typically contain 3 to 4% nitrogen. When compared to other nutrients, this is a much higher concentration. The only other nutrients present in higher concentrations are carbon, hydrogen, and oxygen, which do not play a significant role in most soil fertility management programmes.

Nitrogen is essential because it is a component of chlorophyll, the compound that plants use to convert sunlight energy into sugars from water and carbon dioxide (i.e., photosynthesis). It's also a key component of amino acids, which are the building blocks of proteins.

Plants wither and die in the absence of proteins. Some proteins function as structural units in plant cells, while others function as enzymes, enabling many of the biochemical reactions that underpin life. Nitrogen is found in energy-transfer compounds like ATP (adenosine triphosphate).

Finally, nitrogen is an important component of nucleic acids such as DNA, which is the genetic material that allows cells (and eventually entire plants) to grow and reproduce. There would be no life as we know it if nitrogen did not exist.

Note: Plants that lack nitrogen grow slowly and appear weak and stunted. Older leaves are light green or yellow. Because nitrogen is mobile in the plant and moves to where it is needed, the symptoms of nitrogen deficiency are always seen first on the older and lower leaves.