The Importance of Nitrogen for Plant Growth

The Importance of Nitrogen for Plant Growth: Deficiency, Toxicity, and Fertilizer Management

What is nitrogen, and why is it important for plants?

Nitrogen (N) is a vital chemical element, making up 79% of the atmosphere. However, for plants to absorb it, nitrogen must react with elements like hydrogen and oxygen to be assimilated into the soil. This process occurs through several pathways:

Nitrogen fixation: Bacteria from the Rhizobium family, which form symbiotic relationships with the roots of legumes, convert atmospheric nitrogen into forms that plants can absorb.
The Haber process: In this industrial process, nitrogen is combined with hydrogen under high pressure and temperature to produce ammonia, a key nitrogen source for fertilizers.
Lightning and organic matter: Nitrogen also enters the soil through nitrate precipitation from lightning and organic matter breakdown, such as proteins and amino acids.
Nitrification: Bacteria like Nitrosomonas convert ammonia into nitrites, and Nitrobacter transforms nitrites into nitrates, which plants can absorb more easily.

All these processes are essential for converting inert atmospheric nitrogen into forms that plants can use. Plants mainly absorb nitrogen in the nitrate form and, to a lesser extent, in its ammoniacal form. The presence of nitrogen in the soil meets a significant portion of a plant's nutritional needs.

Nitrogen is essential for plant growth, vigor, productivity, and seed formation. It constitutes 3-4% of plant tissue, which is higher than any other nutrient. Only oxygen, hydrogen, and carbon are present in larger amounts, but these elements provide no direct nutritional value. Nitrogen plays an indispensable role in several processes:

Chlorophyll production: Nitrogen is a component of chlorophyll, which is necessary for photosynthesis, enabling plants to convert sunlight, carbon dioxide, and water into sugars for energy.
Protein and nucleic acid synthesis: Nitrogen is a core component of amino acids, the building blocks of proteins, and nucleic acids (DNA and RNA), which carry genetic information.
Energy transport: Nitrogen is part of important compounds like ATP (adenosine triphosphate), which stores and transports energy within cells.
Vitamins, hormones, and alkaloids: It contributes to the formation of vitamins and hormones, as well as alkaloids that help plants resist diseases like Botrytis and Phytophthora.

Nitrogen Deficiency and Toxicity

Nitrogen Deficiency and Toxicity Nitrogen Deficiency

When plants do not receive enough nitrogen, they develop a deficiency, leading to two main stages of symptoms:

Initial phase: Reduced leaf growth occurs, but photosynthesis is not immediately affected.
Advanced phase: Nucleic acids, proteins, and chlorophyll begin to degrade, resulting in visible symptoms like chlorosis (yellowing leaves), starting from older leaves. The plant’s growth slows, leading to a smaller, stunted appearance.

Nitrogen Toxicity

Excess nitrogen can cause toxicity, delaying plant maturation and reducing both quality and yield. In fruit trees, for example, excessive nitrogen can lead to competition between vegetative growth and fruiting, resulting in poor production. The plant becomes more susceptible to pests and diseases, such as nematodes. In cereals, nitrogen toxicity can cause lodging (bending of the stems), making the plant more vulnerable to damage due to overly large spaces between internodes. Symptoms of nitrogen toxicity in leaves include thickening, darker green coloration, and upward curling of the tips. In severe cases, the leaf tips turn yellow or brown, and the leaves may eventually fall off.

How to Manage Nutritional Disorders

Nutritional disorders include both deficiencies and toxicities caused by imbalances in nutrient levels, such as nitrogen.

Correcting Nitrogen Deficiency

Nitrogen deficiency can be corrected by applying fertilizers. Organic fertilizers include manure, compost, green manure, and urea (a slow-release fertilizer). Inorganic fertilizers are available in granular or crystalline forms, often used with or without nitrification inhibitors. These can be applied through various methods:

Soil application: Fertilizers are spread or mixed into the soil.
Fertigation: Fertilizers are mixed with irrigation water.
Foliar feeding: Fertilizers are sprayed directly onto the plant leaves.

To prevent nitrogen toxicity, the amount of fertilizer applied, the timing, and environmental conditions must all be carefully managed.

Nitrites and Nitrates in the Environment

Nitrates and nitrites are forms of nitrogen naturally found in the soil, but they are also produced by bacteria. Nitrifying bacteria convert ammonia into nitrites and nitrates, which plants absorb. However, excess nitrogen in the form of nitrates can accumulate in plants and leach into groundwater, posing health risks to humans.

Nitrites, in particular, can be harmful because they can react with amines and amides in the body to form carcinogenic compounds known as N-nitrosamines. Nitrites can also oxidize hemoglobin in the blood, reducing its ability to carry oxygen, leading to a condition called methemoglobinemia (blue baby syndrome), especially dangerous for infants. To avoid these health risks, it is important to:

Thoroughly wash vegetables.
Cook them properly.
Store cut vegetables at appropriate temperatures.

This is especially important for leafy vegetables, which can accumulate higher amounts of nitrates due to excessive nitrogen fertilization. Traders and consumers should check for any damage or bruises on vegetables, as these can make them more vulnerable to contamination and disease.

References:

Further reading

Nitrogen (N): Essential for plant growth and yield but can have high costs to farmers, the environment, and health

Biological Nitrogen Fixation and seeding Legumes for Soil Fertility

Key Practices for effective Nutrient Management in Organic Farming

Urea and Slow Release Fertilizers: Importance and Best Use Practices