Nitrogen in the plant: Nitrogen is taken up by plant roots from the soil, and is used in the formation of many plant tissues and compounds such as proteins, chlorophyll (green plant pigment), enzymes, and nucleic acids. Because plants use nitrogen whenever they form new tissue, this nutrient is used in large quantities whenever growth is rapid; plants that are dormant or growing slowly use less nitrogen.
Soil deficient in nitrogen: If the soil is deficient in nitrogen, plants break down the compounds in older leaves to recycle the nitrogen to new growth. This is why the older leaves of plants suffering from nitrogen deficiency turn yellow and die.
High nitrogen levels: High nitrogen levels stimulate leaf growth, often at the expense of root growth, flower and fruit production, and tuber development.
Most plants need to be fertilized regularly with nitrogen fertilizers. While nitrogen is needed all the time a plant is not dormant, it is needed in the greatest amounts when leaves are growing most rapidly. For most plants, this is from spring through early summer. (For more information about nitrogen fertilization, see Nitrogen.)
