When deficient, nitrogen (N) can be the greatest factor limiting growth of a plant. Plants receive nitrogen in either of three ways: organically rotting plant materials or manures provide the element, inorganic or man-made fertilizers are applied to soil or a gas permeates into the soil and is fixed by microorganisms for better absorption by roots. The nitrogen molecule is a basic building block in plant tissues.
All living plant cells contain nitrogen and many proteins, enzymes and metabolic processes in plants need the nitrogen molecule to be created and utilized. When soils lack nitrogen, oldest leaves begin to yellow, leaves look thin and smaller in size, and newest growth is stunted and short. Replenishing the soil with nitrogen allows plants to quickly reach optimum metabolism, including creating new stems and leaves and producing more food during photosynthesis. An overabundance of nitrogen can very quickly lead to lots of leaf and stem production, potentially at the cost of fewer flowers and subsequent fruits and seeds.
Intensification of Green Pigmentation
Closely related to the increased vigor resulting from nitrogen fertilizer application is the physical greening of plant tissues. Nitrogen is a component of the chlorophyll pigment molecule and without it new or replacement pigment molecules are not made. When photosynthesis is reduced, the overall growth of the plant diminishes. With more green chlorophyll, the plants look greener and the food-making machine that drives the growth of plants increases.
Increasing Seed/Fruit Yield
If plants are flowering well, nitrogen fertilizers will ensure roots are absorbing adequate amounts of nitrogen for the growth and expansion of cells in fruits and their seeds. A lack of nitrogen can lead to small-sized fruits and seeds, or seeds that are malformed, aborted or unviable for later germination.