The Primary Nutrients & Plant Growth - Nitrogen's Role

Among the three major nutrients, plants require Nitrogen in the largest amounts as it promotes rapid growth, increases leaf size and quality, hastens crop maturity, and promotes fruit and seed development. Importantly, Nitrogen being a constituent of amino acids, which are required to synthesize proteins and other related compounds, it plays a prominent role in almost all plant metabolic processes. Nitrogen is an integral part of chlorophyll manufacture through photosynthesis (the process through which plants utilize light energy to convert atmospheric carbon dioxide into carbohydrates). Carbohydrates (sugars) provide energy required for growth and development.

The chemical equation for photosynthesis is 6CO2 + 12H2O + 672 Kcal radiant energy = C6H12O6 + 6H2O + 6O2

Indian soils contain low levels of nitrogen and require annual applications to sustain crop growth. Little of the applied nitrogen is carried over to subsequent growing seasons due to crop removal, leaching and denitrification. Of all the elements required for crop production, nitrogen poses the greatest environmental threat due to contamination of surface and ground water.

Nitrogen fertilizer is available in both organic (manures) and inorganic forms. The amount of nitrogen in organic sources varies with source material and its state of decomposition. However, for commercial crop production, the following inorganic fertilizers are primarily used:

Ammonium Nitrate (33.5%N)
Potassium Nitrate (13% N)
Sodium Nitrate (16% N)
Calcium Nitrate (15.5% N), urea (46% N)
Mono-Ammonium Phosphate (18% N)
Di-Ammonium Phosphate (46% N) and
Liquid Nitrogen (30% N and 10-34-0)

Legume crops require little or no nitrogen fertilizer as the beneficial bacteria that live in the roots of these plants, capture nitrogen from the atmosphere. This nitrogen is available for use by the plant. Nitrogen is also used by microbes to break down organic matter.

Nitrogen-deficient plants exhibit slow stunted growth, and their foliage is pale green. Deficiency symptoms generally appear on the bottom leaves first. In severe cases, the lower leaves have a "fired" appearance on the tips, turn brown, usually disintegrate, and fall off.

In leafy crops such as tobacco, vegetables, forage and pasture crops, low nitrogen results in low yield and quality. When grain crops, such as corn and small grains, are deficient, they generally exhibit yellow leaf tips, stunted growth with spindly stalks and low yields of poor quality grain.

In contrast, too much nitrogen causes excessive vegetative growth, delays maturity, increases lodging, fosters disease and poses an environmental threat to surface and ground water. Nitrogen deficiency generally stems from inadequate fertilizer application, denitrification by soil microbes, or leaching loss due to excessive rainfall. Leaching occurs most commonly in sandy-textured coastal plain soils during periods of excessive rainfall. Nitrogen is also lost through volatilization from surface applications during periods of hot, dry weather.

An important point to be considered is that the Nitrogen deficiency can be corrected with an application of nitrogen fertilizer. Crop response to fertilization with nitrogen is generally very prompt, depending on the source of nitrogen, stage of plant growth, rainfall and temperature.