Nitrogen

Nitrogen is a component of every protein and enzyme, of the chlorophyll that drives photosynthesis, and of nucleic acids. Adequate nitrogen supports leaf and canopy development, photosynthetic capacity, and the accumulation of protein in grains and forage.

Because nitrogen is so closely tied to growth, its supply has a large effect on yield potential. However, more nitrogen is not always better: supply must be matched to crop demand and to the soil’s own contribution.

Most soil nitrogen is held in organic matter and becomes plant-available only as microorganisms mineralise it to ammonium and then nitrate. Nitrate is highly soluble and can be lost by leaching below the root zone, while nitrogen can also be lost to the air through denitrification and ammonia volatilisation.

• Biological fixation by legumes and free-living microbes adds nitrogen from the atmosphere
• Mineralisation of organic matter releases plant-available nitrogen
• Leaching, denitrification, and volatilisation are the main loss pathways

Nitrogen is mobile within the plant, so deficiency symptoms appear first on older, lower leaves as the plant moves nitrogen to new growth. Typical signs are uniform pale-green to yellow leaves (chlorosis), reduced growth and tillering, and, if severe, early senescence and lower yield and protein.

Excess nitrogen can produce lush, disease- and lodging-prone growth, delay maturity, and reduce quality in some crops. Nitrogen lost from fields is also an environmental concern: nitrate can reach groundwater and surface water, and nitrous oxide is a potent greenhouse gas.

Nitrogen interacts with other nutrients: sulfur is needed alongside nitrogen for protein synthesis, and an imbalance can limit the response to nitrogen. Adequate phosphorus and potassium are needed for the crop to convert nitrogen into yield. Managing nitrogen well therefore means managing overall nutrient balance.