Nitrogen (N)

What nitrogen does

Nitrogen is part of every protein and of chlorophyll, the green pigment plants use to capture light. It drives leaf and tiller growth, canopy size and, in cereals, grain protein.

Signs of deficiency

Nitrogen moves within the plant, so shortages show first in the older, lower leaves. They turn pale and yellow while newer growth stays greener. Crops are thin, with fewer tillers or branches.

Fertiliser forms

• Urea: the most concentrated common solid nitrogen fertiliser.
• Ammonium forms, such as ammonium sulphate, which also supply sulphur.
• Compound fertilisers that combine nitrogen with phosphorus, potassium, sulphur or other nutrients in each granule.
• Liquid nitrogen products such as UAN.

How nitrogen is lost

• Volatilisation: urea on the soil surface can lose nitrogen to the air as ammonia, especially on warm, moist soil with no follow-up rain, and on alkaline soils.
• Leaching: nitrate moves down with water, below the root zone, on sandy soils and in wet periods.
• Denitrification: in waterlogged soils, nitrate is converted to gases and lost.
• Immobilisation: soil microbes breaking down stubble can temporarily tie up nitrogen.

Why nitrogen goes unused

• Sulphur shortage: plants need sulphur to turn nitrogen into protein, so a crop short of sulphur uses nitrogen less fully.
• Seed-row effects: urea dissolving next to the seed releases ammonia, which can damage germinating seed and thin the stand.
• Late uptake: nitrogen taken up late in cereals goes mostly into grain protein rather than yield.